Fluke RUSKA 2470 User Manual

RUSKA 2470

Gas Lubricated Piston Pressure Gauge

PN 3974971
December 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.

Users Manual

LIMITED WARRANTY AND LIMITATION OF LIABILITY

Each Fluke product is warranted to be free from defects in material and workmanship under normal use and
service. The warranty period is one year and begins on the date of shipment. Parts, product repairs, and
services are warranted for 90 days. This warranty extends only to the original buyer or end-user customer of
a Fluke authorized reseller, and does not apply to fuses, disposable batteries, or to any product which, in
Fluke's opinion, has been misused, altered, neglected, contaminated, or damaged by accident or abnormal
conditions of operation or handling. Fluke warrants that software will operate substantially in accordance
with its functional specifications for 90 days and that it has been properly recorded on non-defective media.
Fluke does not warrant that software will be error free or operate without interruption.

Fluke authorized resellers shall extend this warranty on new and unused products to end-user customers
only but have no authority to extend a greater or different warranty on behalf of Fluke. Warranty support is
available only if product is purchased through a Fluke authorized sales outlet or Buyer has paid the
applicable international price. Fluke reserves the right to invoice Buyer for importation costs of
repair/replacement parts when product purchased in one country is submitted for repair in another country.

Fluke's warranty obligation is limited, at Fluke's option, to refund of the purchase price, free of charge repair,
or replacement of a defective product which is returned to a Fluke authorized service center within the
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To obtain warranty service, contact your nearest Fluke authorized service center to obtain return
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INCLUDING LOSS OF DATA, ARISING FROM ANY CAUSE OR THEORY.

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

Fluke Europe B.V.
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 Introduction ......................................................................................... 1-1

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

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

Compressed Gas ............................................................................................ 1-1

Lubricants and Seals...................................................................................... 1-2

Oxygen Compatibility ................................................................................... 1-2

Heavy Weights .............................................................................................. 1-2

Personal Protective Equipment...................................................................... 1-2

Symbols Used in this Manual ............................................................................ 1-2

Specifications..................................................................................................... 1-3

Accuracy ............................................................................................................ 1-4

2 General Piston Pressure Gauge Considerations ............................. 2-1

Types of Piston Pressure Gauges....................................................................... 2-1

Calculations ....................................................................................................... 2-3

Measurement of Pressure with the Piston Pressure Gauge ................................ 2-3

Elastic Distortion of the Cylinder.................................................................. 2-4

Gravity........................................................................................................... 2-4

Buoyant Effect of the Air .............................................................................. 2-4

Temperature................................................................................................... 2-5

Reference Plane of Measurements ................................................................ 2-5

Crossfloating.................................................................................................. 2-8

Bibliography.................................................................................................. 2-8

3 Description........................................................................................... 3-1

General Information........................................................................................... 3-1

Description of the Mass Set ............................................................................... 3-2

Description of the Gauge Base .......................................................................... 3-2

4 Installation ........................................................................................... 4-1

Introduction........................................................................................................ 4-1

5 Operation ............................................................................................. 5-1

i

RUSKA 2470

Users Manual

Precautions......................................................................................................... 5-1

General............................................................................................................... 5-2

Low Range Piston Assembly............................................................................. 5-2

Mid Range Piston Assembly.............................................................................. 5-7

High Range Piston Assembly ............................................................................ 5-10

Establishing Pressure ......................................................................................... 5-13

Gauge Pressures............................................................................................. 5-13

Automating the Calculations and Data Storage............................................. 5-14

Leaks ............................................................................................................. 5-15

Maintenance of the Gauge ................................................................................. 5-15

6 Piston / Cylinder Cleaning Instructions ............................................ 6-1

General Information and Preparation................................................................. 6-1

Functional Testing of Piston/Cylinder Assemblies............................................ 6-2

Cleaning the Low Range Piston/Cylinder Assembly......................................... 6-2

Mid Range Piston/Cylinder................................................................................ 6-6

Cleaning the High Range Piston/Cylinder Assembly........................................ 6-8

Appendices

A Explanation of "Pressure Calculation Worksheet"...................................... A-1

B Equation A-4 — Air Density....................................................................... B-1

C Glossary....................................................................................................... C-1

ii

List of Tables

Table Title Page

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

iii

RUSKA 2470

Users Manual

iv

List of Figures

Figure Title Page

2-1. Simple Cylinder ..................................................................................................... 2-1

2-2. Re-Entrant Cylinder ............................................................................................... 2-1

2-3. Controlled Clearance Cylinder............................................................................... 2-2

2-4. Reference Plane Determination.............................................................................. 2-6

2-5. Head Correction Measurement............................................................................... 2-7

3-1. Base with 2470 P/C Installed ................................................................................. 3-2

3-2. Base with 2468 P/C Installed ................................................................................. 3-3

4-1. FPI Mounting - RUSKA 2456 Piston Gauge Monitor........................................... 4-2

4-2. Plumbing and Recommended Ancillary Equipment .............................................. 4-3

5-1. Section View, Low Range Piston/Cylinder............................................................ 5-3

5-2. Parts Required For Low Range Piston Operation .................................................. 5-4

5-3. Removing Low Range Piston and Cylinder from Container - Step 1 .................... 5-4

5-4. Removing Low Range Piston and Cylinder from Container - Step 2 .................... 5-5

5-5. Handling the Low Range Piston and Cylinder - Step 3 ......................................... 5-5

5-6. Low Range Piston and Cylinder Showing O-Ring Groove ................................... 5-6

5-7. Positioning the Upper Thrust Washer/Piston Retainer in the Cylinder

Retaining Cap Recess............................................................................................. 5-6

5-8. Section View, Mid Range Piston/Cylinder ............................................................ 5-8

5-9. Parts Required for Operation of the Mid Range P/C.............................................. 5-9

5-10. Mid Range Piston/Cylinder Assembly ................................................................... 5-9

5-11. Retaining Nut and Bearing ..................................................................................... 5-10

5-12. Section View, High Range Piston/Cylinder ........................................................... 5-11

5-13. Parts Required for High Range Piston Operation .................................................. 5-12

5-14. Always Use Two Hands to Carry a Piston/Cylinder Assembly ............................. 5-12

5-15. Order of Installation ............................................................................................... 5-13

5-16. Float Position ......................................................................................................... 5-14

6-1. Materials for Cleaning Low Range Piston/Cylinder .............................................. 6-3

6-3. Preparing the Kim-Wipes - Step 1 ......................................................................... 6-4

6-4. Preparing the Kim-Wipes - Step 2 ......................................................................... 6-5

6-5. Preparing the Low Range Cleaning Tool - Step 1.................................................. 6-5

6-6. Preparing the Kim-Wipes - Step 2 ......................................................................... 6-6

6-7. Materials for Cleaning the Mid Range Piston/Cylinder......................................... 6-6

6-8. Preparing the Mid Range Cleaning Tool................................................................ 6-7

6-9. Materials for Cleaning the High Range Piston Cylinder........................................ 6-8

6-10. Preparing the Kim Wipe for Cleaning the High Range Cylinder - Step 1 ............. 6-10

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RUSKA 2470

Users Manual

6-11. Preparing the Kim Wipe for Cleaning the High Range Cylinder - Step 2 ............. 6-11

6-12. Preparing the Kim Wipe for Cleaning the High Range Cylinder - Step 3 ............. 6-11

6-13. Cleaning the High Range Cylinder ........................................................................ 6-11

6-14. Drying the High Range Cylinder ........................................................................... 6-12

vi

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

Introduction

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

The following are general safety precautions that are not related to any specific
procedures and do not appear elsewhere in this publication. These are recommended
precautions that personnel must understand and apply during equipment operation and
maintenance to ensure safety and health and protection of property.

Compressed Gas

Use of compressed gas can create an environment of propelled foreign matter. Pressure
system safety precautions apply to all ranges of pressure. Care must be taken during
testing to ensure that all pneumatic connections are properly and tightly made prior to
applying pressure. Personnel must were eye protection to prevent injury.

DO NOT use oxygen as a Pressure supply media. Use only dry, clean Nitrogen or
equivalent.

DO NOT exceed the prescribed maximum inlet pressure for this device. See Chapter 1,
Specifications, for more detail.

.

.

.

1-1

RUSKA 2470

Users Manual

Lubricants and Seals

Oxygen Compatibility

Heavy Weights

DO NOT use hydrocarbon lubricants in this device, use only approved lubricants.

Always use replacement parts specified by Fluke.

For more information regarding common replacement parts and recommended lubricants,
see Appendix B.

This Instrument has been designed with components that will not introduce hydrocarbons
into the calibration process. The O-rings and lubricating grease supplied with the
instrument must not be substituted with other laboratory supplies. For more information
regarding common replacement parts and recommended lubricants, see Appendix B.

Cleaning the instrument for oxygen compatibility using HFCs and ultrasonic cleaning
systems is permitted with the EXCEPTION OF THE PISTONS AND CYLINDERS.
Ultrasonic cleaning may damage the crystalline structure of the tungsten carbide pistons
and cylinders. The RUSKA procedures for piston/cylinder cleaning must be followed.
See Chapter 5, Operation, for piston/cylinder cleaning instructions.

Lifting and movement of heavy weights can create an environment of strain and impact
hazards. Care must be taken during testing to ensure that weight masses are lifted in a
manner that avoids over-reaching or twisting, and that the masses are not dropped.
Personnel must wear reinforced safety shoes to prevent injury.

Personal Protective Equipment

Wear eye protection and reinforced safety shoes approved for the materials and tools
being used.

W Warning

If the equipment is used in a manner not specified by the
manufacturer, the protection provided by the equipment may be
impaired.

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 Gas Lubricated Piston
Pressure Gauge or the equipment under test.

Symbols used on the Gas Lubricated Piston Pressure Gauge and in this manual are
explained in Table 1-1.

Table 1-1. Symbols

Symbol Description

B AC (Alternating Current)

J Earth Ground

W Important Information: refer to manual

1-2

X Shock Hazard

Do not dispose of this product as unsorted

~

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

Introduction

Specifications 1

Specifications

Low Range Piston

Pressure Range (Model 2465) 1.4 to 172 kPa (0.2 to 25 psi) psi gauge
Pressure Range (Model 2468) 1.4 to 345 kPa (0.2 to 50 psi) psi gauge
Pressure Uncertainty Rating 0.0010% (10 ppm) or 0.07 Pa (1.0E-05 psi), whichever is greater
Uncertainty Threshold 7 kPa (1 psi)
Resolution 1 ppm or 1 mg, whichever is greater
Precision (Typical Type A Unc.) 3 ppm (3)
Long Term Stability 3 ppm per year
Piston/Cylinder Material 440C Stainless Steel/Tungsten Carbide
Thermal Coefficient 1.5E-05 per deg. C
Sink Rate at Maximum Pressure typical <2 mm per minute, maximum 4 mm per minute

Mid Range Piston

Pressure Range (Model 2465) 12 to 700 kPa (1.7 to 100 psi) absolute or gauge
Pressure Range (Model 2468) 12 to 1380 kPa (1.7 to 200 psi) absolute or gauge
Pressure Uncertainty Rating 0.0010% (10 ppm) or 0.07 Pa (1.0E-05 psi), whichever is greater
Uncertainty Threshold 35 kPa (5 psi)
Resolution 1 ppm or 1 mg, whichever is greater
Precision (Typical Type A Unc.): 3 ppm (3)
Long Term Stability 3 ppm per year
Piston/Cylinder Material Tungsten Carbide/Tungsten Carbide
Thermal Coefficient 9.1E-06 per deg. C
Sink Rate at Maximum Pressure typical <2 mm per minute, maximum 4 mm per minute

High Range Piston

Pressure Range 700 to 20680 kPa (100 to 3000) absolute or gauge
Pressure Uncertainty Rating 0.0030% (30 ppm) or 3.7 Pa (5.4E-04 psi), whichever is greater
Uncertainty Threshold 248 kPa (36 psi)
Resolution 1 ppm or 1 mg, whichever is greater
Precision (Typical Type A Unc.) 3 ppm (3)
Long Term Stability 3 ppm per year
Piston/Cylinder Material Tungsten Carbide/Tungsten Carbide
Thermal Coefficient 9.1E-06 per deg. C
Sink Rate at Maximum Pressure typical <2 mm per minute, maximum 4 mm per minute

Mass Set

Approximate Total Mass 17.8 kg
Approximate Carrier Mass 0.58 kg
Smallest Increment 5.9 gram
Mass Material 300 Series, Non-magnetic, Austenitic, Stainless Steel

(3)

Adjustment Method Completely machined with no fill cavities
Mass Uncertainty 0.0005% (5 ppm) or 5E-07 kg, whichever is greater
Optional Fine Increment Trim Set ASTM Class 1, 20g to 1 mg

(1)

Absolute mode uncertainty higher due to reference pressure sensor.

(2)

Approximate 95% level of confidence (Refer to Chapter 1, Accuracy, and to Calibration Report.)

(3)

Mass carrier composite construction 300 Series, Non-magnetic, Austenitic, Stainless Steel and other

non-magnetic material.

(1,2)

(1,2)

(1,2)

1-3

RUSKA 2470

Users Manual

Temperature Range

Operating

Storage

Humidity Range

Operating

Storage

Pressure Medium Clean dry gas, Nitrogen or equivalent, regulated to a pressure Compatible

Pressure

Maximum Working Pressure

2470 piston/Cylinder installed in 2470 Column adapter

With High Range Piston/Cylinder 3000 psig

2465 piston/Cylinder installed in 2465 Column adapter

With Mid Range Piston/Cylinder 100 psig (Do Not Exceed 6.31 Kg Mass Load)
With Low Range Piston/Cylinder 25 psig (Do Not Exceed 6.31 Kg Mass Load)

2468 piston/Cylinder installed in 2465 Column adapter

18 ºC to 28 ºC

-40 ºC to 70 ºC when thermometer and low range piston & Cylinder are
stored separately from each other.

20% to 75% noncondensing

0% to 90% noncondensing

with each particular Piston/cylinder assembly. Dew Point of less than or
equal to -60 ºF

With Mid Range Piston/Cylinder 200 psig (Do Not Exceed 12.31 Kg Mass Load)
With Low Range Piston/Cylinder 50 psig (Do Not Exceed 12.31 Kg Mass Load)

Note

The pressurized housing with the 2470 adapter has been tested to 4000 psig.
The pressurized housing with the 2465 adapter has been tested to 1000 psig.

Accuracy

The gauge is capable of measuring pressures to the accuracy indicated below. See the
calibration report for the actual accuracy of this gauge. The claim for accuracy is valid
only when the gauge is operated according to the instructions provided with the
equipment. In addition, the claim is valid when the value of gravity acting upon the
weights is known to +/-0.001 cm sec

Accuracy is defined as the departure of the measured pressure from the true pressure.
The value is based on a simple error analysis of the calibration experiment and represents
the sum of the systematic errors and two standard deviations of the random variability of
the measurement process.

Gauge Pressure (0.0035% Class)

High Range Piston
(0.013 sq in area)

Mid Range Piston
(0.13 sq in area)

0.0035 percent of reading
or 0.002 psi, whichever is greater

0.0035 percent of reading
or 0.0002 psi, whichever is greater

2

.

1-4

Low Range Piston
(0.52 sq in area)

0.0035 percent of reading
or 0.00005 psi, whichever is greater

General Piston Pressure Gauge

Types of Piston Pressure Gauges

The piston pressure gauge is sometimes regarded as an absolute instrument because of
the principle by which it measures pressure. An absolute instrument is defined here as
one capable of measuring a quantity in the fundamental units of mass, length, time, etc.
It may be suggested that only certain types of piston pressure gauges qualify in this
category.

Figures 2-1, 2-2, and 2-3 illustrate the three most common types of cylinder arrangements.

Chapter 2

Considerations

A

PR ESS URE IN

Figure 2-1. Simple Cylinder

B

PRESS UR E IN

Figure 2-2. Re-Entrant Cylinder

glg01.eps

glg44.eps

2-1

RUSKA 2470

Users Manual

SECONDARY
PRESSURE

PRESS UR E IN

glg45.eps

Figure 2-3. Controlled Clearance Cylinder

When the simple cylinder of 2-1 is subjected to an increase in pressure, the fluid, exerting
a relatively large total force normal to the surface of confinement, expands the cylinder
and thus increases its area. A pressure-drop appears across the cylinder wall near point A,
resulting in an elastic dilation of the cylinder bore.

It can be shown that the effective area of the piston and cylinder assembly is the mean of
the individual areas of the piston and of the cylinder; therefore as the pressure is
increased, the cylinder expands and the effective area becomes greater. The rate of
increase is usually, but not always, a linear function of the applied pressure. The piston
also suffers distortion from the end-loading effects and from the pressure of the fluid, but
to a much lesser extent than the cylinder. It is evident then, that the simple cylinder of 2-1
would be inadequate for a primary piston pressure gauge unless some means of
predicting the change in area were available.

The increase in the effective area of the simple cylinder is also accompanied by an
increase in the leakage of the fluid past the piston. Indeed, the leakage becomes so great
at some pressures that insufficient floating time can be maintained for a proper pressure
measurement.

In Figure 2-2, the pressure fluid is allowed to surround the body of the cylinder. The
pressure drop occurs across the cylinder wall near the top of the cylinder at point B, but
in the opposite direction to that of the simple cylinder in Figure 2-1. In consequence, the
elastic distortion is directed toward the piston, tending to decrease the area of the cylinder.

Again, the change in area with changing pressure places a limit on the usefulness of the
cylinder in 2-2 for it as a primary instrument. But some benefit results from the use of
this cylinder in the construction of a piston pressure gauge because higher pressures may
be attained without a loss in float time. A small sacrifice is made in the float time at
lower pressures because the total clearance between piston and cylinder must necessarily
be greater at low pressure for the cylinder in 2-2 than for the cylinder in Figure 2-1.

In the controlled-clearance design of Figure 2-3, the cylinder is surrounded by a jacket to
which a secondary fluid pressure system is connected. Adjustment of the secondary, or
jacket, pressure permits the operator to change the clearance between the cylinder and
piston at will. A series of observations involving piston sink rates at various jacket
pressures leads to the empirical determination of the effective area of the assembly.
Throughout the world, the controlled-clearance piston pressure gauge is an accepted
standard of pressure.

Piston pressure gauges having very high resolutions may be made by using simple and
reentrant cylinders. A determination of the distortion coefficients of such gauges may be
made by direct comparison with a controlled-clearance gauge. Most piston pressure
gauges have some elastic distortion, but some, used in the very low pressures, have only
small coefficients and, in some instances, correction for distortion may be neglected.

2-2

General Piston Pressure Gauge Considerations

P

Calculations 2

Measurement of pressure with the piston pressure gauge is subject to uncertainties
resulting from effects other than those of elastic distortion. But, it was appropriate that
the subject of elastic distortion be discussed first, since this characteristic is largely
responsible for the various designs that have been developed.

Measurement processes proposed for high accuracy are disturbed by limitations in the
performance of the equipment, by small changes in the environment, and by operational
procedures. The disturbances can be reduced to a degree by exercising control of the
apparatus. Some of the disturbances are difficult to control; it is easier to observe their
magnitudes and apply corrections for their effects.

The factors that affect a pressure measurement process when conducted with a piston
pressure gauge are described below. It is important that the operator is acquainted with
these factors and become accustomed to recognizing their presence. The success of the
measurement will depend upon the degree to which control has been maintained, or to the
completeness by which corrections were applied for these factors.

• Elastic distortions of the piston and cylinder.

• Effects of gravity on the masses.

• Temperature of the piston and cylinder.

• Buoyant effect of the atmosphere upon the masses.

• Hydraulic and gaseous pressure gradients within the apparatus.

Calculations

For a consolidation of these various corrections, see Appendix A of this manual.
Appendix A contains a Pressure Calculation Worksheet (both SI and English units) with
instructions. The Pressure Calculation Worksheet will step the user through the necessary
corrections as applied to calibrations with a piston pressure gauge.

Measurement of Pressure with the Piston Pressure Gauge

Pressure results from the application of a force onto an area. Numerically, it is the
quotient of the force divided by the area onto which it is applied:

F

P =

Where:

F Represents the force

A Represents the area

A

Represents the pressure

2-3

RUSKA 2470

P

Users Manual

Elastic Distortion of the Cylinder

Gravity

As the pressure is increased within a piston pressure gauge, the resulting stress produces
a temporary and reversible deformation of the cylinder. The net effect is a change in the
effective area of the piston-cylinder combination. If the change in the area is a linear
function of the applied pressure, the relationship may be described by the equation:

()

e

1 PbPbAA

2

++=

210

Where:

is the nominal pressure

A is the effective area at a pressure, P

e

A is the area of the piston-cylinder assembly at a reference pressure level

0

& bb are coefficients of elastic distortion which are determined experimentally

21

Since pressure is defined as force per unit area, anything that changes the force applied to
the piston of a piston pressure gauge also changes the pressure produced by that gauge.
Therefore, the effects of gravity on the masses loaded on the piston must be considered.
The gravity correction is usually very significant and must be used during calculations to
achieve the advertised accuracy of the piston pressure gauge.

Confusion has resulted from the English System of units concerning the terms, mass and
weight. The International System of units does not leave room for ambiguity and should
be used whenever possible.

It is recognized that some facilities still operate under the English System of units.
Therefore, this manual provides calibration data and calculation instructions in both the
English and the International System of units.

Corrections for local gravity can vary by as much as 0.5% thus it is very important to
have a reliable value for the local acceleration of gravity. A gravity survey with an
uncertainty better than 0.00001 m/s

Buoyant Effect of the Air

According to Archimedes's principle, the weight of a body in a fluid is diminished by an
amount equal to the weight of the fluid displaced. The weight of an object (in air) that has
had its mass corrected for the effects of local gravity is actually less than that corrected
value indicates. This reduction in weight is equal to the weight of the quantity of air
displaced by the object, or the volume of an object multiplied by the density of the air.
But the volume of an irregular shaped object is difficult to compute from direct
measurement. Buoyancy corrections are usually made by using the density of the material
from which the object is made. If the value of mass is reported in units of apparent mass
vs. brass standards rather than of true mass, the density of the brass standards must be
used. Apparent mass is described as the value the mass appears to have, as determined in
air having a density of 0.0012 g/cm³, against brass standards of a density of 8.4 g/cm³,
whose coefficient of cubical expansion is 5.4 x 10
mass in value (see reference 4).

2

is recommended.

-5

/ ºC, and whose value is based on true

2-4

General Piston Pressure Gauge Considerations

g

Measurement of Pressure with the Piston Pressure Gauge 2

Although the trend is swinging toward the use of true mass in favor of apparent mass,
there is a small advantage in the use of the latter. When making calculations for air
buoyancy from values of apparent mass, it is unnecessary to know the density of the mass.
If objects of different densities are included in the calculation, it is not necessary to
distinguish the difference in the calculations. This advantage is obtained at a small
sacrifice in accuracy and is probably not justified when considering the confusion that is
likely to occur if it becomes necessary to alternate in the use of the two systems.

A satisfactory approximation of the force on a piston that is produced by the load is
given by:

Where:

Temperature

Piston pressure gauges are temperature sensitive and must, therefore, be corrected to a
common temperature datum.

Variations in the indicated pressure resulting from changes in temperature arise from the
change in effective area of the piston due to expansion or contractions caused by
temperature changes. The solution is a straightforward application of the thermal
coefficients of the materials of the piston and cylinder. The area corresponding to the new
temperature may be found by substituting the difference in working temperature from the
reference temperature and the thermal coefficient of area expansion in the relation as
follows:

p

p

AIR

BRASS

⎞
⎟

g

⎟
⎠

⎛
⎜

MF

−= 1

A

⎜
⎝

F is the force on the piston

M

is the mass of the load, reported as "apparent mass vs. brass

A

p

Is the density of the air

AIR

p Is the density of brass (8.4 g/cm³)

BRASS

is the acceleration due to local gravity

standards"

[]

rt

)(0)(0

()

rtcAA

−+= 1

Where:

is the effective area at temperature, t

A

)(0 t

is the effective area at zero pressure and reference temperature, r

A

)(0 r

c is the coefficient of thermal expansion

Reference Plane of Measurements

The measurement of pressure is linked to gravitational effects on the pressure medium.
Whether in a system containing a gas or a liquid, gravitational forces produce vertical
pressure gradients that are significant and must be evaluated. Fluid pressure gradients and
buoyant forces on the piston of a pressure balance require the assignment of a definite

AFP /=

position at which the relation

exists.

2-5

RUSKA 2470

Users Manual

It is common practice to associate this position directly with the piston as the datum to
which all measurements made with the piston are referenced. It is called the reference
plane of measurement, and its location is determined from the dimensions of the piston.
If the submerged portion of the piston is of uniform cross section, the reference plane is
found to lie conveniently at the lower extremity as shown in 2-4. If, however, the portion
of the piston submerged is not uniform, the reference plane is chosen at a point where the
piston, with its volume unchanged, would terminate if its diameter were uniform.

The reference plane of the standard is the effective bottom of the measurement piston.
This location can be correlated to the index on the mass stack using the L1 dimension
(found on Calibration Report for the Piston/Cylinder) and the D Dimension (found on
Calibration Report for the Mass set).

Figure 2-4. Reference Plane Determination

glg02.bmp

When a pressure for the piston pressure gauge is calculated, the value obtained is valid at
the reference plane. The pressure at any other plane in the system may be obtained by
multiplying the distance of the other plane from the reference plane by the pressure
gradient and adding (or subtracting) this value to that observed at the piston reference
plane.

2-6

General Piston Pressure Gauge Considerations

Measurement of Pressure with the Piston Pressure Gauge 2

Figure 2-5. Head Correction Measurement

glg03.bmp

()

AIRmH

ghPPP

**−=

Where:

h is the vertical distance between the reference plane of the Standard and

the reference plane of the DUT (Device Under Test)

ρ

is the density of the air

air

ρ

is the density of the test media

m

g is the acceleration due to local gravity

L1 is the vertical distance from the mass loading location to the effective

bottom of the piston.

D is the vertical distance from the mass loading location to the bottom of

the Hanger Mass

Note

For instances where the reference plane of the DUT is LOWER than the
reference plane of the standard, the h is a negative number and therefore

becomes a negative number.

P

H

In addition, gas lubricated piston pressure gauge calculations should account for the fact
that the pressure gradient mentioned in the preceding paragraph changes as system
pressure is changed. This is because the specific gravity of gas varies as a function of
pressure, not remaining approximately constant, as does a hydraulic fluid.

2-7

RUSKA 2470

Users Manual

Crossfloating

For good work, a piston pressure gauge should be provided with an index mark for
associating the reference of the piston with other planes of interest within a system. The
design of this index will vary with the design and manufacture of the instrument, it may
be in the form or an index rod with scribed lines on it, an index groove on the column of
the instrument, or, other type of fixed indicator. Not only does the mark serve to establish
fixed values of pressure differences through a system, it indicates a position of the piston
with respect to the cylinder at which calibration and subsequent use should be conducted.
If the piston is tapered, it is important to maintain a uniform float position for both
calibration and use. This Position is referred to as the “Mid-Float” position as it
represents the middle of the calibrated range of the Piston/Cylinder.

In normal operation, the system is pressurized until the piston is in a floating position
slightly above the index mark. After a period of time, the piston and its load will sink to
the line at which time the conditions within the system are stable. If there is a question as
to the error that may be produced by accepting a float position that is too high or too low,
the error will be equivalent to a fluid head of the same height as the error in the float
position. This statement assumes that the piston is uniform in area over this length.

It was mentioned earlier that some piston pressure gauges must be calibrated against a
standard gauge. In the jargon of the laboratory, this process is called crossfloating. When
crossfloating one gauge against another, the two are connected together and brought to a
common balance at various pressures. The balancing operation is identical with that
employed on an equal-arm balance where the mass of one object is compared to another.
In each instance the operator must decide when the balance is complete. In a crossfloat,
the two gauges are considered to be in balance when the sink rate of each is normal for
that particular pressure. At this condition there is no pressure drop in the connecting line,
and consequently no movement of the pressure medium. The condition can be difficult to
recognize, particularly if there is no means of amplification in the method of observing.
The precision of the comparison will depend directly upon the ability of the operator to
judge the degree to which the balance is complete. This procedure is repeated for several
pressures, and the values of areas obtained are plotted against the nominal pressure for
each point. A least-squares line is fitted to the plots as the best estimate value of the area
at any pressure.

2-8

There are two accepted methods for determining the balance of the two pressures. First,
the sink rates can be observed and graphed using high sensitivity sensors. Second, a
sensitive null-pressure transducer can be interposed which will display small pressure
differences directly.

When using a suitable amplifying device, the scatter in the plotted areas from a good
quality piston gauge should not exceed a few parts per million.

Bibliography

1. Bridgman, P. W., The Physics of High Pressure, G. Bell & Sons, London, 1952.

2. Cross, J. L., "Reduction of Data for Piston Gauge Pressure Measurements". NBS

Monograph 65 (1963).

3. Dadson, R. S., "The Accurate Measurement of High Pressures and the Precise

Calibration of Pressure Balances", Proc. Conf. Thermodynamic and Transport
Properties of Fluids, London, pp. 32-42, 1957, Institute of Mechanical Engineers.

4. "Design and Test of Standards of Mass", NBS Circular No. 3 (Dec., 1918), Included

in NBS Handbook 77, Volume III.

5. Johnson, D. P., J. L. Cross, J. D. Hill, and H. A. Bowman, "Elastic distortion Error in

the Dead Weight Piston Gauge", Ind. Engineering Chem., 40, 2046 (Dec., 1957).

General Piston Pressure Gauge Considerations

Measurement of Pressure with the Piston Pressure Gauge 2

6. Johnson, D. P., and D. H. Newhall, "The Piston Gauge is a Precise Measuring

Instrument", Trans. of ASME, April, 1953.

7. Newhall, D. H. and L. H. Abbot, "Controlled-Clearance Piston Gauge",

Measurements and Data, Jan.-Feb. 1970.

8. "Pressure Measurement", Measurements & Data Home Study Course, No. 17,

Measurements and Data. September-October, 1969.

9. Tate, D. R., Gravity Measurements and the Standards Laboratory, National Bureau of

Standards Technical Note No. 491 (1969).

10. Heydemann and Welch, Chapter 4, Part 3, "Pure and Applied Chemistry",

Butterworths.

11. Kirk K. Mosher, Ruska Instrument Corporation, "The Traceability Chain of the

Piston Pressure Gauge to NIST", presented at the Canadian National Conference of
Standards Laboratories, 1991.

12. Ken Kolb, Ruska Instrument Corporation, "Reduced Uncertainty and Improved

Reliability for the Pneumatic Piston Pressure Gauge through Statistical Process
Control" published in the "Proceedings" for the Annual Measurement Science
Conference, 1991.

2-9

RUSKA 2470

Users Manual

2-10

General Information

The RUSKA Gas Lubricated Piston Pressure Gauge, model 2470 is a pneumatic pressure
standard designed for the accurate generation and measurement of gas pressures to
3000 psig. This measurement is accomplished in the basic manner of using the
fundamental pressure equation PRESSURE = FORCE/AREA (see Chapter 2, General
Piston Pressure Gauge Considerations, for more information). The gauge is used as the
precision measuring device in the RUSKA Gas Lubricated Piston Pressure Gauge System.

It may be seen from the above general equation that when a known force produced by a
known mass is applied to a piston of a known area, a pressure will be produced that may
be calculated (see Appendix A for detailed information). The RUSKA gauge is arranged
for the application of carefully determined masses on a piston of known area.

Chapter 3

Description

A key feature of the gauge is its ability to accurately reproduce its performance at the
lower pressures. The low viscosity of the gas provides excellent lubrication for the
close-fitting piston/cylinder assembly. Relative motion between the piston and cylinder is
necessary and is obtained by hand rotation of the masses and table which will then
distribute the gas molecules throughout the annulus of the assembly. It is the relative
absence of friction between piston and cylinder walls that characterizes the performance
for which the gauge is so highly respected.

The nominal range of pressure (interval) over which the gauge is capable of operating is
the span from 1.4 kPa (0.2 psig) to 20.6 MPa (3000 psig). This interval is covered by
three interchangeable piston/cylinder assemblies having sufficient overlap for
establishing continuity of measurement and for making detailed investigations of
subintervals within the total range (span).

Some of the most important industrial uses of the gas lubricated piston pressure gauge is
that of a standard for calibrating transducers, Bourdon-tube type gauges, manometers,
and other dead weight gauges. Frequently, the gauge is used in combination with the
pressure null transducer (RUSKA model 2413 or similar) for cross float calibrations
between gaseous and hydraulic media.

3-1

RUSKA 2470

Users Manual

Description of the Mass Set

Description of the Gauge Base

All masses of the Mass Set as supplied with this gauge are made of non-magnetic,
austenitic (series 300) stainless steel (1). They are machined from rolled stock or forgings,
and the removal of any metal is performed in such a way as to maintain balance about the
centerline. Final mass adjustment is usually accomplished by drilling a symmetrical
pattern of holes concentric with the axis.

The Gauge base incorporates simple, sturdy construction and is equipped with three
adjustable feet.

1. Mass carrier composite construction 300 Series, Non-magnetic, Austenitic, Stainless

Steel and other non-magnetic material.

The base has an integral thermowell to accommodate a glass thermometer, or
precision temperature probe (Platinum Resistance Thermometer). The thermowell
allows for accurate temperature measurement of the test media and Piston/Cylinder.

The base features pre-drilled holes to facilitate installation of inductive Float Position
sensors (RUSKA Model 2456 Piston Gauge Monitor or equal).

The most exciting feature of the base is the “Split-Column” design. This design
allows for piston cylinders of different configuration to be mounted on the same base
and operated with the same mass set. The Split Column allows for the operation of
the base with the standard 2470 Piston/Cylinder as shown in Figure 3-1 and with the
2468 Low-Range or the 2468 Mid-Range Piston/Cylinders as shown in Figure 3-2.

3-2

Figure 3-1. Base with 2470 P/C Installed

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