Omega Products PX771A Installation Manual

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PX771A

Differential Pressure Transmitter

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An OMEGA Technologies CompanyAn OMEGA Technologies Company

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Operator's Manual

Operator's Manual

M-3602/1101

M-3602/1101

Remove the Packing List and verify that you have received all equipment, including the following
(quantities in parentheses):

PX725A Gauge Pressure Transmitter (1)
Operator’s Manual (1)
If you have any questions about the shipment, please call the OMEGA Customer Service

Department.
When you receive the shipment, inspect the container and equipment for signs of damage. Note

any evidence of rough handling in transit. Immediately report any damage to the shipping agent.

M-3602

DIFFERENTIAL PRESSURE TRANSMITTERS - SERIES PX771A

TABLE OF CONTENTS

SECTION TITLE PAGE #

Section 1 - INTRODUCTION

1.1 PRODUCT DESCRIPTION............................................................................................1-1

1.2 TRANSMITTER FEATURES.........................................................................................1-1

1.3 MODELS APPROVED FOR HAZARDOUS AREAS...................................................1-2

1.4 USING THIS MANUAL.................................................................................................1-3

Section 1A – DIFFERENTIAL PRESSURE TRANSMITTERS Series PX771A

1A.1 PRODUCT DESCRIPTION.........................................................................................1A-1

1A.2 THEORY OF OPERATION.........................................................................................1A-1

1A.3 IDENTIFYING TRANSMITTER OPTIONS..............................................................1A-3

1A.4 TRANSMITTER MOUNTING....................................................................................1A-7

Standard Process Flange.............................................................................................1A-7

Optional Process Manifold Block ................................................................................1A-8

Vent Plug......................................................................................................................1A-8

1A.4.1 Transmitter Housing Rotation....................................................................................1A-8

1A.5 DP MEASUREMENT APPLICATIONS...................................................................1A-10

Liquid Application......................................................................................................1A-10

Gas Application..........................................................................................................1A-10

Steam Application......................................................................................................1A-11

Liquid Level Application ...........................................................................................1A-12

1A.6 SERVICE CHECKS...................................................................................................1A-13

1A.7 TRANSMITTER SPECIFICATIONS........................................................................1A-14

Section 2 - INSTALLATION

2.1 INSTALLATION NOTES...............................................................................................2-1

2.2 INSTALLATION IN HAZARDOUS AREAS.................................................................2-1

2.3 ELECTRICAL WIRING NOTES ...................................................................................2-2

2.4 WIRING OF 4-20mA SIGNAL/POWER LOOP.............................................................2-3

2.5 WIRING OF 1-5V SIGNAL/POWER LOOP..................................................................2-6

2.6 EFFECTS OF LEAD & LOAD RESISTANCE & SUPPLY VOLTAGE.......................2-6

Section 3 - CALIBRATION

3.1 CALIBRATION SETUP..................................................................................................3-1

3.2 ACCESS TO ADJUSTMENTS.......................................................................................3-1

3.3 EXTERNAL CHECK PROCEDURE .............................................................................3-4

3.4 CALIBRATION ADJUSTMENTS .................................................................................3-4

3.5 TYPES OF RANGE CALIBRATION.............................................................................3-5

3.6 ZERO-BASED CALIBRATION......................................................................................3-8

3.7 ELEVATED ZERO CALIBRATION ..............................................................................3-8

3.7.1 Zero Elevation Example .................................................................................................3-9

3.8 SUPPRESSED ZERO CALIBRATION..........................................................................3-9

3.8.1 Zero Suppression Example...........................................................................................3-11

3.9 SELECTABLE DAMPING...........................................................................................3-11

M-3602 Contents / 0 - 1

M-3602

DIFFERENTIAL PRESSURE TRANSMITTERS - SERIES PX771A

TABLE OF CONTENTS

SECTION TITLE PAGE #

Section 4 - SERVICE

4.1 GENERAL.......................................................................................................................4-1

4.2 TROUBLESHOOTING...................................................................................................4-1

4.3 FACTORY REPAIRS......................................................................................................4-1

Section 5 - SPECIFICATIONS

5.1 FUNCTIONAL SPECIFICATIONS...............................................................................5-1

Current Loop Mode: .................................................................................................5-1

Voltage Mode: ...........................................................................................................5-1

Calibration Adjustments:.........................................................................................5-1

Response Time & Damping......................................................................................5-1

Linearity:...................................................................................................................5-2

5.2 PERFORMANCE SPECIFICATION.............................................................................5-2

Accuracy:...................................................................................................................5-2

Resolution: ................................................................................................................5-2

Long Term Stability: ................................................................................................5-2

Ambient Temperature Effect:..................................................................................5-2

Power Supply Effect:................................................................................................5-2

Ripple and Noise:......................................................................................................5-2

5.3 ENVIRONMENTAL SPECIFICATION ........................................................................5-2

Temperature Limits: ................................................................................................5-2

Humidity Limits:......................................................................................................5-3

EMI Effect:................................................................................................................5-3

Surge Protection:......................................................................................................5-3

Vibration Effect: .......................................................................................................5-3

5.4 PHYSICAL SPECIFICATIONS.....................................................................................5-3

Fill Media:.................................................................................................................5-3

Electrical Housing:...................................................................................................5-3

Electrical Connections:.............................................................................................5-3

APPENDICES

Class I, Division 2 Hazardous Locations .................................................................................. Appendix A

Loop Powered Indicator Option ................................................................................................ Appendix B

Material Safety Data Sheets ......................................................................................................Appendix Z

0 - 2 / Contents M-3602

Section 1

INTRODUCTION

1.1 PRODUCT DESCRIPTION

Series PX771A Differential Pressure Transmitters convert pressure measurements into a
proportional 4-20 mA or a 1 - 5 Vdc output signal that functions as the input to a controller,
recorder, indicato r or similar device. T hese transmitters find a pplication in the g as, water,
and process industries that require accurate measurements over a wide range of
environmental conditions.

1.2 TRANSMITTER FEATURES

The features that follow are common to all transmitter models are described in the following
listing:.

E Pressure Sensor. Strain gauge, piezo-resistive sensors perform pressure

measurements.

E Signal-Power Loop. The transmitter requires a nominal 24 Vdc power source to

operate the signal loop, a 2- wire 4-20 mA output.

E Available Voltage Output. For low power applications, a 1-5 Vdc output is user

configurable via an internal jumper.

E Adjustable Ranges. Transmitters are provided with coarse switch and fine pot

adjustments for range calibration. Span is adjustable from 16 to 100% of the upper range
limit, while zero is adjustable from -600 to 600% of the lower range limit for elevation
and suppression.

E Damping. A circuit jumper selects damping periods of .275 sec or 1 ms to control trans-

mitter response to a change of the measured variable.

E Mechanical Assembly. The transmitter electronics enclosure is constructed of cast

aluminum with an epoxy finish. The materials offered for diaphragms, process
connections, flanges, bolts, etc. are given in Section 1A.

E Fill System. The transmitter's diaphragm and sensor operate in a sealed fluid system.

These systems are furnished with DC 200 as the fill fluid.

E Electrical Conduit Port. Two 1/2 inch NPT female ports are provided for electrical

conduit.

M-3602 Introduction / 1-1

Figure 1-1 – PX771A Differential Pressure Transmitter

1-2 / Introduction M-3602

1.3 MODELS APPROVED FOR HAZARDOUS AREAS

Transmitter mode l s ce rti fi e d fo r operation in h a za rdo us areas by Un de rwri ters Laborato ri e s
(UL) will have the appropriate logo inscribed on the instrument data plate. These models
are intended for use in t he following hazardous locations:

Explosion-proof for Class I, Division 1, Groups C and D.
Nonincendive for Class I, Division 2, Groups A, B, C and D.

The National Electric Code, Article 500, defines the above classes and divisions as follows:

Class I Atmospheres: Contains flammable gases or vapors.
Class II Atmospheres: Contains combustible dust particles.
Class III Atmospheres: Contains ignitable fibers or flyings.

Division 1:

Where continuous expo su re, o r th reat o f fire o r ex plo sio n m ay b e presen t d u e to accid en t o r
uncommon occurrence.

Division 2:

Where threat of fire or explosion is not normally present, and not likely to result from
abnormal occurrence.

Groups A through D:

Cover various flammable gases an d liquids such as ethyl- ether vapor, gasoline, acetone,
etc.

Groups E through G:

Cover various combustible dusts such as dust fro m metalw o rk in g , co al, co k e carb o n b lack ,
grain, etc.

1.4 USING THIS MANUAL

Section 1A provides information relevant to product description, types of mounting,
measurement applications, service checks, and specificat ions.

Sections 2 through 5 describe installation, calibration, service and general specifications.
The Loop Powered Indicator option is covered in Appendix B.

M-3602 Introduction / 1-3

Section 1A

DIFFERENTIAL PRESSURE TRANSMITTERS

Series PX771A

1A.1 PRODUCT DESCRIPTION

The Series PX771A Differential Pressure (DP) Transmitter measures the pressure
differential existing across an orifice plate or similar type device and converts it into a
proportional 4-20 mA or a 1-5 Vdc signal that can be applied to the input of a device such as
a flow compute r, controlle r, recorder, etc. Thi s transmitter i s typically u sed with gas, w ater
and chemical processes to provide accurate measurements under extreme environmental
conditions.

The DP transmitter is offered in ranges from 0-100 inH
for the Series PX771A is given in Table 1A-A .

The transmitter can be installed on a DP pressure manifold or it may be specified with a
universal mounting bracket. The bracket permits the unit to be clamped to a two-inch pipe
or secured to a support structure.

The transmitter electronics enclosure is constructed of cast aluminum. The diaphragm,
flanges, flange bolts and the manifold are offered in two materials; stainless steel, and
Hasteloy C.

O to 0-300 psi. A listing of ranges

2

1A.2 THEORY OF OPERATION

The main assemblies of the DP transmitter are the electronics housing, sensor module and
process flanges as noted in Figure 1A-2. The electronics housing encloses the amplifier
board and the field wiring terminals as shown in the schematic of Figure 1A-3. The sensor
module contains the pressure sensor, two sealed fluid systems, an overpressure diaphragm,
and two isolation diaphragms. The flanges provide the HI and LO port connections and also
function as the outer wall of the pressure input chambers.

The electronic pressure sensor located at the upper part of the sensor module is mounted on
a micro diaphragm that serves as a divider between the two fluid systems. One fluid system
corresponds to the HI pressure input, and the other to the LO pressure input. The isolation
diaphragm of each syst em is olates the fluid system from the input pressure.

When a differential pressure is applied across the HI and LO ports, both isolation
diaphragms will compress or retract in response to the change of differ ential. The movement
of these diaphragms causes similar pressure changes in each of the sealed fluid systems
that are detected by the sensor.

If the differential pressure appl ied to the HI-LO ports acci dentally ex ceeds the uppe r limits
of the transmitter, an overpressure diaphragm mechanism takes control of the situation.
The action of this mechanism prevents the overpressure from reaching the sensor, thereby
minimizing the risk of damage.

M3602 DP Transmitters / 1A-1

Figure 1A-1 – Series PX771A

Figure 1A-2 - Tr ansmitter Assemblies

1A-2 / DP Transmitters M3602

Figure 1A-3 - Simplified Diagram

Implanted on the sensor's micro-machined surface are four strain gauge resistors connected
in a bridge configuration. This circuit, which is powered by a constant current supply on the
amplifier board, produces a millivolt output that is equal to the difference between the two
pressure inputs.

The output of the sensor circuit is wired to a high-gain, linear amplifier that converts the
millivolt signals to a 4-20 mA current output. Figure 1B-3 shows the transmitter output
wired to a typical external loop circuit. This circuit uses a 250-ohm load resistor and a 24 V
dc power source. The 4-20 mA amplifier current flowing through the load resistor produces a
1-5 V input signal for the exter nal device.

The amplifier circu it in clud es a fine -ga in a nd fi ne-o ffset po ten tio mete r for pe rformi ng min or
calibration adjustments. Transmitters also include internal coarse zero and coarse span
switches for more extensive range conversion.

1A.3 IDENTIFYING TRANSMITTER OPTIONS

A data plate affixed to the transmitter body lists the model number, serial number, and
instrument range. To identify the features and options furnished with your model, refer to
the complete model number contained in the sales order. This n umber includes a sequence
of suffix numbers that are identified in Table 1A-A.

M3602 DP Transmitters / 1A-3

TABLE 1A-A - MODEL NUMBER BREAKDOWN FOR SERIES PX771A

PX771A - (1) – (2) ßßßß (see Codes below)

(1) INPUT RANGE (2) OPTIONS

CODE RANGE

CODE DESCRIPTION

100WCGI 0-17 to 0-100 inH
300WCGI 0-50 to 0-300 inH
025GI 0-4 to 0-25 psi
100GI 0-17 to 0-100 psi
300GI 0-50 to 0-300 psi

MB Mounting bracket

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M Local digital indicator

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H Hastelloy C wetted parts

Figure 1A-4A - Overall Dimensions for Series PX771A Transmitter

(With Flange Type Moun ting Bracket

1A-4 / DP Transmitters M3602

Figure 1A-4B - Overall Dimensions for Series PX771A Transmitter

(With Neck Type Mounting Bracket

M3602 DP Transmitters / 1A-5

Figure 1A-4C - Dimensions for Series PX771A Transmitter

(With Manifold Adapter Option)

1A-6 / DP Transmitters M3602

1A.4 TRANSMITTER MOUNTING

The transmitter may be mounted in any position. However, when it leaves the factory it is
calibrated for operation in the upright position with the electronics enclosure at the top and
the DP connections at the bottom as shown in Figure 1B-2. If it is installed in a different
position, the transmitter may require a slight zero adjustment. This procedure is described
in Section 3 Calibration.

The transmitter provides connection ports on the process flange as the standard
arrangement. Optional manifold blocks may also be specified. Both arrangements are
described as follows:

Standard Process Flange. Two process flanges containing the connection ports are
assembled to the transmitter. Th e port designation s (L and H) are stamped o n the body of
the flanges. The ports accept 1/4-18 NPT pipe connections on 2-1/8 in. centers for connection
to the orifice taps or a standard three-valve manifold. The process flange connections are
illustrated at the t op of Figure 1A-5.

Figure 1A-5 - Proce ss Flange and Optional Manifold Block Connectors

The two process flange assemblies are held in place by four bolts and nuts. When the bolts
are removed, the flanges can be repositioned so that the connections can emanate from the

M3602 DP Transmitters / 1A-7

front, rear or bottom of the transmitter. Care should be taken not to damage the sensor
module assembly during this procedure. Once the flange has been positioned, the bolts
should be tightened in an alternat ing sequence to about 20-30 foot-pounds of torque.

Optional Process Manifold Block. Process manifold blocks may be installed on the
transmitter to permit the use of connector assemblies having different connection centers.
The manifold blocks, which are oval in appearance, mate with the transmitter's process
flange. The blocks may be installed in several positions to achieve different connection
centers as shown in Figure 1A-5.

Vent Plug. Each process flange includes a 3/8 inch vent plug to bleed pressure lines. To
vent the unit, l oose n the i nne r 5/32” H ex screw 1/4 turn . To pe rform cal ibrati on b y applyi ng
pressure to the flange, remove the plug with a 7/16” Hex Wrench and install a 1/4” NPT
fitting. Be sure to secure both plugs upon completion.

Warning! Both vents may be under high pressure! Never lo osen th em mo re than 1/4 tu rn to
bleed the lines. Secure both vents after bleeding is complete.

1A.4.1 Transmitter Housing Rotation

Once mounted, the Transmitter Housing can be rotated up to 180° in either direction, i.e.,
clockwise or counterclockwise. The Transmitter Housing must not be rotated from its
shipped position any more than 180° clockwise or counterclockwise. CAUTION: Trans-

mitter will be damaged if the Trans mitter Housing is rotated more than 180° from
its shipped position.

To rotate the T ransmitte r Housin g, the set scre w that lock s the Pressu re Tran sducer to the
Transmitter Housing must be removed with a 3mm Hex Wrench. Once the Transmitter
Housing has been turned to the desired position, be sure to replace and tighten the set
screw (see Figure 1A-6).

1A-8 / DP Transmitters M3602

Figure 1A-6 - Transmitter Housing Rotation Diagram

M3602 DP Transmitters / 1A-9

1A.5 DP MEASUREMENT APPLICATIONS

The Series PX771A transmitter measures the differential pressure of pressurized liquids,
gases, or steam. They can also be used to measure a column of liquid in a tank or vessel. A
discussion of some basic applications follows:

Liquid Application. When measuring the diff erential of pressurized liquids, mount the DP
transmitter bel ow the orifice plate to min imize en try o f air into the tran smitter and its con ­necting lines. As shown in Figures 1A-7 and 1A-8, the HI side of the transmitter must con­nect to the upstream side of the orifice otherwise transmitter readings will be reversed.

Figure 1A-7 - Liquid, Horiz. Pipe Figure 1A-8 - Liquid, Vert. Pipe

These installations should include process shutoff and bypass valves as shown in the
illustrations. For normal transmitter operation, both process valves must be open and the
bypass valve closed. The lines are bled by using the vent plugs on both flanges of the
transmitter.

When calibrating the transmitter, a zero differential can be generated by opening the
bypass valve and closing both shutoff valves. Calibration is covered in Section 3.

Gas Application. The gas industry typically measures differential pressure, static
pressure, temperature, and other variables associated with gas flow. Figures 1A-9 and 1A­10 show a Series PX771A Transmitter measuring the differential pressure across an orifice
plate. A Series PX725A measures the static pressure at the upstream side of the plate. In
both illustrations the transmitters are mounted above the orifice plate so that liquids and
sediment drain away by gravity. It is essential that the HI side of the DP Transmitter
connect to the upstream side of the plate otherwise the DP readings will be incorrect.
Similarly, if the GP Transmitter is connected to the downstream side, SP readings will be
incorrect.

These installations should include process shutoff valves and bypass valves. The shutoff
valves permit the transm itter to be checked or servi ced without disru pting the process. F or
normal transmitter operation, both process valves are open and the bypass valve is closed.
Vent plugs on both flanges of t he transmitter may be used to bleed the lines.

1A-10 / DP Transmitters M3602

An on-site, zero test signal for calibration purposes can be generated by opening the bypass
valve and closing both shutoff valves . Calibration information will be found in Section 3.

Figure 1A-9 - Gas, Horiz, Pipe Figure 1A-10 - Gas, Vert. Pipe

Steam Application. When measuring steam pressure, the maximum temperature of the

transmitter's electronic circuitry must be strictly observed. Temperatures above the
specified limit (see topic 2.1, Environmental Temperature) will cause output errors and
possibly resul t in dama ge to th e tran smitter. On e meth od o f prote cting the transmi tter ca n
be achieved by installing an extended, liquid-filled connecting line as shown in Figures 1A­11 and 1A-12. The liquid functions as a buffer and prevents live steam from entering the
transmitter.

When using a liquid-filled syst em, the connecting line must be installed in a descending step
so that the transmitter is below the level of the process pipe tap and filling tee; this slope
will maintain the liquid in the connecting line and prevent it from being drawn into the
process pipe. Liquid-filled lines must also be properly filled and bled, and checked on a
regular basis.

Figure 1A-11 - Steam Process, Horiz. Pipe Figure 1A-12 - Steam Process, Vert. Pipe

M3602 DP Transmitters / 1A-11

A liquid-filled line is one way to isolate the transmitter from a steam process. As an al­ternate method, a steam trap may be installed in the connecting line. Several
manufacturers offer traps for this application.

Liquid Level Application. DP Transmitters can be used to measure the head pressure of
a column of liquid in an open tank. Typical tank configurations are shown in Figures 1A-13
to 1A-15.

Open Tank. For the application of Figure 1A-13, the transmitter is operated as a
gauge pressure device . T h e H I si de connects to the b o tto m o f th e ta n k w h i l e th e L O si de
is vented to the atmosphere. The span of the transmitter is calibrated to correspond
with maximum (100%) and minimum (0%) tank levels.

Closed Tank. In the closed tan k applica tion, th e HI si de of the transmi tter con nects to
the bottom of the tank whi le the LO si de conne cts to the to p as shown in Figu re 1A-15 .
The transmitter measures the differential pressure between maximum and minimum
tank levels. The wet leg in the upper connection provides a liquid head for all
measurements below the maximum tank level.

Closed Tank w/ Purge Line. If the process medium contained in a closed tank is a
substance that reacts with the transmitter's construction materials, a purge line may be
installed as shown in Figure 1A-14. This line consists of a pipe or dip tube that extends
from the bottom of the tank to the HI side of the transmitter. The LO side pipe
connection is at the very top of the closed tank and is several inches above the
maximum (100%) measurement line.

Figure 1A-13 - Open Tank Measurement Figure 1A-14 - Closed Tank Measurement

1A-12 / DP Transmitters M3602

Figure 1A-15 - Closed Tank with Air Purge

The purge meter (pressure regulator) maintains a pressure on the dip tube that is about
5% above the maximum pressure of a full tank. This prevents the liquid from backing
up the dip tube i nto the HI side of the tran smitter. For this application , the liquid in
the tank should not be allowed to exceed the maximum (100%) measurement level or
else it may backflow into the LO side of the transmitter.

Head Error. A head error results whenever the transmitter is positioned above or
below the minimum (0%) measurement level of t he tank. Head errors will add to or sub­tract from the tank zero readings. These errors can be corrected during range
calibration to obtain the proper output readings. See Section 3 for calibration
information.

1A.6 SERVICE CHECKS

General troubleshooting hints are listed in Table 1A-B. Some of these checks will require a
digital multimeter (DMM). The DMM may be connected across the (+) and (V) terminals to
measure current directly without opening the current loop. See Section 4 - Service for
details.

M3602 DP Transmitters / 1A-13