Fluke RUSKA 2417, RUSKA 2416, RUSKA 2413 User Manual

RUSKA 2413, 2417 &

2416

Differential Pressure Null Indicator

Users Manual

PN 3966494
November 2010
© 2010 Fluke Corporation. All rights reserved. Printed in USA. Specifications are subject to change without notice.
All product names are trademarks of their respective companies.

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P.O. Box 9090
Everett, WA 98206-9090
U.S.A.

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P.O. Box 1186
5602 BD Eindhoven
The Netherlands

11/99

To register your product online, visit register.fluke.com

Table of Contents

Chapter Title Page

1 Description and Specifications.......................................................... 1-1

Introduction........................................................................................................ 1-1

How to Contact Fluke ........................................................................................ 1-1

Safety Information ............................................................................................. 1-1

Symbols Used in this Manual ............................................................................ 1-1

General Description ........................................................................................... 1-2

Specifications..................................................................................................... 1-2

2 Applications......................................................................................... 2-1

Introduction........................................................................................................ 2-1

Gas-to-Gas..................................................................................................... 2-1

Liquid-to-Gas ................................................................................................ 2-1

Liquid-to-Liquid............................................................................................ 2-1

3 Preparation for Use ............................................................................. 3-1

Preparation for Use ............................................................................................ 3-1

Bleeding Lower Chamber in a Liquid-To-Liquid System................................. 3-2

4 Operation ............................................................................................. 4-1

Operating Instructions........................................................................................ 4-1

5 Performance Observations ................................................................ 5-1

Actual Sensitivity versus Apparent Sensitivity.................................................. 5-1

Calibration ......................................................................................................... 5-1

Detecting Leaks ................................................................................................. 5-2

6 Maintenance......................................................................................... 6-1

Servicing the Instrument.................................................................................... 6-1

Diaphragm ......................................................................................................... 6-1

RUSKA 2413 Differential Pressure Null Indicator —Replacement of

Differential Transformer.................................................................................... 6-2

Failure Diagnoses .......................................................................................... 6-2

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RUSKA 2413, 2417 & 2416

Users Manual

RUSKA 2413 — Replacement of Transformer................................................. 6-2

7 Parts List .............................................................................................. 7-1

RUSKA 2413 Differential Pressure Null Indicator — Parts List ...................... 7-1

RUSKA 2417 Differential Pressure Transducer — Parts List........................... 7-5

RUSKA 2417-706 Differential Pressure Cell 40,000 PSI — Parts List ............ 7-5

8 General Information ............................................................................ 8-1

Introduction........................................................................................................ 8-1

9 Functional Circuit Description ........................................................... 9-1

Introduction........................................................................................................ 9-1

0Test Procedure .................................................................................... 1

Introduction........................................................................................................ 1

Setup Procedure ................................................................................................. 1

RUSKA 2417 Transducer — Replacement of Differential Transformer ...... 6-3

Failure Diagnoses .......................................................................................... 6-4

Appendices

A Explanation of Test Report.......................................................................... A-1

ii

List of Tables

Table Title Page

1-1. Symbols.................................................................................................................. 1-2

6-1. Failure Diagnosis ................................................................................................... 6-2

6-2. Resistance Measurements for the Windings .......................................................... 6-4

7-1. RUSKA 2413 Differential Pressure Cell - Parts List ............................................. 7-4

7-2. RUSKA 2417 Differential Pressure Cell - Parts List ............................................. 7-5

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RUSKA 2413, 2417 & 2416

Users Manual

iv

List of Figures

Figure Title Page

7-1. RUSKA 2413-711 Differential Pressure Null Indicator - Side View .................... 7-2

7-2. RUSKA 2413-711 Differential Pressure Null Indicator - Top View ..................... 7-3

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RUSKA 2413, 2417 & 2416

Users Manual

vi

Description and Specifications

Introduction

This manual covers the operation and maintenance of the RUSKA 2413, RUSKA 2416
and RUSKA 2417.

How to Contact Fluke

To order accessories, receive operating assistance, or get the location of the nearest Fluke
distributor or Service Center, call:

• Technical Support USA: 1-800-99-FLUKE (1-800-993-5853)

• Calibration/Repair USA: 1-888-99-FLUKE (1-888-993-5853)

• Canada: 1-800-36-FLUKE (1-800-363-5853)

• Europe: [phone]

• China: [phone]

• Japan: [phone]

• Singapore: [phone]

• Anywhere in the world: [phone]

Chapter 1

Or, visit Fluke's website at www.fluke.com

To register your product, visit http://register.fluke.com

To view, print, or download the latest manual supplement, visit

http://us.fluke.com/usen/support/manuals

.

.

.

Safety Information

W Warning

The control box/null indicator must be set for the proper line
voltage prior to connection to a power source.

Symbols Used in this Manual

In this manual, a Warning identifies conditions and actions that pose a hazard to the
user. A Caution identifies conditions and actions that may damage the Differential
Pressure Null Indicator.

Symbols used on the Differential Pressure Null Indicator and in this manual are explained in
Table 1-1.

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RUSKA 2413, 2417 & 2416

Users Manual

General Description

The Differential Pressure Null Detector, composed of a Differential Pressure Transducer
(RUSKA 2413 or RUSKA 2417) and Electronic Null Indicator (RUSKA 2416), is
designed to sense small pressure differences in both low and high pressure systems.

The transducer consists of two pressure chambers, separated by a thin diaphragm. A
difference in pressure in the two chambers causes a deflection of the diaphragm and a
resultant signal to the electronic circuit. The signal is obtained as the output of a
differential transformer whose movable core is attached to the diaphragm. The signal is
not a linear function of the difference in pressure; therefore, use of the instrument for
accurate evaluation of pressure differences is limited to small deflections of the
diaphragm. The principle use of the instrument is intended as a null sensor/indicator with
which the pressure of one medium may be precisely adjusted to that of another. Some of
the advantages of the instrument are its high sensitivity, high working pressure
(15,000 psi for RUSKA 2413 Series cells and 40,000 psi for RUSKA 2417 Series), and
its ability to withstand the full working pressure across the diaphragm without injury
(15,000 psi maximum over-range pressure both series).

Table 1-1. Symbols

Symbol Description

B AC (Alternating Current)

J Earth Ground

W Important Information: refer to manual

Do not dispose of this product as unsorted

~

municipal waste. Go to Fluke’s website for
recycling information.

Specifications

Inaccuracy Inaccuracy is defined as the error in the null indication. It is

expressed as the ratio of ΔP actually existing when the meter
indicates a null, to the total cell pressure, in parts per million,
or as constant ΔP — whichever is greater.

PPM ΔP PSI

Error with calibration

corrections

Error without calibration

corrections

Sensitivity The sensitivity is continuously variable from 2* 10-4 psi ΔP

per meter division to 0.01 psi ΔP per meter division. The
maximum value may exceed 2 *10-4 psi/div. because of
variations in diaphragm characteristics and circuit parameters.

Operating Pressure 15,000 psi liquid or gas for RUSKA 2413 Series cells; 40,000

psi for RUSKA 2417 Series cells (See pressure media for
limitations).

Static Test Pressure 22,000 psi for five minutes with nitrogen. The ungasketed

5 0.01

20 0.1

1-2

Description and Specifications

Specifications 1

metal seals act as relief valves when pressures exceed 22,000
psi. The bolts yield to the increased lead and permit the
excess pressure to escape. All attempts at destructive testing
of these units have failed. 50,000 psi for RUSKA 2417 Series
cells.

Over-Range Pressure 15,000 psi ΔP either side of diaphragm for both RUSKA

2413 and RUSKA 2417.

Construction Material Basic material of the transducer is one of the 400 Series

Stainless Steels.

Pressure Media Lower Chamber of pressure cell — Dry air, nitrogen,

mercury, or any fluid inert to 400- or 300-Series Stainless
Steels.

Upper Chamber — Dry air, nitrogen, or any fluid inert to

400- or 300-Series Steels, low-carbon iron, brass, copper,
PVC, cadmium-plated steel or soft solder. Electrolytes may
not be used in the upper chamber.

It is not recommended to use fluids in either cavity

containing free hydrogen. The use of such fluids is
hazardous because of possible hydrogen embrittlement of
the cell body. (Consult the manufacturer for cells of special
materials.)

Temperature Range 40º F to 160º F

Construction Details and Parameters:*

Note

Replacing diaphragm or transformer voids calibration.

Change in Null with Working

See Specifications

Pressure

The stress from the applied pressure produces a

displacement of the core within the transformer even though
the pressure across the diaphragm may be zero. The
displacement results in a shift of the apparent null with the
true null and is approximately a linear function of the
pressure. A calibration curve is supplied with each
instrument to indicate the magnitude of the null shift.

Change in Null

<0.05 psi

with Over-Range Pressure

The null change with over-range pressure arises from

dimensional variations within the cell body. The value
shown represents the maximum expected change when the
cell is over-ranges from alternate sides of the diaphragm. In
practice, a procedure is used that permits intentional over­ranging, from only one side. After several such applications
of over-range pressure from the same side, null indication
becomes stable. If the cell is accidentally over-ranged from
the *opposite side, there is no harm except for a temporary
loss of the original null setting. The cell must be over-ranged
from the original side to reestablish the true null.

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Users Manual

Approximate Range of ΔP: +/-2 psi

Volumes of Cavities: Upper — 29.5 cc

Lower — 0.6 cc

Effective Diameter

1.9 inches

of Diaphragm:

Thickness of Diaphragm: 0.001 inch

Types of Fittings: For RUSKA 2413 Series, RUSKA 60 degree cone with

3/8 24 straight thread male cone on fitting, female cone in
body of cell. Adapters are provided to go to DH500
(equivalent to AE F250C, HIP HF4). For RUSKA 2417
Series, DH500 (equivalent to AE F250C, HIP HF4).

*Values shown under this heading are nominal at time of this publication and are not to be

considered as binding specifications. They are subject to change with improvements in design
and technology.

1-4

Introduction

The RUSKA 2416 may be employed as a null detector/indicator in the following manner:

Gas-to-Gas

The instrument may be used with dry air, nitrogen, carbon dioxide, some hydrocarbons,
and the noble gases, but not with gases containing free hydrogen or oxygen. Although
oxygen will not directly attack the materials of the lower cavity, there is the danger that
an accidentally perforated diaphragm will permit the oxygen to enter the upper cavity.
The organic materials in the upper cavity propose a hazard in the presence of compressed
oxygen. In all instances where a gas is used, no liquid vapors should be permitted to enter
the diaphragm cavity, as the surface tension effects of condensed vapors will surely spoil
the performance of the diaphragm.

Chapter 2

Applications

Liquid-to-Gas

The instrument is used to separate a liquid pressure medium from a gas. When used with
a dead-weight gage, the transducer affords a means of calibrating elastic sensors with
inert gases. The sensors, such as transducers and bourdon-tube gages may then be used in
systems containing oxygen.

Liquid-to-Liquid

The Differential Pressure Transducer may be employed as a null detector between two
liquid systems. For instance, when calibrating elastic sensors prepared for oxygen
service, it is sometimes more convenient to use a liquid pressure medium than to use a
gas. The liquid medium, of course, must be chemically inactive in the presence of oxygen
in all concentrations. Mixtures of the volatile fluorocarbon solvents are frequently used
for this purpose. The system containing the fluorocarbon may be balanced against the oil
dead-weight gage system to pressures as high as 40,000 psi. Such systems are somewhat
more economical than equivalent liquid-to-gas systems, since the pressurizing apparatus
is less expensive.

With the possible exception of use with the highly volatile fluorocarbons, it is not
recommended that a cell be purchased for alternate use in liquid-to-liquid and
liquid-to-gas service. In order for the cell to perform properly, the diaphragm cavity must
be either completely filled with liquid, or it must be completely dry. A trace of liquid in
the otherwise dry cavity will upset the performance as quickly as will an air bubble in the
liquid cavity. In each instance, the surface tension effects are greater than the ΔP error
signal being observed.

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RUSKA 2413, 2417 & 2416

Users Manual

Whenever a liquid is used in either side of the transducer, an open tube manometer must
be connected in such a way that the pressure across the diaphragm may be adjusted to
zero and that the meter may also be adjusted to indicate zero. In a liquid-to-liquid system,
two manometers must be used — one in each of the liquid systems. Manometers suitable
for this purpose are available.

A special application of the differential pressure null indicator is one in which the unit is
used when cross-floating two dead-weight gages. A by-pass valve arrangement is
provided for the purpose of directly connecting the two gags while making preliminary
balancing adjustments. When the two gages are at pressure and approximately balanced,
the valve is opened and the electrical zero adjusted. The valve is then closed and the
balancing operation continued, while observing the residual pressure difference on the
meter. As the pressures become more nearly equal, the valve is opened to verify the
correct zero adjustment and then closed and opened alternately until no difference in
meter readings is observed when the valve is either open or closed. The resolution of the
entire system is quickly determined by placing a small weight on one gage and observing
the effect on the meter. When using the transducer for this purpose, calibration of the null
shift with working pressure is unnecessary.

2-2

Preparation for Use

Normally, when a transducer is shipped from the factory, it has been calibrated with
nitrogen and is dry in both cavities. Before installation, a quick performance tests may be
made by first connecting the box to the cell, with power on, adjusting the sensitivity to
maximum and the meter to zero. By pressing against the end of the open fittings with the
finger, the meter will be seen to deflect. The effect will be less when pressing on the
upper fitting, since the upper cavity has a volume some fifty times greater than the lower
cavity. At maximum sensitivity, it should be relatively easy to deflect the meter from zero
to full scale when pressing on the lower fitting.

Because of the small volume, it is of some advantage to connect the lower cavity to the
gas portion in a liquid-to-gas system. When so connected, less work will be done in
raising the gas pressure.

Chapter 3

Preparation for Use

All fluids should be filtered before their introduction into the pressure system. A small,
hard particle, such as metal chip, in the diaphragm cavity will perforate the diaphragm
when the cell is over-ranged. Every effort should be made to keep contaminating particles
out of the transducer. In charging the upper cavity with a liquid, it is important to displace
most of the air with the liquid. There are many traps in the cavity which may retain small
air bubbles. If these bubbles remain in contact with the diaphragm or stem which carries
the transformer core, the performance will be erratic. The fact that the air bubbles
dissolve in the liquid when the pressure is increased may be used to an advantage. With
the vent plug removed, the liquid is pumped into the upper chamber until it appears at the
vent port. The plug is replaced and the pumping continued until the pressure in the liquid
system reaches 150 bar (2175 psi). At this pressure, the entrapped bubbles dissolve in the
liquid, forming a concentrated solution in the vicinity of the trap. Some time should be
allowed for the solution to diffuse so that, when the pressure is released, the bubbles will
not reappear in the same trap. The bubbles must reappear at some new point where they
may rise to the top of the chamber and be expelled through the vent port. The presence of
a bubble in the top of the cell cavity does not affect the measurement significantly, but it
does affect the response. It is therefore convenient to work the air out of the cavity as
much as is practical.

The cavity may also be charged by first evacuating and then admitting the liquid to the
evacuated chamber. Usually, some small bubbles still remain because of the difficulty in
reducing the internal pressure sufficiently through the small-bore tubing.

The presence of remaining air in the cavity may be measured if the liquid pressure
generator is a screw-type displacement pump and the system contains a Bourdon-tube
reference gage. It is first necessary to measure the air that exists in the portion of the
liquid system other than the transducer. To make this measurement, it is necessary to
isolate the liquid system from the cell and the dead-weight gage (if one is used). If there

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RUSKA 2413, 2417 & 2416

Users Manual

is no valve on the line to the cell, the line must be temporarily disconnected and stopped
off. The valve to the liquid supply reservoir is opened and the plunger advanced
somewhat to remove the backlash in the pump spindle nut. With the reservoir valve
closed, the screw crank is slowly rotated until the Bourdon gage pointer is observed to
move a perceptible amount. The quantity of motion of the screw is noted. The motion of
the crank should be small — something like one-quarter turn or less. After reattaching the
differential transducer to the system and pressurizing the opposite cavity to several
hundred psi, the experiment with the screw pump is repeated. The difference in rotation
of the screw crank in the two experiments represents the quantity of air remaining in the
cell. In these experiments, the gage pointer must not be resting against a pin at zero
pressure. It is obvious that pressurizing the opposite cavity will prevent the flexible
diaphragm from spoiling the experiment. It is not difficult to keep the free air in the
differential pressure transducer below 0.05 cc.

Bleeding Lower Chamber in a Liquid-To-Liquid System

When charging the small cavity beneath the diaphragm, a bias pressure is placed in the
upper cavity to force the diaphragm against the lower cavity surface. After the plug
beneath the cell is loosened, some liquid is forced into the lower fitting until the liquid
appears around the plug threads. This method is adequate in most instances. A small
bubble is trapped in the vertical section of the input port to the lower cavity; but after
pressurizing the liquid for a period and repeating the process, the bubble is mostly
displaced or dissolved. For more thorough displacement of the air, the cell should be
inverted.

3-2

Operating Instructions

In comparing the pressure of one system to that of another, it must first be established
that the comparator or indicating device is adjusted correctly. The adjustment must assure
the operator that all hydraulic and pneumatic heads have been accounted.

With the transducer connected between two systems and prepared for operation, the
power is turned on and the circuit allowed to warm up for ten minutes. A sequence of
operations must be adopted in which one of the systems is always at a higher pressure
than the other during the period of change from one pressure to another. If there is a
choice, it is of some advantage, in a liquid-to-gas system, to maintain the higher pressure
in the liquid system during the period of change. This procedure is not difficult to execute
for both increasing and decreasing changes in pressures. If it is intended to raise both
system pressures from one level to a higher one, the liquid pressure is raised first to a
value somewhat below the final one. The diaphragm of the differential pressure cell is
driven to the lower cavity surface where it supports the excess liquid pressure. The
operator is then free to concentrate on raising the gas pressure to, but not in excess of, the
liquid pressure. As the final pressure is approached, it is usually possible to raise both
systems simultaneously, while keeping them sufficiently balanced for the meter pointer to
remain on scale.

Chapter 4

Operation

Before starting a measurement on a liquid-to-gas system, the differential pressure
transducer is intentionally over-ranged in the direction proposed by the adopted
procedure; i.e.; from the liquid side. The pressure is allowed to remain for a minute or so
and then released. In some manner, the liquid system must be opened to atmosphere at a
point level with the diaphragm. An open-tube manometer valve opened and the gas
system also opened to atmosphere, the liquid is adjusted to stand in the tube at the height
of the diaphragm. Under these conditions, the pressure across the diaphragm is zero. The
electrical circuit, with sensitivity set at maximum or whatever value has been chosen,
may then be adjusted for the meter to indicate zero ΔP. As the manometer valve is closed,
the pumping action of the stem causes the liquid to rise slightly in the tube and the meter
pointer to deflect. The deflection is a normal one which results from the disturbance of
the liquid in the tube.

Before the measurement is begun, the sensitivity is reduced by placing the shunt switch
in the ON position. The shunt switch reduces the gain of the circuit by a factor of
approximately 1000. First the liquid pressure and then the gas pressure is raised in the
manner described above. As the gas pressure becomes approximately equal to that of the
liquid, it will be observed that the two pressures will rise simultaneously as the increase
in gas pressure is continued. At this time, the diaphragm is being forced away from the
lower cavity surface by the gas. The displacement of the diaphragm increases the
pressure in the liquid system. Although the two pressures are approximately equal, a

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RUSKA 2413, 2417 & 2416

Users Manual

signal will not appear on the meter until the gas pressure is within 2 psi of the liquid,
since this figure is the limiting value of the indicated differential pressure. Some liquid
must be withdrawn from the differential pressure cell, allowing the diaphragm to move
toward the center of the cavity whereupon the meter signal will approach the zero. If a
dead weight gage is connected in the system, the pressure in the liquid may build up high
enough to float the weights. With a slight excess of gas pressure, the diaphragm will then
move freely across the cavity; the weights will be seen to rise rapidly. After the
sensitivity is increased, by placing the shunt switch in the OFF position, the two pressures
may be brought to a satisfactory balance.

The increase in pressure of the two fluids is accompanied by an increase in temperature.
As the fluids give up their excess heat to the apparatus, each suffers a reduction in
energy. While the piston gage is floating, however, it acts as a regulator and holds the
pressure of the liquid approximately constant. The shrinkage of the liquid from its loss in
heat is reflected as an increase in the normal sink rate of the piston. The gas, being
confined to a single-ended system, suffers a loss in pressure as it gives up its excess heat.
The net effect is an unstable condition in which the indicator will signal a continuous
reduction in the gas pressure as through the system were leaking. For rather large changes
in the pressure level, the balance indication will approach a high state of excitement for
the first minute or so. Complete stabilization will require a period of up to one hour but,
for calibration purposes, manual control of the gas will be possible after only a few
minutes.

In reducing the pressure, the procedure is reversed. The gas pressure is first reduced and
then followed by the liquid pressure.

At the conclusion of the measurement, some time must be allowed for the transducer to
recover before the zero-pressure conditions are verified. Particularly, if the last reduction
in pressure is of one or more thousand psi, the recovery period may be as much as
5 to 10 minutes. A considerable quantity of heat is exchanged in the reduction process.

The procedure for operating a liquid-to-liquid system is much the same as described
above, except that a second manometer is required in the second liquid portion of the
system. When adjusting the differential pressure unit at the beginning of the test, both
manometers must be opened to atmosphere and each liquid adjusted to the height of the
diaphragm. It must be remembered that the density of the one liquid is often different
from that of the other; the total head correction must consider the two densities with their
interface at the diaphragm.

4-2

Performance Observations

Actual Sensitivity versus Apparent Sensitivity

Although the differential pressure indicator is regarded as a null-indicating instrument,
the degree to which a true null may be achieved depends upon the readability of the error
signal displayed on the meter. In order to obtain a readable error signal, the diaphragm
must move. The sensitivity of the instrument is expressed as the change in pressure
divided by the corresponding change in meter reading — the change in meter reading
being a function of the motion or displacement of the diaphragm. The sensitivity must be
determined in such a way that the tension in the diaphragm, resulting from the applied
pressure, is the only restoring force which re-establishes equilibrium

When one side of the diaphragm is opened to atmosphere and a small increment of
pressure is applied to the other side, the diaphragm will move under the influence of the
applied pressure. The motion will continue until the forces tending to move the
diaphragm are equally opposed by the forces of tension in the diaphragm tending to resist
the motion. The sensitivity is then equal to ΔP divided by the change in meter reading.

Chapter 5

When one side of the diaphragm is connected to a single-ended system containing a gas
under pressure, the circumstances are different. The forces of an applied pressure
increment tend to move the diaphragm as before, but the forces resisting the motion are
greater than before. As the diaphragm moves, the volume of the single-ended system is
reduced and its pressure increased. The ΔP that was applied to the diaphragm is
automatically diminished and the instrument sensitivity appears to be less than before.

An example of the extreme case is one in which the single-ended system is completely
filled with a non-compressible liquid. As the pressure is increased on the opposite side of
the diaphragm, the liquid will not permit the diaphragm to move. In this instance, the
sensitivity will appear to be very poor, but the actual sensitivity is no different than when
measured under ideal conditions.

Calibration

The calibration procedure consists of determining the pressure coefficient of the
transducer, the maximum sensitivity, and the zero shift that accompanies alternate
over-ranging pressures on the diaphragm.

The pressure coefficient is usually small — on an average, being less than 10-5/psi.
When the transducer is used in a bi-fluid system, for the calibration of elastic
pressure-measuring devices, the error of the transducer can often be disregarded. When
used in an apparatus for basic PVT studies, the coefficient is significant and its
expression is of more value if reported in units of diaphragm displacement per unit of
pressure level rather than as a change in pressure differential per unit of pressure level.
For very small samples, the displacement of the diaphragm can result in an intolerable

5-1

RUSKA 2413, 2417 & 2416

Users Manual

change in the sample volume, and the error will not be corrected by an adjustment of the
pressure in the amount indicated by a pressure correction curve. A calibrating procedure
in which the diaphragm presumably can be restored to its isostatic position by a physical
adjustment of the electrical sensing-indicating circuit has been adopted. The procedure
involves simultaneous pressurization of both sections of the transducer from a common
source and measuring the correction required to maintain a null indication throughout a
range of pressures.

The correction is applied as a change in the ten-turn zero adjusting potentiometer, the
shaft of which is equipped with a turn-counting graduated dial. In practice, the dial knob
is set near one end of its range and the transformer of the pressure cell adjusted to
indicate an approximate null when the diaphragm is exposed to atmospheric pressure on
each side. At each pressure level of operation, the dial is changed by an amount obtained
from the calibration curve. The curve is plotted as the change in dial units as a function of
operating pressure. Usually, it is necessary to decrease the dial registration as the pressure
is increased.

The advantage of maintaining a more uniform volume of the sample by accepting the
method of calibration just described outweighs the convenience of correcting the data by
a computer adjustment of the errors in pressure resulting from the strain in the transducer.
Manual adjustment of the potentiometer becomes a part of each measurement and must
not be overlooked.

Detecting Leaks

The differential pressure unit may be used to indicate a change in pressure of one system
with respect to that of another. The change may result from a leak or from a change in
temperature. When the instrument is used for detecting leaks in a system, sufficient time
must be allowed to eliminate temperature effects. Also, a leak in a liquid system will
have a different rate indication than a leak of the same magnitude in a gas system. Some
caution must be exercised when interpreting the results of this type of test.

5-2

Servicing the Instrument

Failure of the instrument may result from a malfunction of the following components:

• Diaphragm

• Differential Transformer

• Electrical Feed-Thrus

• Connecting Cable

• Electronic Circuit

Diaphragm

In most instances, failure of the transducer has been traced to perforations in the
diaphragm caused by particle contamination of the fluids. When the cell is over-ranged
from one side, the diaphragm is driven to and pressed firmly against the surface of the
opposite cavity. If a particle of sufficient size is present in that cavity, the impression of
the particle against the diaphragm will cause a perforation. The diaphragm must, of
course, be replaced.

Chapter 6

Maintenance

A perforated diaphragm may be detected from one or more of the following symptoms:

1. Liquid-to-Gas System

a. Otherwise unexplained presence of gas in the liquid portion of the cell.

b. Continuous increase in liquid pressure when the cell is over-ranged from the

gas side.

c. Erratic behavior of the cell caused by traces of liquid in the gas cavity.

d. A wet bore in the fitting to the gas cavity — an indication that the liquid has

migrated as far as the fitting.

2. Liquid-to-Liquid System

Periodic physical tests on one of the liquids to detect the presence of contamination
by the other liquid; i.e., a fluorescent residue from an evaporated sample of Freon in
a Freon-to-oil system.

The instrument should be returned to the manufacturer for replacement of the diaphragm.

6-1

RUSKA 2413, 2417 & 2416

Users Manual

RUSKA 2413 Differential Pressure Null Indicator —
Replacement of Differential Transformer

A serious effort has been directed toward the construction of a differential transformer
which is capable of continuous operation at high ambient pressures. Notwithstanding a
thorough inspection and performance test at the factory, a transformer occasionally
breaks down under the severe conditions within the cell.

Failure Diagnoses

Some transformers have been known to fail at high pressure and to resume operation at a
lower pressure. Failure is usually traced to an opening in one of the windings. A
symptom-remedy table is given which describes tests to be made for defective
transformers.

Symptom Reason for Failure Test & Correction Procedure

Table 6-1. Failure Diagnosis

Transducer cell insensitive to a
change in pressure. Meter can be
zeroed with zero adj.

Meter pointer hard against pin.
Cannot be brought to zero with
zero adj. Cell insensitive to
pressure

Open Primary Measure DC resistance between pin-socket

A (Ground) to B. If resistance greater than
1000 ohms, primary is open. Normal
primary resistance 310 ohms.* Replace
transformer.

Open Secondary Measure DC resistance between pin-socket

C (Ground) to D and C to F. Normal
resistance of one secondary winding is
105 ohms. A high resistance of either
winding indicates coil is open. Replace
transformer

The occasion on which the transformer windings open at high pressure will exhibit the
above symptoms temporarily above some value of pressure. The meter pointer will be
seen to dive to the pin as the pressure is increased. When the meter becomes
uncontrollable at high pressure, the cable connector may be removed from the cell (or the
box) and the above tests made while the cell is under pressure.

Occasionally, after a prolonged period of over-ranging pressure of the diaphragm in the
direction of a liquid medium, it is possible for the diaphragm to stick to the cavity
surface. With the diaphragm in this position, the cell displays the same symptoms as
those of a defective transformer. The resistance tests will indicate which of the
conditions exist.

RUSKA 2413 — Replacement of Transformer

Refer to RUSKA 2413 Parts List and Sectional Diagram (Figure 7-1 and 7-2).

Replacement of the transformer should be performed on a clean bench.

1. Wash the cell down thoroughly on the outside with a volatile solvent and dry with

clean air.

2. Grasp the flats of the cell firmly in a vise, the jaws of which are covered by brass or

aluminum plates. Pieces of paper against the flats will help prevent the polished
surfaces from becoming scratched. The cell should be grasped beneath the cable
receptacle.

3. Remove the upper body, Part 2413-1-1, and lay it aside, inverted, on a clean piece

of paper.

6-2

Maintenance

RUSKA 2413 — Replacement of Transformer 6

4. Remove the cell from the vise and pour the liquid from the cavity. While holding the

cell semi-inverted — about 45# with bottom up — over a rather large container,
direct a stream of solvent (Freon TF or TRICHLOR**) into the cavity and
particularly around the feed-through terminals. A 2- or 3-ounce plastic ear syringe is
suitable for this purpose. The reason for inverting the cell while flushing is to prevent
any solid contaminates that may have come to rest in the transformer chambers from
being washed into the diaphragm cavity.

5. Mark the sectional diagram with the proper wire colors for the transformer leads.

Carefully lift the wires off the feed-through terminals, using a 25-watt pencil type
soldering iron. As each wire is lifted, measure the resistance of the feed-through
terminal to ground. The resistance should be greater than 1 megohm. The terminals
should not be overheated with the iron, since the gasket will be damaged. If there is
any reason to believe that the resistance measurements in the previous section could
be confused by an abnormally low resistance to ground by way of the feed-through
terminals, the resistance measurements should be repeated at the transformer leads
before the transformer is completely removed.

6. Remove the wires from ground.

7. Remove part (symbol) 5, detent.

8. Turn the pinion, symbol 16, counterclockwise while preventing transformer from

rotating with the other hand. Do not bend the wires where they are attached to the
transformer. Continue to turn until the transformer is free.

9. Remove the transformer and flush the cell again as in Operation 2. Do not disturb the

core on the diaphragm stem.

10. Insert the new transformer. The wires may be somewhat longer than the original

ones, but it makes no difference; it is not necessary to shorten them.

11. Solder the wires to the terminals following the color code on the diagram

(See Operation 5). Try to prevent oxides and excess flux from dropping into the
cavity. If necessary, flush the cavity when the soldering is complete. Replace the
detent.

12. Make the primary and secondary resistance tests.

13. Plug the box in, and allow the circuit to warm up.

a. Set the gain to maximum.

b. Set zero control to 800 or so.

c. Rotate the pinion (16) until the meter reads zero.*

14. Wash the upper body and bolts and replace the O-ring in the cap if it shows evidence

of extrusion. Do not use a hard, metal object to remove the O-ring: use a toothpick or
wooden match.

15. With the cell in the vise, assemble the upper body and torque the bolts down evenly

to 35 foot-pounds.

16. Connect the box to the cell and test the performance by pressing the finger against the

end of the lower fitting. At maximum sensitivity, it should be possible to pin the
meter from zero by pushing against the fitting; this is a very rough quantitative test.

RUSKA 2417 Transducer — Replacement of Differential Transformer

A serious effort has been directed toward the construction of a differential transformer
that is capable of continuous operation at high ambient pressures. Notwithstanding a
thorough inspection and performance test at the factory, a transformer occasionally
breaks down under the severe conditions within the cell.

6-3

RUSKA 2413, 2417 & 2416

Users Manual

Failure Diagnoses

Some transformers have been known to fail at high pressure and to resume operation at a
lower one. Failure is usually traced to an opening in one of the windings.

If an instrument fails to display a difference in pressure as expected, a simple and quick
test will indicate if the cause is in the electronic circuit or in the differential transformer.
When the connecting cable is removed from the transducer, the meter in the control box
circuit should be capable of being brought to zero with the zero potentiometer. If this
condition is true, the problem may then lie in the transformer.

A resistance measurement of the windings will establish the condition of the windings.

The measurements may be made at the receptacle on the transducer as follows:

Table 6-2. Resistance Measurements for the Windings

Connection Winding

D-C (Gnd) Secondary 100

F-C Secondary 100

B-A (Gnd) Primary 300-2000*

*A series resistor is sometimes added to the primary winding at the transducer for matching purposes. An

open primary should show infinite resistance.

Approximate

Resistance (ohms)

In some instances, after a prolonged period of over-ranging pressure on the diaphragm in
the direction of a liquid medium, it is possible for the diaphragm to stick to the cavity
surface. With the diaphragm in this position, the cell displays the same symptoms as
those of a defective transformer. The resistance tests will indicate which of the conditions
exist. This condition may be encountered, for example, when making phase studies with
gases. Under this circumstance application of a small liquid overpressure against the
diaphragm with the intent of dislodging it will be unsuccessful. The area of the
diaphragm exposed to the pressurized liquid is so small that only a small, dislodging
force actually exist. If the pressure is increased with the expectation of results, the
diaphragm may break loose with a shock and disturb the core on the stem. The lodging
pressure should not exceed 4 or 5 atmospheres and should be left applied until the
diaphragm is released.

1. Remove the circular base, supporting legs, tie bolts (Symbol 27) and top cap (1) as

described in this section on replacement of the diaphragm. Take note of any solid
debris that may have accumulated on the floor of the exposed transformer chamber.
Every effort must be directed to prevent any of the particles from being washed into
the diaphragm cavity during exchange of the transformer.

2. Pour the oil from the chamber. With tweezers, slide the plastic sleeves back from the

connections at the feed-thru terminal. While holding the cell semi-inverted — about
45o with bottom up — over a container, direct a stream of solvent (Freon TF or
mineral spirits) into the cavity and particularly around the feed-thru connections.
A 2 or 3 ounce plastic ear syringe is suitable for this purpose. The reason for
inverting the cell while flushing is to prevent any solid contaminants that may have
come to rest in the transformer chamber from being washed into the diaphragm
cavity. Do not disturb any of the wires while performing this operation.

6-4

Maintenance

RUSKA 2413 — Replacement of Transformer 6

3. The feed-thru wires are very brittle and must be treated with the greatest care. Do not

bend these short stubs when exchanging the transformer connecting wires. While
using a 25-watt pencil type soldering iron, lift the transformer connecting wires from
the feed-thru wires. As each wire is lifted, make a note of the wire color and the
socket pin number to which the wire was connected. After the wire is lifted,
continuity may be traced to the correct pin by use of an ohmmeter.

4. Remove the 2 wires from ground.

5. Remove part (14), detent.

6. Turn the pinion, (12), counterclockwise while preventing transformer from rotating

with the other hand. Do not bend the wires where they are attached to the
transformer. Continue to turn until transformer is free.

7. Remove the transformer and flush the cell again as in Operation 2. Do not disturb the

core on the diaphragm stem.

8. Insert the new transformer. The wires may be somewhat longer than the original

ones, but it makes no difference; it is not necessary to shorten them.

9. Solder the wires to the terminals following the color code on the diagram (see

Operation 3). Try to prevent oxides and excess flux from dropping into the cavity. If
necessary, flush the cavity when the soldering is complete. Replace the detent.

10. Make the primary and secondary resistance test.

11. Plug the box into the mains and allow the circuit to warm up a few minutes.

a. Set the GAIN to maximum and set the SHUNT switch ON.

b. Set the ZERO control to midpoint (500 counts).*

c. Rotate the pinion (12) until the meter indicates zero. Turn the pinion 1/2 to 1

revolution counterclockwise (CCW) and repeat the zero adjustment while
approaching the setting in a clockwise direction. All final adjustments of the
pinion should be made in a clockwise direction to remove the backlash from the
elevating nut threads. The adjustment will then be more permanent.

12. Wash the cap and bolts and replace the O-ring in the cap if it shows evidence of

extrusion. Do not use a hard metal object to remove the O-ring: use a toothpick or
wooden match.

13. Assemble the cap and torque the bolts down evenly to 250-foot-pounds.

14. Connect the box to the transducer. Set gain to maximum, shunt switch OFF and

adjust the meter to zero. Pressing the finger against either of the input ports should
deflect the meter.

* Or to a value that will allow full adjustment of the potentiometer when correcting from

the strain that is given on the calibration report.

6-5

RUSKA 2413, 2417 & 2416

Users Manual

6-6

Chapter 7

Parts List

RUSKA 2413 Differential Pressure Null Indicator — Parts
List

When ordering parts, refer to Figures 7-1 and 7-2 to determine reference symbol.
RUSKA stock number corresponding to the reference symbol will be found listed on
Table 7-1.

Serial number of the instrument and RUSKA stock numbers of parts required must
accompany all orders for replacement.

When returning the instrument to the factory for servicing, the indicator/control circuit
must always accompany the transducer.

7-1

RUSKA 2413, 2417 & 2416

Users Manual

Figure 7-1. RUSKA 2413-711 Differential Pressure Null Indicator - Side View

7-2

gmo01.bmp

Parts List

RUSKA 2413 Differential Pressure Null Indicator — Parts List 7

Figure 7-2. RUSKA 2413-711 Differential Pressure Null Indicator - Top View

gmo02.bmp

7-3

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Users Manual

Table 7-1. RUSKA 2413 Differential Pressure Cell - Parts List

Item

Number

1 Upper Body 1 2413-1-8

2 Socket Cap Screw 12 70-229 (3/8 x 1-1/4)

3 Socket Cap Screw 3 70-122 (#4 x 3/4)

4 Terminal Block 1 2413-1-16

5 Detent 1 2413-1-4

6 Round Head Screw 1 71-132 (#4 x 5/16)

7 Differential Transformer Assy. 1 2413-020-0

8 Adjustment Gear 1 2413-1-3

9 Transformer Armature 1 86-396

10 Washer 1 2413-1-14

11 Wave Washer 1 2413-1-11

12 Washer 1 2413-1-13

13 Diaphragm Assembly 1 2413-012

14 Tube Plug 2 2118-1-1

Description Qty. Part Number

15 "O" Ring 1 54-900-035

16 Adjustment Pinion 1 2413-1-5

17 Middle Body 1 2413-1-6

18 Dowel Pin 2 59-119

19 Transformer Block 1 2413-1-1

20 Lower Body 1 2413-1-7

21 Socket Cap Screw 12 70-230 (3/8 x 1-1/2)

22 Round Head Screw 2 70-120-05-06 (#4 x 1/2)

23 Lead Thru Assembly 3 2413-023

24 Coupling Body 2 2103-1

25 Tube Nut 2 2113-1-1

N/Shown Receptacle 1 14-814

N/Shown Round Head Screw 1 70-110-05-06 (#2 x 3/16)

N/Shown Screwdriver 1 71-490

N/Shown Transducer Support Assy. 1 2413-3

7-4

Parts List

RUSKA 2417 Differential Pressure Transducer — Parts List 7

RUSKA 2417 Differential Pressure Transducer — Parts List

When ordering parts, refer to cross sectional drawings to determine reference symbol.
RUSKA stock number corresponding to the reference symbol will be found listed on
Table 7-2.

Serial number of the instrument and RUSKA stock number of parts required must
accompany all orders for replacement.

The transducer and its electronic indicating circuit are adjusted and calibrated as a unit.
If the transducer is returned to the manufacturer for servicing, the indicating circuit must
also be returned.

RUSKA 2417-706 Differential Pressure Cell 40,000 PSI —
Parts List

When ordering parts, refer to cross-sectional drawings 7-1 and 7-2 to determine reference
symbol. RUSKA stock number corresponding to the reference symbol will be found
listed on Table 7-2 .

Serial number of the instrument and stock numbers of parts required must accompany all
orders for replacement.

Table 7-2. RUSKA 2417 Differential Pressure Cell - Parts List

Item

Number

1 1 Top Cap 2417-1-7

2 1 Cover 2417-1-12

3 1 Connector 14-814

4 3 Gland Nut 25-045

5 1 Connector 2417-5

6 1 Diaphragm Assembly 2413-15

7 1 Retainer 2417-1-23

8 1 Bottom cap 2417-1-19

9 2 Pin 99074-306-625

10 1 Washer 2413-1-14

11 1 XFMR Block 2417-1-22

12 1 ADJ Pinion 2417-1-24

13 1 Transformer 2413-20

Qty Description Part Number

14 1 Detent 2413-1-4

15 1 Screw 71-132-501

16 1 DJ Gear 2413-1-3

17 1 Gland Nut 25-057

18 1 Plug 25-062

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RUSKA 2413, 2417 & 2416

Users Manual

Item

Number

19 1 Tube Plug 2118-1-1

20 2 Adapter 1-211

21 2 Plug 25-047

22 2 Sleeve 25-046

23 2 Wave Washer 2413-1-11

24 1 Washer 2413-1-13

25 1 Body 2417-1-21

26 1 Pin 59-137

27 6 Screw 2417-1-26

28 6 Allen Nut 53-950

29 2 Screw 70-120-501

30 2 Screw 70-152-4

31 3 Screw 70-179-2100

32 1 O-ring 54-700-140

Qty Description Part Number

33 2 O-ring 54-900-33

7-6

Introduction

This instrument employs contemporary time-proved analog techniques. The components
were chosen for their long-term stability and reliability. Numerous test points exist on the
circuit board that facilitate trouble-shooting, when necessary. The operational amplifiers
are plug-in, which allows for easy installation.

The 15-volt DC system is powered by a modular power supply which is, in turn, fully
isolated from line voltages through a dual primary input isolation transformer. The input
line voltage, 120 Vac or 230 Vac at 50 Hz or 60 Hz, is selectable by means of a small
circuit board located internal to the power cord receptacle. Removal of the power cord
allows access to the fuse and circuit board compartment.

Chapter 8

General Information

The front panel controls include a power switch and indicator, a 3.5-inch null meter, a
ten-turn ZERO potentiometer with calibrated dial, a ten-turn GAIN potentiometer with
calibrated dial, and a SHUNT switch that allows discrete high and low gain selection.

8-1

RUSKA 2413, 2417 & 2416

Users Manual

8-2

Introduction

The excitation signal to the primary coils of the LVDT is generated by an oscillator made
of Z2A and Z2B. The output of the oscillator (at TP7) is a 24 volt peak-to-peak sinusoid
at a frequency of 5.1 KHz. This signal is attenuated by R5 and R6 and buffered through
Z1B. The excitation signal (TP6) to the primary coil has a magnitude of 4 volts (P-P).
The output of the LVDT is measured differentially and amplified by Z4B. This error
signal is then again amplified by Z4A. The information present at the output of Z4A is a
sinusoid whose amplitude and phase (0o or 180o) is a function of the position of the core
in the LVDT. This information is half-wave rectified by Q1. Q1 is operated by the
squarewave generator Z1A. The result at TP5 is a positive or negative half-wave sinusoid
whose amplitude is a function of core displacement from the null position and whose
polarity is a function of the direction of the core displacement. The signal at TP5 is then
integrated by R8 and C5. This DC level is then amplified through Z3. The gain of Z3 is
controlled by the GAIN adjustment. The ZERO adjustment provides a bias voltage added
algebraically to the error signal via Z3.

Chapter 9

Functional Circuit Description

Resistor R29 and diodes CR3 and CR4 provide current and voltage limiting to the null
meter. R30 is a current limiter to the recorder output jack. Resistors R25, R26, R27, and
R28 determine the ZERO control sensitivity and span.

9-1

RUSKA 2413, 2417 & 2416

Users Manual

9-2

Introduction

Reference to Drawing Nos. 2416-60-200 and 2416-63 is recommended.

A DC voltmeter with .001 volt resolution is sufficient for the single required
potentiometric adjustment.

For trouble-shooting, an oscilloscope will be required. All measurements are made with
respect to TEST POINT 2 (TP2).

Setup Procedure

1. Set Gain control to maximum (full clockwise).

Chapter 10

Test Procedure

2. Set ZERO control to mid-point.

3. Set SHUNT to ON.

4. Connect the transducer.

5. Turn power ON.

6. Observe the voltage at TP4. Adjust P1 to yield minimum DC volts.

7. Adjust the transformer in the transducer until the meter indicates null. It must be

noted here that, for oil-filled transducers, insertion of the screwdriver will affect null
position. The oil displaced by the screwdriver will create a pressure head. The head
must be considered. The head pressure will become more important as the test
procedure progresses to the stages of higher amplification.

8. Set SHUNT to OFF. This condition provides maximum amplification of null sensing.

9. The final adjustment of the transducer transformer prepares the instrument for use.

The accuracy with which the adjustment is made and the permanency that may be
expected of it depend upon the performance characteristics of the individual
transducer and the manner of its preparation for adjustment. The permanency of the
adjustment may be associated with:

a. The magnitude of the zero shift of the pressure null indication with alternate

overranging pressures and the direction of application of the last previous
overranging pressure.

b. The direction in which the pinion is rotated for completion of the adjustment.

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RUSKA 2413, 2417 & 2416

Users Manual

c. The presence of an unrecognized pressure bias on the diaphragm at the time of

The accuracy to which the adjustment must be made will depend upon:

1. The skill of the operator in neutralizing, or otherwise accounting for, the abnormal

pressure bias on the diaphragm as described in C, above.

2. The number of potentiometer dial units that span the change in null indication from

zero to full working pressure. This information is obtained from the calibration
report. The curve representing the change in potentiometer dial counts versus
operating pressure level indicates the total number of counts that must be available to
the operator for completion of his process. The zero adjustment potentiometer must
be offset at the initial adjustment to accommodate this range of operation.

adjustment. If the top section of the transducer contains a liquid at the time of
adjustment, removal of the pinion access plug and insertion of the adjusting
screwdriver will create an abnormal pressure bias on the diaphragm; liquid
pressure heads are different than for normal operations. Allowance must be made
for the magnitude of the head when making the adjustment either by trial and
error or by adjusting the external heads to match the internal ones.

Bias pressures from other sources may be present and, unless they remain
constant throughout the measurement process, will affect the stability of the null
pressure adjustment.

10-2

Explanation of Test Report

Explanation of Test Report

When the RUSKA Differential Pressure Null Transducer is operated at an elevated
pressure, the body is in a state of temporary strain. Even though the pressure on each side
of the diaphragm may be equal, as they are when the two cavities are connected together,
the output display meter indicates that a difference in pressure does exist. This erroneous
indication is caused by a displacement of the transformer with respect to the diaphragm
and is a result of the strain in the body. Since the position of the diaphragm is of great
interest when making PVT experiments, the value of the change must be determined by
calibration. The reported result of the calibration is the apparent change in the original
setting of the zero-adjusting potentiometer as a function of the pressure within the
transducer.

Appendix A

In practice, after the necessary proof-pressure and leak tests have been completed, a
common gas pressure is applied to both sides of the diaphragm. As the pressure is
increased in uniform increments, the output meter is restored to its null position by
adjustment of the zero potentiometer. The value of the counter-type dial on the
zero-adjusting knob is recorded for each increment. The difference in these values and
that of the original zero is plotted against their corresponding internal pressures and a
smooth curve is fitted to the set of points. Unless otherwise stated on the report, the curve
represents values observed at the maximum gain of the indicating circuit. Negative values
for the abscissae imply that the observed readings of the dial are less than that of the
original zero. Curves are constructed for both increasing and decreasing internal
pressures.

In use, it is necessary to determine from the curve, the change in dial setting for the
pressure at which the cell has become stable, subtract this number from that observed
when the cell was exposed to the atmosphere, and set the dial to the new figure. When the
two mediae are balanced, the diaphragm will be in its original position, and the pressure
across it will be zero.

The gain of the electronic circuit is such that, on an average, a displacement of the
diaphragm of 0.75 microinch is equivalent to one unit indicating meter. This information
is of use in making an error analysis of the measurement process.

The shift in the zero position of the diaphragm as a result of alternate overranging
pressures is obtained by pressurizing one side of the diaphragm to 2000 psi or so for a
few minutes. The pressure is then released and the transducer allowed time to recover.
After setting the meter to null, the opposite side of the diaphragm is pressurized by the
same amount. Upon release of the latter pressure and recovery of the transducer, a small

A-1

RUSKA 2413, 2417 & 2416

Users Manual

U-tube manometer is connected to the appropriate side and the pressure required to
restore the original null is measured. This figure is characteristic of the diaphragm and, in
some instances, is small enough to allow indiscriminate overranges.

The sensitivity at maximum gain is obtained by measuring the differential pressure
required to sweep the meter pointer across the scale several times in a stepping fashion.
The pressure is established with the manometer and is measured with respect to the
atmosphere. After the meter pointer is set to the appropriate end of the scale, pressure is
applied until the pointer sweeps full scale to the opposite end. While the pressure is
maintained, the pointer is returned to the original end of the scale and the process
continued until a substantial head has been established in the manometer. The total
pressure divided by the total number of units through which the pointer has passed is the
sensitivity in pressure units per meter unit.

The sensitivity decreases as the working pressure increases. At elevated pressures, the
sensitivity is difficult to measure—the observed value being, at best, only an estimate.
The experience i using the null transducer as a balance indicator when crossfloating two
piston gages indicates that the sensitivity is reduced to 1/2 or 1/3 of its original value
when operated near its maximum pressure. The reduction is not serious, however,
because as the pressure is increased, the sensitivity becomes greater as a percentage of
the total pressure.

A-2