Sierra 780S Series Flat-Trak User manual

Series 780S Instruction Manual Table of Contents
IM-78S-B 0-1
Sierra 780S Series Flat-Trak™
Mass Flow Meter
Instruction Manual
Part Number IM-78S
07/99 Revision B
5 Harris Court, Building L Monterey, CA 93940
[phone] [phone] Fax [phone]
http://www.sierrainstruments.com
Sierra Instruments b.v. Bolstoen 30A 1046 AV Amsterdam The Netherlands
[phone] Fax [phone]

Table of Contents Series 780S Instruction Manual
0-2 IM-78S-B
Customer Notice
Sierra Instruments, Inc. is not liable for any damage or personal injury, whatsoever,
resulting from the use of Sierra Instruments standard mass flow meters for oxygen
gas. You are responsible for determining if this mass flow meter is appropriate for
your oxygen application. You are responsible for cleaning the mass flow meter to
the degree required for your oxygen flow application.
© COPYRIGHT SIERRA INSTRUMENTS 1998
No part of this publication may be copied or distributed, transmitted, transcribed, stored in
a retrieval system, or translated into any human or computer language, in any form or by
any means, electronic, mechanical, manual, or otherwise, or disclosed to third parties
without the express written permission of Sierra Instruments. The information contained in
this manual is subject to change without notice.
TRADEMARKS
Flat-Trak™, Steel-Trak™ and Smart Interface™ software are trademarks of Sierra Instru-
ments, Inc. Other product and company names listed in this manual are trademarks or
trade names of their respective manufacturers.
All Sierra products are Year 2000 compliant.

Series 780S Instruction Manual Table of Contents
IM-78S-B 0-3
Table of Contents
Chapter 1 Introduction
Series 780S Flat-Trak Mass Flow Meters ..........................1-1
Using this Manual.................................................1-1
Note and Safety Information.....................................1-2
Receipt of System Components .................................1-2
Technical Assistance..............................................1-2
The Series 780S Flow Sensing Principle...........................1-3
Smart Electronics Features............................................1-4
Enclosure Options .....................................................1-5
Smart Interface™Software............................................1-5
Chapter 2 Installation and Wiring
Installation Overview..................................................2-1
Unobstructed Flow Requirements...............................2-2
Installing the Flow Meter .............................................2-2
Changing Display Orientation....................................2-3
Wiring Connections ...................................................2-4
Input Power Wiring...............................................2-5
Output Signal Wiring .............................................2-7
Alarm Output Wiring.............................................2-10
Remote Sensor Probe Wiring...................................2-11
Range Selection Wiring..........................................2-13
Chapter 3 Operating Instructions
Flow Meter Start Up...................................................3-1
Using the Smart Electronics Basic Features........................3-2
Hazardous-Area Enclosure Optional LCD Display............3-2
LCD Display Programming Menu...............................3-3
Single-Digit LED Programming Menu..........................3-4
Entering Alarm Parameters.......................................3-5
K-Factor Adjustment..............................................3-6
User Full Scale Adjustment......................................3-7
Time Response Delay Adjustment...............................3-8
Totalizer Reset.....................................................3-9
Using the Smart Electronics Advanced Features..................3-11
Voltage Zero Adjustment........................................3-11
Voltage Span Adjustment........................................3-11
Current Zero Adjustment........................................3-12
Current Span Adjustment........................................3-12
Instrument Validation.................................................3-13
Electronics Validation Procedure...............................3-14
Sensor Validation Procedure....................................3-15

Table of Contents Series 780S Instruction Manual
0-4 IM-78S-B
Chapter 4 Troubleshooting and Repair
Troubleshooting the Flow Meter.....................................4-1
Returning Equipment to the Factory.................................4-3
Appendix A Product Specifications
List of Figures
1-1. Series 780S Flow Sensing Principle .......................1-3
2-1. Flow Meter Orientation.......................................2-2
2-2. Wiring Access NEMA 4X Enclosures .....................2-4
2-3. Wiring Access Hazardous-Area Enclosures...............2-4
2-4. AC Input Power Connections...............................2-5
2-5. Ferrite Clamp Installation....................................2-5
2-6. DC Input Power Connections (NEMA 4X) ...............2-6
2-7. DC Input Power Connections (Hazardous-Area).........2-6
2-8. DC Output Signal Connections (NEMA 4X)..............2-7
2-9. DC Output Signal Connections (Hazardous-Area)........2-7
2-10. Load Resistance Versus Input Voltage.....................2-8
2-11. Isolated 4-20 mA Loop (NEMA 4X).......................2-9
2-12. Non-isolated 4-20 mA Loop (NEMA 4X).................2-9
2-13. Isolated 4-20 mA Loop (Hazardous-Area).................2-9
2-14. Non-isolated 4-20 mA Loop (Hazardous-Area)...........2-9
2-15. Isolated Alarm Connections (NEMA 4X).................2-10
2-16. Non-Isolated Alarm Connections (NEMA 4X) ..........2-10
2-17. Isolated Alarm Connections (Hazardous-Area) ..........2-11
2-18. Non-isolated Alarm Connections (Hazardous-Area) ....2-11
2-19. Remote Electronics to Sensor (NEMA 4X)...............2-12
2-20. J Box to Remote Enclosure (NEMA 4X).................2-12
2-21. Remote Electronics to Sensor (Hazardous-Area) ........2-12
2-22. J Box to Remote Enclosure (Hazardous-Area)...........2-13
2-23. Range Selection Wiring (NEMA 4X) .....................2-13
2-24. Range Selection Wiring (Hazardous-Area)...............2-13
3-1. Smart Electronics Device Locations ........................3-1
3-2. Magnetic Switch Operation..................................3-2
3-3. Electronics Validation Component Locations.............3-13
3-4. Sensor Validation Component Locations .................3-15
List of Tables
2-1. Pipe Length Requirements for Installation.................2-2
3-1. Electronics Validation Results..............................3-15
3-2. Sensor Validation Results...................................3-16

Series 780S Instruction Manual Table of Contents
IM-78S-B 0-5
Warnings and Cautions
Warning! Agency approval for hazardous location installations varies between flow
meter models. Consult the flow meter nameplate for specific flow meter approvals be-
fore any hazardous location installation.
Warning! All wiring procedures must be performed with the power off.
Warning! To avoid potential electric shock, follow National Electric Code safety practices
or your local code when wiring this unit to a power source and to peripheral devices. Fail-
ure to do so could result in injury or death. All AC power connections must be in accor-
dance with published CE directives.
Warning! Do not power the flow meter with the sensor jumper wires disconnected. This
could cause over-heating of the sensors and/or damage to the electronics.
Warning! Before attempting any flow meter repair, verify that the line is de-pressurized.
Warning! Always remove main power before disassembling any part of the mass flow meter.
Caution! Changing the length of cables or interchanging sensors or sensor wiring
will affect the accuracy of the flow meter. You cannot add or subtract wire length with-
out returning the meter to the factory for re-calibration.
Caution! When using toxic or corrosive gases, purge the line with inert gas for a
minimum of four hours at full gas flow before installing the meter.
Caution! The AC wire insulation temperature rating must meet or exceed 71°C (158°F).
Caution! Before making adjustments to the Smart Electronics device, verify the flow meter
is not actively monitoring or reporting to any master control system. Adjustments to the
electronics will cause direct changes to flow control settings.
Caution! Printed circuit boards are sensitive to electrostatic discharge. To avoid
damaging the board, follow these precautions to minimize the risk of damage:
• before handling the assembly, discharge your body by touching a grounded,
metal object
• handle all cards by their edges unless otherwise required
• when possible, use grounded electrostatic discharge wrist straps when handling
sensitive components

Table of Contents Series 780S Instruction Manual
0-6 IM-78S-B

Series 780S Instruction Manual Chapter 1 Introduction
IM-78S-B 1-1
Chapter 1 Introduction
Series 780S Flat-Trak™Mass Flow Meters
Sierra’s Series 780S Flat-Trak Mass Flow Meter provides a reliable
solution for monitoring gas mass flow rate. Low-flow sensitivity,
fast response and outstanding rangeability have made this model the
instrument of choice for many critical gas flow applications. The
Series 780S meter solves the problems associated with monitoring
flow in installations where long runs of straight pipe are not avail-
able. The instrument’s built-in flow conditioner creates a uniform
velocity profile by means of two stainless-steel perforated plates
welded into the body between the sensor and the inlet connection.
With flow conditioning, upstream piping requirements are reduced
to less than three diameters after most common flow disturbances.
The flow meter’s versatile microprocessor-based transmitter integrates
the functions of flow-range adjustment, meter validation and diagnos-
tics in a probe-mounted or remotely mounted housing. Mass flow rate
and totalized flow, as well as other configuration variables are dis-
played on the meter’s optional LCD display. The meter provides an
optical/galvanic isolated flow output, two alarm outputs and one con-
tact input for range or gas selection. The programmable transmitter is
easily configured via RS-232 and Sierra’s Smart Interface™software
or through three push buttons built into the device.
The Series 780S Mass Flow Meter’s simple installation combines
with an easy-to-use interface that provides quick set up, long term
reliability and accurate mass flow measurement over a wide range of
flows and conditions.
Using This Manual
This manual provides the information you need to install and operate
the Series 780S Mass Flow Meter. The four chapters of this manual
cover the following areas:
• Chapter 1 includes the introduction and product description
• Chapter 2 provides installation and wiring instructions
• Chapter 3 describes system operation and programming
• Chapter 4 covers troubleshooting and repair
The product specifications are found in Appendix A.

Chapter 1 Introduction Series 780S Instruction Manual
1-2 IM-78S-B
Note and Safety Information
We use note, caution and warning statements throughout this book
to draw your attention to important information.
Warning! Caution! Note
This statement appears with
information that is important to
protect people and equipment
from damage. Pay very close
attention to all warnings that
apply to your application.
This statement appears with
information that is important
for protecting your equipment
and performance. Read and
follow all cautions that apply
to your application.
This statement appears with
a short message to alert you
to an important detail.
Receipt of System Components
When receiving a Sierra mass flow meter, carefully check the outside
packing carton for damage incurred in shipment. If the carton is dam-
aged, notify the local carrier and submit a report to the factory. Re-
move the packing slip and check that all ordered components are pre-
sent. Make sure any spare parts or accessories are not discarded with
the packing material. Do not return any equipment to the factory
without first contacting Sierra Customer Service.
Technical Assistance
If you encounter a problem with your flow meter, review the con-
figuration information for each step of the installation, operation and
setup procedures. Verify that your settings and adjustments are con-
sistent with factory recommendations. Refer to Chapter 4, Trouble-
shooting, for specific information and recommendations.
If the problem persists after following the troubleshooting procedures
outlined in Chapter 4, contact Sierra Instruments by fax or by E-mail
(see inside front cover). For urgent phone support you may call (800)
866-0200 or [phone] between 8:00 a.m. and 5:00 p.m. PST.
In Europe contact Sierra Instruments bv at [phone]. When
contacting Technical Support, make sure to include this information:
• the flow range, serial number and Sierra order
number (all marked on the meter nameplate)
• the software version (visible at start up)
• the problem you are encountering and any corrective
action taken
• application information (gas, pressure, temperature
and piping configuration)

Series 780S Instruction Manual Chapter 1 Introduction
IM-78S-B 1-3
The Series 780S Flow Sensing Principle
Sierra’s unique Steel-Trak™sensor probe is responsible for the un-
surpassed accuracy, ruggedness and reliability of Sierra industrial
flow meters. The immersible Steel-Trak sensor consists of two
sensing elements–a velocity sensor and a temperature sensor that
automatically corrects for changes in gas temperature.
When power is applied to the flow meter, the transducer electronics
heats the velocity sensor to a constant temperature differential above
the gas temperature and measures the cooling effect of the gas flow.
The electrical power required to maintain a constant temperature dif-
ferential is directly proportional to the gas mass flow rate.
Both sensors are reference-grade platinum resistance temperature
detectors (RTDs). The platinum RTD wire is wound on a rugged ce-
ramic mandrel for strength and stability. Steel-Trak sensors are clad
in a rugged, sealed 316 stainless steel encasement.
Figure 1-1. Series 780S Flow Sensing Principle

Chapter 1 Introduction Series 780S Instruction Manual
1-4 IM-78S-B
Smart Electronics Features
Instrument Validation
Two simple tests offer full “field-validation” of your Smart mass
flow meter. The first test checks the system electronics, linearization
and microprocessor functionality and is performed by injecting a
known input value and confirming that the flow meter outputs the
expected value. The second test verifies that the instrument’s pri-
mary sensing elements have not drifted or shifted from their original
calibration and is accomplished by measuring the resistance of the
velocity and temperature sensors and comparing the results to the
NIST-traceable calibration data provided with the flow meter. To-
gether, these tests confirm that your meter is working correctly and
the calibration variables did not drift, shift or change values.
Dual Range or Dual Gas Calibration (Optional)
Select one of two factory calibrated flow ranges using a simple ex-
ternal customer-supplied single contact closure.
User Full Scale Flow Rate
Field-configure from 50% to 100% of the factory full scale setting
(factory full scale is normally set to 125% of the user-specified maxi-
mum flow rate). This adjustment can be made for each flow range.
Alarms
Program high and low or window alarm limits independently for
each flow range. The solid state contacts are optically isolated.
K-Factor Correction
Change the calibration correction factor to compensate for flow pro-
file disturbances or specific application conditions. The K-factor is a
multiplication factor applied to the linearized flow signal. You may
set the K-factor individually for each flow range.
Dual Output Signals
The flow meter offers two separate linear output signals propor-
tional to flow, 0-5 VDC (0-10 VDC optional) and 4-20 mA. The
4-20 mA output can be field-configured as an active loop powered
by the flow meter or an optically isolated passive loop requiring an
external power supply.

Series 780S Instruction Manual Chapter 1 Introduction
IM-78S-B 1-5
Totalizer
With the optional LCD display, actual mass flow appears on line 1
and the totalized flow on line 2 both in the user-specified engineer-
ing units. The totalizer counts only the selected range and when
ranges are switched, the value of the non-selected range is stored in
memory. You may reset the totalizer using the membrane buttons or
by using a hand-held magnet.
Zero and Span Outputs
Validate and adjust the settings to ensure output circuits are correct.
Time Response Delay
Select from a low response for faster tracking to a high response
for a smoother output.
Enclosure Options
Flow meter electronics are available mounted directly to the flow
body, or remotely mounted up to 200 feet away. The electronics
housing may be used indoors or out, including wet environments.
Display options include a 2 x 12 character LCD display of mass flow
rate including totalized mass, or a single-digit LED located on the de-
vice printed circuit board. Local operation and reconfiguration is ac-
complished using the three push buttons operated via finger touch.
Smart electronics include nonvolatile memory that stores all configu-
ration information. The memory allows the flow meter to function
immediately upon power up, or after an interruption in power.
Smart Interface™Software
Sierra’s Smart Interface Windows™-based software is available for
connecting your PC directly to the mass flow meter. An RS-232 se-
rial cable along with floppy disks containing the program and sys-
tem files are available from the factory. See the Smart Interface User
Guide included with the software package for operating instructions.
(Order code for this package is 780-SIP.)

Chapter 1 Introduction Series 780S Instruction Manual
1-6 IM-78S-B

Series 780S Instruction Manual Chapter 2 Installation
IM-78S-B 2-1
Chapter 2 Installation
Installation Overview
The Series 780S Flat-Trak™is available with ANSI or DIN flanges,
NPT or butt-weld connections. For ease of installation, the meter is
pre-assembled with the sensor probe installed in the flow body.
When selecting an installation site, make sure that:
1. Line pressure and temperature will not exceed the flow meter
rating. Temperature should not vary more than 200°F (100°C)
around the calibration temperature. Line pressure should not
vary more than 50 psi (3.4 bar) around the calibrated pressure.
2. The location meets the required minimum number of pipe
diameters upstream and downstream of the sensor head (see
Table 2-1).
3. Safe and convenient access with adequate clearance. Also, verify
the meter is located where the gas is clean and dry and the meter
is calibrated for the gas to be measured.
4. When using a CSA, FM or EEx approved flow meter, verify that
the cable entry into the instrument meets the specific standard re-
quired for that approval.
5. For remote installations, verify the supplied cable length is suffi-
cient to connect the flow meter sensor to the remote electronics.
(Do not extend or shorten the supplied cable between the probe
and the electronics.)
Also, before installation check your flow system for anomalies
such as:
•leaks
•valves or restrictions in the flow path that could create dis-
turbances in the flow profile that might cause unexpected
flow rate indications
•heaters that might cause rapid excursions in the measured
temperature
Warning!
Agency approval for hazardous
location installations varies
between flow meter models.
Consult the flow meter name-
plate for specific flow meter
approvals before any hazard-
ous location installation.

Chapter 2 Installation Series 780S Instruction Manual
2-2 IM-78S-B
Unobstructed Flow Requirements
Select an installation site that will minimize possible distortion in the
flow profile. Valves, elbows, control valves and other piping compo-
nents may cause flow disturbances. Check your specific piping condi-
tion against the examples shown below. To achieve accurate and re-
peatable performance, install the flow meter using the recommended
number of straight run pipe diameters upstream of the sensor.
Piping Condition Upstream(1) Requirements
Single 90° elbow or T-piece 1 D
Reduction (4:1) 3 D
Expansion 3 D
After a control valve 3 D
Two 90° elbows (in same plane) 3 D
Two 90° elbows (different planes) 5 D
(1) Number of diameters (D) of straight pipe required between upstream disturbance and the flow meter sensor.
Table 2-1. Pipe Length Requirements for Installation
Installing the Flow Meter
SIERRA
Flow direction indicator
should point downstream
in the direction of flow
FLOW
Enclosure
adjustable to any
viewing position
Figure 2-1. Flow Meter Orientation (Flange Connection Shown)
When positioning the flow meter, refer to the flow direction indicator
attached to the probe. For proper operation, install the meter with the
flow direction indicator pointing downstream in the direction of flow.
Installing the meter opposite this direction may result in inaccurate
flow measurement.

Series 780S Instruction Manual Chapter 2 Installation
IM-78S-B 2-3
To install the flow meter:
1. Turn off the flow of process gas. Verify that the line is not pres-
surized. Confirm that the installation site meets the minimum up-
stream pipe diameter requirements shown in Table 2-1.
2. Prepare the fluid connection fittings or flanges in the pipeline.
(Fitting components should be blown clean with filtered gas be-
fore use.) Mount in a vertical position. For horizontal pipelines
having a process gas temperature above 300° F (130°C), mount the
flow meter at a 90-degree angle to avoid overheating the electron-
ics enclosure.
3. Seat the flow meter level and square on the mating fluid connec-
tions with a gasket in place for each side for flange types. (Make
sure both gaskets are smooth and even with no gasket material
protruding into the flow profile; obstructions in the pipeline could
cause inaccurate flow measurement.) Make sure that the flow di-
rection indicator is pointing downstream in the direction of flow.
4. Secure the fluid connections as specified in the technical guidelines
furnished by the fitting or flange manufacturer.
5. If needed, adjust the optional display to the desired orientation,
as follows.
Changing Display Orientation (Hazardous-Area Enclosures Only)
Depending on installation requirements, you may need to change the
position of the optional display. To rotate the display board:
1. Use a 1/16-inch hex key to loosen the set screw securing the
larger end of the enclosure. Turn cover counterclockwise and
remove.
2. Remove 4 screws and standoffs from the display. Release the
latch securing the display ribbon cable connector from the dis-
play board.
3. Rotate the display board to the desired position. Re-connect the
ribbon cable to the display board.
4. Replace the 4 screws and standoffs. Replace the enclosure
cover. Tighten set screw.
Caution!
When using toxic or cor-
rosive gases, purge the
line with inert gas for a
minimum of four hours at
full gas flow before in-
stalling the flow meter.

Chapter 2 Installation Series 780S Instruction Manual
2-4 IM-78S-B
Wiring Connections
For NEMA 4X enclosures, use TB2 for power and signal connec-
tions, TB1 is for sensor connections. (The terminal designations are
labeled inside the enclosure cover.)
Wiring connections
inside cover
To access components:
1. Loosen 4 screws.
2. Remove cover.
NEMA 4X enclosure
terminal blocks
TB1
TB2 1
3
5
7
9
11
13
15
2
4
6
8
10
12
14
16 6
4
2
5
3
1
Figure 2-2. Wiring Access NEMA 4X Enclosures
For Hazardous-Area enclosures, use the terminal blocks located in-
side the smaller end of the flow meter enclosure for all wiring con-
nections. (The terminal designations are labeled inside the enclosure
cover.) Make sure to observe all CE compliance requirements for
AC wiring connections given on the next page.
Hazardous-Area enclosure
terminal blocks
Set screw
Wiring
connections
To access components:
1. Use 1/16 inch hex key to loosen set screw.
2. Turn cover counterclockwise to remove.
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
Figure 2-3. Wiring Access Hazardous-Area Enclosures
Warning!
To avoid potential electric
shock, follow National Elec-
tric Code safety practices or
your local code when wiring
this unit to a power source
and to peripheral devices.
Failure to do so could result
in injury or death. All AC
power connections must be
in accordance with pub-
lished CE directives.

Series 780S Instruction Manual Chapter 2 Installation
IM-78S-B 2-5
Input Power Wiring
AC Power Wiring
The AC power wire size must be 26 to 16 AWG with the wire
stripped 1/4 inch (6 mm). Connect 100 to 240 VAC (300 mA load,
maximum) to the Neutral and Line terminals on the small, two-
position terminal block. Connect the ground wire to the safety ground
lug. Torque all connections to 4.43 to 5.31 in-lbs (0.5 to 0.6 Nm).
For all installations not using solid metal conduit, the ferrite clamp
must be installed just above the input power wire entry into the en-
closure (Figure 2-5). All EEx installations must use an approved
EEx fitting at both cable entries into the enclosure. If conduit seals
are used, they must be installed with 18 inches of the enclosure.
The Hazardous-Area enclosure has two separate conduit entries to
maintain separation between AC input power and output signal wir-
ing. To eliminate the possibility of noise interference use a separate
cable entry for the AC power and signal lines.
Enclosure
safety
ground lug
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
AC Power (Line)AC Power (Neutral)
Hazardous-Area Enclosures
Figure 2-4. AC Input Power Connections
Figure 2-5. Ferrite Clamp Installation
Warning!
All wiring procedures must
be performed with the power
Off.
Caution!
The AC wire insulation tem-
perature rating must meet or
exceed 71°C (158°F).

Chapter 2 Installation Series 780S Instruction Manual
2-6 IM-78S-B
DC Power Wiring
The DC power wire size must be 26 to 16 AWG with the wire
stripped 1/4 inch (6 mm). Connect 18 to 30 VDC (625 mA load,
maximum) to the terminals marked PWR+ and PWR– on the terminal
block. Torque all connections to 4.43 to 5.31 in-lbs (0.5 to 0.6 Nm).
All EEx installations must use an approved EEx fitting at both cable
entries into the enclosure. If conduit seals are used, they must be in-
stalled within 18 inches of the enclosure.
1
2
15
16
DC Power (+)
DC Power (–)
NEMA 4X Enclosures
Figure 2-6. DC Input Power Connections
DC Power (+)
DC Power (–) 1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
Hazardous-Area Enclosures
Figure 2-7. DC Input Power Connections
Warning!
All wiring procedures
must be performed with
the power Off.

Series 780S Instruction Manual Chapter 2 Installation
IM-78S-B 2-7
Output Signal Wiring
Output signal cable should be completely screened with a 100%
shield. You must use metal cable glands that provide cable screen
clamping. The cable screen should be connected to the gland and
shielded at both ends over 360 degrees. The shield should be ter-
minated to an earth ground.
Flow meters are equipped with either a calibrated 0-5 VDC (0-10
VDC optional) or a calibrated 4-20 mA output signal. This linear
output signal represents 0-100% of the flow meter’s user full
scale.
DC Output Wiring
The 0-5 VDC (0-10 VDC optional) signal can drive a minimum load
of 1000 Ohms. Note: the optional 0-10 VDC output signal is not
available for power sources below 15 VDC.
For 0-5 VDC or 0-10 VDC connections, connect to the terminals
marked Vout (+) and Vout (–) as shown below.
1
2
15
16
0-5 V out (+)
0-5 V out (–)
NEMA 4X Enclosures
Figure 2-8. DC Output Signal Connections
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
0-5 V out (+)
0-5 V out (–)
Hazardous-Area Enclosures
Figure 2-9. DC Output Signal Connections

Chapter 2 Installation Series 780S Instruction Manual
2-8 IM-78S-B
4-20 mA Output Wiring
The 4-20 mA current loop output can be self-powered (non-isolated)
or externally powered (isolated). To use the 4-20 mA isolated output,
an external 12 to 36 VDC power supply is required. The maximum
loop resistance (load) for both types of current loop outputs are de-
pendent upon the supply voltage and are given in Figure 2-10.
Rload is the total resistance in the loop, including the wiring resis-
tance. To calculate Rmax, the maximum Rload for the loop, use the
maximum loop current, 20 mA. The voltage drop in the loop due to
resistance is 20 mA times Rload and this drop is subtracted from the
input voltage. Thus:
Rmax the maximum load resistance = 50 *(Vsupply – 7.5V)
To use an external power supply for an isolated 4-20 mA output, con-
nect as shown in Figure 2-11 or Figure 2-13. For an internally pow-
ered, non-isolated 4-20 mA output, connect as shown in Figure 2-12
or Figure 2-14.
Figure 2-10. Load Resistance Versus Input Voltage
VsupplyRmax
(Volts) (Ohms)
11 (min) 175
12 225
15 375
18 525
24 825
30 1,125
36 (max) 1,425