Cameron Barton 752 User manual
BARTON®MODEL 752 & 752A
DIFFERENTIAL PRESSURE
TRANSMITTERS
For Nuclear Service
User Manual
Part No. 9A-C10820, Rev. 02
November 2015
Contents
Safety............................................................................................................ 2
Section 1—Introduction................................................................................. 3
General ......................................................................................................... 3
Product Description....................................................................................... 3
Differential Pressure Unit ......................................................................... 3
Electronic Transmitter................................................................................ 4
Power Supply ............................................................................................ 4
Zero and Span Control.................................................................................. 4
Zero Control .............................................................................................. 4
Span Control ............................................................................................. 5
Specications ............................................................................................... 5
Qualication............................................................................................... 7
Section 2—Theory of Operation.................................................................... 9
Basic Components........................................................................................ 9
Differential Pressure Unit (DPU) ............................................................... 9
Electronic Transmitter...............................................................................11
Basic Operation ...........................................................................................11
Surge Voltage Protection Circuit .............................................................11
Reverse Polarity Protection......................................................................11
Regulator..................................................................................................11
Strain Gage Bridge Network ................................................................... 12
Signal Amplier........................................................................................ 12
Current Amplier...................................................................................... 12
Temperature Compensation........................................................................ 12
Section 3—Installation, Startup, and Shutdown........................................ 13
Overview..................................................................................................... 13
Unpacking/Inspection.................................................................................. 13
Initial Calibration Check .............................................................................. 13
Mounting ..................................................................................................... 13
Wall or Rack Mounting ............................................................................ 13
Piping Guidelines........................................................................................ 14
Electrical Connections ............................................................................... 14
Loop Resistance Calculations................................................................. 16
Maximum Loop Resistance..................................................................... 17
EMI/RFI Shielding....................................................................................... 17
2
Safety
Before installing this product, become familiar with the installation instruc-
tions presented in Section 3 and all safety notes throughout.
! WARNING:Thissymbolidentiesinformationaboutpracticesorcircum-
stances that can lead to personal injury or death, property damage, or
economic loss.
CAUTION: Indicates actions or procedures which if not performed correctly
may lead to personal injury or incorrect function of the instrument
or connected equipment.
IMPORTANT: Indicates actions or procedures which may affect instrument operation or
may lead to an instrument response that is not planned.
Startup Procedure....................................................................................... 17
Shutdown Procedure .................................................................................. 18
Section 4—Calibration and Maintenance................................................... 19
General Field and Periodic Maintenance.................................................... 19
Electronic Transmitter.............................................................................. 19
Differential Pressure Unit (DPU) ............................................................. 19
Calibration .................................................................................................. 19
Electrical Connections for Calibration ..................................................... 20
Calibration Checkpoints .......................................................................... 21
Calibration Procedure ............................................................................ 21
DPU Inspection and Cleaning..................................................................... 22
Troubleshooting .......................................................................................... 23
Section 5—Assembly Drawing and Parts List........................................... 27
Section 6—Dimensional Drawings ............................................................. 31
Appendix A...................................................................................................A-1
Safety Precautions.....................................................................................A-1
Flow Application .....................................................................................A-1
Liquid Level Applications........................................................................A-1
Typical Piping/Startup Examples ...............................................................A-2
Gas Flow, DPU Above Run ....................................................................A-2
Gas Flow, DPU Below Run ....................................................................A-3
Gas Flow, Hydrates Present...................................................................A-4
Steam Flow, DPU Below Run.................................................................A-5
Liquid Flow, DPU Above Run .................................................................A-6
Liquid Flow, DPU Below Run .................................................................A-7
DPU Below Tank with Reference Leg: Hot or Cool Liquids....................A-8
DPU Level with Tank Bottom: Cool Liquids with Pressurized Tank ........A-9
DPU Below Tank Bottom: Cool Liquids with Pressurized Tank ............A-10
Product Warranty .....................................................................................A-11
Product Brand............................................................................................................
A-11
3
Model 752 and 752A Differential Pressure Transmitters Section 1
Section 1—Introduction
General
The Model 752 and 752A Differential Pressure Transmitters provide a 4-20
mA or 10-50 mA signal that is proportional to differential pressure and trans-
mits it to remote receiving, control, or readout devices. Sources of differential
pressure include liquid level and specic gravity changes in vessels; ow of
liquids and gases through orice plates, nozzles or venturis; pressure drop
across lters and static line pressures, etc.
Product Description
The Model 752 and 752A transmitters combine a differential pressure unit
(DPU) with an electronic circuit. The 4-20 mA or 10-50 mA output is compat-
ible with a wide range of electronic receiving, control, and readout equip-
ment. The instrument utilizes miniaturized hybrid electronic circuits and a
molecular-bonded strain gage sensing cantilever beam, actuated directly by
the bellows' travel within the DPU. In many applications, the electrical con-
nections are contained within a junction box, as shown in Figure 3.1, page 15.
However, the junction box is optional.
Differential Pressure Unit
The mechanical actuating device for the Model 752 and 752A transmitters
is a dual bellows assembly enclosed by a set of two pressure housings. The
assembly (Figure 1.1 below and Figure 2.1, page 9) consists of two internally-
connected bellows, a center block, overrange valves, a temperature compen-
sator, a strain gage assembly, and range springs. The internal volume of the
bellows and center block is lled with a clean, non-corrosive, non-conductive
liquid with a low freezing point, and sealed. The motion-sensing cantilever
beam is also sealed within this environment.
Figure 1.1—Bellows unit assembly (BUA)
4
Section 1 Model 752 and 752A Differential Pressure Transmitters
Electronic Transmitter
The electronic transmitter supplies a 4-20 mA or 10-50 mA direct current out-
put signal that is proportional to the differential pressure sensed by the DPU.
The output signal is transmitted over a two-wire transmission line to remote
receiving devices.
Power Supply
A regulated direct current (DC) power supply is required to operate the trans-
mitting loop. The voltage required will depend on the total loop resistance
(load resistor, cable wiring, and any other resistance in the loop) as shown in
Figure 3.3, page 16. Table 3.1, page 16 shows the resistances in ohms per 1000
feet of wiring for the various cable wire sizes. Once the total loop resistance
has been determined, the power supply voltage can be calculated as follows:
• For 4-20 mA output: VDC = 12 VDC + 2 VDC per 100-ohms load
• For 10-50 mA output: VDC = 12 VDC + 5 VDC per 100-ohms load
Exercise care when calculating the power supply voltage. A power supply
specied at 50 VDC ±1 volt must be considered a 49 VDC source to insure
the minimum required voltage at the transmitter. Use the actual value when
available. Otherwise, use the "worst case" value.
For power supply wiring instructions, refer to the electrical connections
shown in Figures 3.1 and 3.2, page 15.
Zero and Span Control
The transmitter has two 10-turn potentiometers—one for zero adjustments,
the other for span control. With these two controls, measurement can be made
between any two points within the rated transmitter span. However, to ensure
a high level of accuracy, combined zero and span adjustments should never
exceed ±5% of the factory calibration.
IMPORTANT Combined zero and/or span eld adjustments exceeding ±5% of the fac-
tory calibration can alter transmitter performance in direct proportion to
the changes to the factory calibration. For example, if combined adjust-
ments to zero and span change the factory calibration by a factor of 2,
transmitter performance may be decreased by a factor of 2.
Zero Control
During calibration, the zero control is used to adjust the instrument’s output
signal to 4 mA or 10 mA at the minimum pressure setting of the instrument.
5
Model 752 and 752A Differential Pressure Transmitters Section 1
Span Control
When a transmitter leaves the factory, it has a xed range—0-120”w.c., 0-63
psi, etc. Typically the output from the transmitter varies from 4-20 mA or 10-
50 mA. This output is linear with the measured variable, as shown in Figure
1.2.
10
50
OR
ELECTRONIC SIGNAL mA
4
20
20 40 60 80 100
% OF FULL SCALE DP RANGE
Figure 1.2—Output calibrated to upper limit of DPU range
During calibration, the span control is used to adjust the instrument’s output
to 20 mA or 50 mA output signal at the maximum pressure setting of the
instrument.
Specications
Input Range.................................... 0-30 inches (water column) to 0-500 psid
(Consult factory for other ranges)
Output............................................. 4-20 mA or 10-50 mA, direct or reverse acting
Reference Accuracy*...................... ±0.5% of factory-calibrated span, including effects of
non-linearity, hysteresis, and repeatability
(±0.25% accuracy optional)
Zero/Span Adjustments .................. Combined zero/span eld adjustments are limited to
±5% of factory-calibrated span. See Zero Suppres-
sion and Custom Span for additional options.
Zero Suppression ........................... Available as an option.
0% to 50% suppression of factory-calibrated span.
Custom Span.................................. Available as an option.
20% to 100% of factory-calibrated span. Minimum
span is 30” w.c
Sensitivity*...................................... ±0.01% of factory-calibrated span
Power Requirements
(See Figure 3.3, page 16)
4-20 mA....................................... 12 VDC plus 2 VDC per 100-ohms load (to 70 VDC
maximum)
10-50 mA..................................... 12 VDC plus 5 VDC per 100-ohms load (to 70 VDC
maximum)
6
Section 1 Model 752 and 752A Differential Pressure Transmitters
Specications(cont'd)
Load Range
(includes line and receiver; see Figure 3.3, page 16)
4-20 mA....................................... 50 ohms per volt above 12 VDC (to 2900 ohms
maximum)
10-50 mA..................................... 20 ohms per volt above 12 VDC (to 1160 ohms
maximum)
Load Effect*
4-20 mA....................................... ±0.025% of factory-calibrated span per 100-ohms
change
10-50 mA..................................... ±0.05% of factory-calibrated span per 100-ohms
change
Power Supply Effect*
4-20 mA....................................... ±0.025% of factory-calibrated span per 1 Volt
change
10-50 mA..................................... ±0.05% of factory-calibrated span per 1 Volt change
Noise*............................................. 0.2% (peak-to-peak) maximum of factory-calibrated
span
Thermal Effect*(combined effect
on zero and span)........................... ±1.0% of factory-calibrated span per 100ºF change
within the operating temperature range selected
Operating Temperature................... 40ºF to 135ºF (standard), -15ºF to +135ºF (optional)
Max. Safe Working Pressure.......... 3000 psig
Static Pressure Effects*
1-30 psid range............................ ±0.2% of factory-calibrated span per 1000 psig
30-200 psid range........................ ±0.5% of factory-calibrated span per 1000 psig
200-500 psid range...................... ±1.0% of factory-calibrated span per 1000 psig
Overpressure Effects*
1-30 psid range............................ ±0.5% of factory-calibrated span per 1000 psig
30-200 psid range........................ ±1.5% of factory-calibrated span per 1000 psig
200-500 psig range...................... ±3.0% of factory-calibrated span per 1000 psig
Overpressure limit .......................... Up to 3000 psig on either side of DPU without
damage to unit
Process Connections...................... 1/4" and 1/2" NPT (female) on both high and low
pressure sides
Weight ............................................ 8 lb (basic)
Electrical Interface.......................... 1/2 inch conduit connections to internal screw termi-
nals (external junction box optional)
*Note: Turndown has a directly proportional effect on the indicated specications. Zero
or span eld adjustments beyond ±5% may affect indicated performance. Calibration is
by the end-point method with zero and full scale outputs held to ±0.5% of true calibrated
values.
IMPORTANT: The Model 752 and 752A transmitters have no integral electronic interfer-
ence suppression features. If an instrument is to be installed in an area
containing EMI/RFI sources and this interference cannot be tolerated,
take precautions to protect the transmitter signal. See also EMI/RFI
Shielding, page 17. An optional EMI/RFI lter system is available upon
request.
7
Model 752 and 752A Differential Pressure Transmitters Section 1
Qualication
The Model 752 and 752A transmitters have been subjected to IEEE-344
qualication testing that demonstrates that the unit will not lose its pressure
boundary or structural integrity when subjected to loadings associated with
seismic accelerations up to 12 Gs.
8
Section 1 Model 752 and 752A Differential Pressure Transmitters
9
Model 752 and 752A Differential Pressure Transmitters Section 2
Section 2—Theory of Operation
Basic Components
Differential Pressure Unit (DPU)
HP Housing
Valve Stem
HP Bellows
LP Housing
LP Bellows
Figure 2.1—DPU cutaway view
The differential pressure range of the dual-bellows type DPU is determined
by the force required to move the bellows through their normal range of
travel. To provide for various ranges, range springs are incorporated into the
Bellows Unit Assembly (BUA). The range springs, which are available in
various factory assemblies, accurately balance the differential pressure ap-
plied to the DPU.
In operation, the two bellows (which are connected by the valve stem shown
in Figure 2.1) move in proportion to the difference in pressure applied across
the BUA. The linear motion of the bellows is picked up by the tip of the sili-
cone strain gage beam, which is actuated directly by the valve stem connect-
ing the two bellows. If the bellows are subjected to a pressure greater than the
differential pressure range of the DPU, they will move through their normal
range of travel, plus a small additional amount of "overtravel," until the valve
on the stem shaft seals against its valve seat. As the valve closes on the seat, it
"traps" the ll liquid in the bellows, protecting the unit from damage or shift
in calibration.
Since the ll uid is essentially non-compressible, the bellows are fully sup-
ported and cannot rupture regardless of the over-pressure (up to the full rated
pressure of the instrument) applied to the unit. Furthermore, since the unit
contains opposed valves, protection against "overrange" in either direction is
provided.
Draining or Venting. Pressure connections on the top and bottom of the high
and low pressure DPU housings provide a drain when the unit is used in gas
installations, or a vent when the unit is used in liquid installations, when
installed in accordance with standard practices.
10
Section 2 Model 752 and 752A Differential Pressure Transmitters
Temperature Compensation. The high pressure side of the DPU has extra
bellows convolutions to provide for expansion and contraction of the ll
liquid caused by ambient temperature changes. These extra convolutions are
connected to the measuring bellows by a passageway to permit the ll liquid
to change volume without materially affecting the internal pressure or the
physical relationship of the measuring bellows.
Bellows. The bellows used in the DPU were specically developed for use
in sensing and measuring instruments. They are designed to provide exacting
linearity characteristics as well as long life, and to be free of the effects of
work hardening. Individual bellows diaphragms are stamped from special or-
der Type 316 ELC (Extra Low Carbon) stainless steel sheets. The diaphragms
are assembled and seam welded to form the bellows.
Strain Gage Assembly. The strain gage assembly (Figure 2.2) consists of
a strain gage beam and a glass-to-metal seal feed-through assembly. Strain
gages are bonded to opposite sides of the strain gage beam. The end of the
strain gage beam is installed directly into a cutout in the valve stem con-
necting the two bellows of the DPU. Any movement of the bellows in either
direction causes a corresponding linear movement of the strain gage beam
which acts upon the strain gages. Any action of the strain gages is monitored
by the electronic transmitter circuit.
Tension Strain Gage
Compression Strain Gage
Beam & Strain Gage Assembly
Figure 2.2—Strain gage assembly
Range Springs. The range springs act with the bellows to balance the differ-
ential pressure applied to the unit. The springs are fabricated of a material that
is compatible with the specic bellows ll uid used. The number of springs
and their rate depends on the differential range desired.
11
Model 752 and 752A Differential Pressure Transmitters Section 2
Electronic Transmitter
The DPU senses the difference in pressure applied across the bellows unit
assembly and the electronic circuit converts to a 4-20 mA or 10-50 mA output
signal. The pressure causes a linear motion of the bellows which is mechani-
cally transmitted to the strain gages by the strain gage beam. Motion of the
end of the strain gage beam applies tension to one gage and compression on
the other. The gage in tension increases in resistance, while the one under
compression decreases in resistance. The two gages are connected to form
two active arms of a bridge circuit.
Basic Operation
The electronic transmitter is basically a loop current regulating device, where
loop current is controlled by an input of mechanical force or motion. The
block diagram (Figure 2.3, page 12) shows the relationships of the various
stages and the main ow of the electrical currents. As shown, the transmitter,
power supply, and load (line plus receiving device) are connected in series.
The current from the power supply enters the transmitter, passes through the
reverse polarity protecting diode, then divides into two separate paths. The
main current ows through the current amplier stage and returns to the loop.
The remainder of the current passes through the electronic regulator where it
divides into two paths, through the bridge circuit and the voltage amplier.
The current is then returned to the loop. The total loop current ows through
the load and back to the power supply.
Surge Voltage Protection Circuit
Two gas discharge tubes and a Zener diode are placed in the input circuit to
prevent transient voltages from entering the transmitter circuit.
Reverse Polarity Protection
Reverse input polarity protection is provided by the forward-conducting
diode. In the event the polarity of the input is reversed, the diode blocks the
input and prevents the reversed input power from damaging the electronic
circuit components. The diode can accommodate a maximum of 80 Volts
without damage.
Regulator
This stage of the circuit regulates that portion of the loop current which is not
calibrated at the current amplier stage, and provides stabilized voltage for
bridge excitation and power for the signal amplier.
12
Section 2 Model 752 and 752A Differential Pressure Transmitters
Figure 2.3—Operational block diagram
Strain Gage Bridge Network
The strain gage bridge network consists of two silicone piezo-resistive strain
sensors, the zero adjusting potentiometer, bridge completion resistors, and the
temperature compensation components.
Signal Amplier
The signal amplier is an integrated circuit operational amplier which pro-
vides amplication of the strain gage bridge network output voltage.
Current Amplier
The current amplier circuit converts the signal amplier output voltage to
current. The amount of current is precisely regulated with the feedback net-
work to make it proportional to the bridge output.
Temperature Compensation
The Model 752 and 752A are temperature-compensated at the factory. Only
those repairs described in Section 4 of this manual may be performed in the
eld without voiding the qualications certication.
13
Model 752 and 752A Differential Pressure Transmitter Section 3
Section 3—Installation, Startup, and Shutdown
Overview
This section describes the steps required to install the instrument so that it
will perform to its original factory calibration condition. Installation tasks
include
• initial calibration check
• mounting the transmitter
• installing piping
• installing eld wiring
Unpacking/Inspection
The instrument should be inspected at the time of unpacking to detect any
damage that may have occurred during shipment.
IMPORTANT: The unit was checked for accuracy at the factory. Do not change any of
the settings during examination or accuracy will be affected.
The transmitter is shipped in a polyethylene bag to protect the instrument
from contamination. Remove this bag only in a clean area.
Initial Calibration Check
The Model 752 and 752A transmitters are factory-calibrated. However, to
ensure that the calibration is intact following shipping, a calibration check
is recommended prior to operating the instrument. See Calibration, page 19,
for step-by-step instructions. Record the "as found" values and recalibrate, if
necessary.
Mounting
Mount the transmitter so that the pressure housings are in a horizontal posi-
tion and when the operator is facing the transmitter cover, the controls are on
the right side. Use mounting structures that are designed to minimize vibra-
tion and avoid resonance and/or keep resulting amplication below 33 Hz.
Support connected process tubing and conduit using the same mounting as the
instrument base to minimize relative motion of the instrument and connec-
tions.
Wall or Rack Mounting
1. Locate and drill four bracket mounting holes in the mounting surface.
2. Attach the instrument to the wall using 5/16" (8 mm) bolts, Grade 5 or
better, and torque to 10-20 ft-lb.
14
Section 3 Model 752 and 752A Differential Pressure Transmitters
Piping Guidelines
Observe the following practices when piping for ow and liquid level ap-
plications.
1. Install the transmitter as near the primary metering device as possible,
and choose a piping diameter accordingly. For distances up to 50 feet,
use 1/4-inch pipe or tubing. For runs of 50 to 100 feet, use 1/2-inch pipe
or tubing.
IMPORTANT: Distances greater than 100 feet should be used only if an air purge or
blow-back system is installed.
2. Slope all piping at least one inch per linear foot to avoid liquid or gas
entrapment in the lines or the instrument.
• Slope all piping downward from the transmitter when used in gas
applications to prevent liquid entrapment.
• Slope all piping upward from the transmitter when used in liquid ap-
plications to prevent liquid entrapment.
3. If the process temperature exceeds 135ºF, provide a minimum of 2 feet
of uninsulated piping between the transmitter and the primary metering
device for each 100 degrees in excess of +135ºF.
4. Install a suitable pulsation dampening device upstream of the transmitter.
Where severe pulsation is present, the accuracy of the ow measurement
will be affected.
5. For ease of operation and maintenance, install manifolds to allow sensing
lines to be shut off while removing the instrument from the line or per-
forming a calibration. Appendix A shows examples of typical installation
congurations.
6. Locate all shutoff valves and bypass valves so that they are readily acces-
sible from the front of the instrument. Locate block valves at the source
of differential pressure lines.
7. Prevent leakage by using a suitable sealing compound on all joints. Mea-
surement errors can be caused by leaks in the piping.
Electrical Connections
!WARNING: Ensure that the condulet cover is secure before applying
power to instrument when used in hazardous areas. Failure to do this
may result in personal injury or property damage.
Flexible cable is recommended for electrical connections to the instrument.
15
Model 752 and 752A Differential Pressure Transmitter Section 3
Perform the following steps to complete eld wiring.
1. Connect the power supply and the receiver to the transmitter as shown in
Figures 3.1 and 3.2.
2. Determine the total loop resistance required for the installation, using
Figure 3.3, page 16, for reference. The total loop resistance must be less
than the maximum calculated value. Table 3.1, page 16, provides loop
resistance values for various cable wire sizes.
3. Install a load resistor sized for the application.
!WARNING: Failure to properly calculate power supply DC output voltage
may result in inaccurate transmitter readings, possibly leading to safety
system performance degradation during design basis events. To avoid
equipment inaccuracy hazards, follow the examples and tables in this
section for determining the proper power supply DC output voltage.
+
GND
Receiver
Power Supply
+
+
Junction Box
Load Resistor
Model
752/752A
Figure 3.1—Typical eld wiring connections for Model 752 and 752A with junction box
Receiver
Power Supply
+
+
Load Resistor
Model
752 /
752A
+
Figure 3.2—Typical eld wiring connections for Model 752 and 752A without junction
box
16
Section 3 Model 752 and 752A Differential Pressure Transmitters
Table3.1—CableSpecications
Loop Resistance/1000 ft
(ohms@20°C)
Cable Wire Size
5.06 14 AWG, 2 Wires
8.04 16 AWG, 2 Wires
12.78 18 AWG, 2 Wires
20.30 20 AWG, 2 Wires
4-20 mA
10-50 mA
200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000
2900
1160
70
60
50
40
30
20
10
Total Loop Resistance (Ohms) (Load Transmission Line)
Power Supply Range (Volts)
Any voltage or resistance within the
shaded area for the respective
transmitter output is acceptable.
Figure 3.3—Power supply and loop resistance
Care must be exercised when calculating the power supply output voltage. A
power supply specied as 50Vdc ±1 volt must be considered a 49Vdc source
to ensure the minimum required voltage at the transmitter. Use the actual
value when available; otherwise, use "worst case" value.
Use Figure 3.3 as a reference to determine if the maximum calculated value
of RT= RLine + RLoad + RExt is correct.
Loop Resistance Calculations
Use the following method to calculate the loop resistance value.
Transmitter Current = IDC (20 mA or 50 mA)
RT= VDC - TVDC
IDC
Total Loop Resistance (RT) = RLine + RLoad + RExt
Power Supply Voltage = VDC (70 V max. for 4-20 mA or 10-50 mA Systems)
Maximum Transmitter Voltage = TVDC (70 V for both 4-20 mA and 10-50 mA Systems)
Minimum Transmitter Voltage = TVDC (12 V for both 4-20 mA and 10-50 mA Systems)
17
Model 752 and 752A Differential Pressure Transmitter Section 3
Maximum Loop Resistance
Example 1: (Maximum loop resistance for 10-50 mA system):
VDC = 70 Vdc TVDC = 12 Vdc
IDC = 50 mA RT= = 1160 Ohms
0.05
70-12
Example 2: (Maximum loop resistance for 4-20 mA system):
VDC = 70 Vdc TVDC = 12 Vdc
IDC = 20 mA RT= = 2,900 Ohms
0.02
70-12
Example 3: (Calculation to determine maximum loop resistance with
power supply ≥12 Vdc, but ≤ 70 Vdc for 10-50 mA and 4-20 mA systems):
VDC = 40 Vdc TVDC = 12 Vdc
IDC = 50 mA RT= = 560 Ohms
0.05
40-12
IDC = 20 mA RT= = 1400 Ohms
0.02
40-12
EMI/RFI Shielding
IMPORTANT: The 752 and 752A transmitters have no integral electronic interference
suppression features. If an instrument is to be installed in an area con-
taining EMI/RFI sources and this interference cannot be tolerated, take
precautions to protect the transmitter signal. An optional EMI/RFI lter is
available upon request. Contact the factory for information.
The following precautions are recommended to limit EMI/RFI interference:
1. Run signal wires in solid conduit or use high quality shielded cable to
connect the transmitter to the power equipment.
2. House the transmitter leads in solid conduit up to the junction box where
the shielded cable is connected to the leads.
3. Ground the electronic transmitter, junction box (including the cover),
conduit, and cable shield.
Startup Procedure
To operate the transmitter, perform the following steps. See the installation
diagrams in Appendix A, page A-1, for typical valve locations.
1. Locate the block valves and make sure they are closed.
NOTE: The block valve is normally installed at the facility for the purpose of iso-
lating the pressure lines (process being monitored) from the monitoring
instruments.
2. Congure the test manifold’s control valves to connect the input pressure
ports of the DPU to the appropriate pressure lines (process being
18
Section 3 Model 752 and 752A Differential Pressure Transmitters
monitored). Follow the guidelines in Appendix A that are specic to your
piping conguration.
3. Open the block valves if applicable (recommended for liquid service, but
not for gas).
NOTE: For gas service, it is recommended that a zero check be performed with
both block valves closed. If the gas ow is pulsating, there may be a
standing wave effect in the process line which can displace the indicator
and appear as a zero error.
4. Apply electrical power to the transmitter loop.
5. Check the transmitter calibration across all checkpoints, using the
instructions provided in Calibration, page 19. If re-adjustment of the zero
and/or span is necessary, perform all 12 steps described in the Calibration
Procedure, page 21.
6. Check the manifold and piping for leaks as follows:
a. Open the bypass valve(s), then open one shutoff valve to pressurize
the instrument.
b. Close the shutoff valve and the bypass valve.
c. Any leakage will be indicated by a change (increase or decrease) in
the transmitter output.
NOTE: Be careful not to subject the DPU to unnecessary shock or overrange
pressure during operations.
Shutdown Procedure
To shut down operations, perform the following steps. See the installation
diagrams in Appendix A, page A-1, for typical valve locations.
1. Remove electrical power from the transmitter loop.
2. Close the transmitter shut-off valves.
3. Close the main block valves at the process connections.
4. Open the transmitter drain valves and remove all pressure from the unit.
19
Model 752 and 752A Differential Pressure Transmitters Section 4
Section 4—Calibration and Maintenance
General Field and Periodic Maintenance
Electronic Transmitter
The electronic transmitter circuits are basically maintenance-free and do not
require routine preventative maintenance other than a periodic check of cali-
bration. See Calibration below for details.
Differential Pressure Unit (DPU)
The eld maintenance schedule for the DPU will depend on the purpose for
which it is used. Periodic cleaning of the DPU is required if the instrument
is used in a system where solids or semi-solids can accumulate in the DPU
housings. Follow the guidelines in DPU Inspection and Cleaning, page 22, and
take the necessary safety precautions to avoid damage to the bellows.
Calibration
Each transmitter is calibrated at the factory prior to shipment. A 9-point cali-
bration check is recommended upon receipt, and again before the transmitter
is operated for the rst time. If "as found" values are not within the specied
range, a full calibration should be performed.
The transmitter should be recalibrated at periodic intervals, determined
primarily by the usage of the transmitter, historical performance, the desired
accuracy of the output signal, or indications that the instrument may be out of
calibration. If a transmitter is installed after an extended period of storage, a
calibration test should be performed before operating the transmitter to ensure
correct performance.
Test Equipment
To perform the calibration procedure for an instrument with 0.5% accuracy,
the test equipment should meet or exceed the requirements listed in Table 4.1,
page 20. For calibrating instruments, the pressure source should have at least 4
times the accuracy of the instrument being tested. In the event equipment is
substituted that does not meet these requirements, the accuracy of the recali-
brated transmitter must be derated accordingly.
20
Section 4 Model 752 and 752A Differential Pressure Transmitters
Table 4.1—Calibration Equipment
Equipment Requirements
Digital Voltmeter ±0.05% of reading accuracy at 10 VDC scale
Power Supply 12-70 Vdc, 60 mA minimum, regulation 3%, ripple 1% (see Electrical
Connections, page 14)
Precision Load Resistor 200 ohms, ±0.05%, 1W (10-50 mA transmitter) 500 ohms, ±0.05%,
1W (4-20 mA transmitter)
Pressure Source Provides zero to full scale pressure (Accuracy: 4 times the accuracy
of the instrument under test)
Electrical Connections for Calibration
The electrical connections required for calibrating the transmitter are shown
in Figure 4.1.
!WARNING: Ensure that the condulet cover is secure before applying
power to instrument when used in hazardous areas. Failure to do this
may result in personal injury or property damage.
+
GND
Precision
DC Voltmeter
Power Supply
+
+
Load
Resistor
500 ohms, 4-20 mA systems
200 ohms, 10-50 mA systems
Model
752/752A
+
Milliammeter
(optional)
Figure 4.1—Electrical connections for calibration
Flexible cable is recommended for electrical connections to the instrument.
Perform the following steps to complete eld wiring.
1. Connect the power supply and the DC voltmeter to the transmitter as
shown in Figure 4.1.
2. Connect a milliammeter as shown, if desired.
3. Determine the total loop resistance required for the installation, using
Figure 3.3, page 16, for reference. The total loop resistance must be less
than the maximum calculated value. Table 3.1, page 16, provides loop
resistance values for various cable wire sizes.
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