Acromag TT351-07 0 Series User manual

Model TT351-07x0, Bridge Input Transmitter
(Strain Gauges, Load Cell, or ±4mV to ±165mV DC)
USB Programmable, DIN Rail Mount, DC-Powered
Single Channel Transmitter w/
Sourcing Current or Voltage Output
USER’S MANUAL
ACROMAG INCORPORATED Tel: [phone]
30765 South Wixom Road
Wixom, MI 48393-2417 U.S.A. email: [email protected]
Copyright 2018, Acromag, Inc., Printed in the USA.
Data and specifications are subject to change without notice. 8501140D

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 2 -
http://www.acromag.com
- 2 -
https://www.acromag.com
Table of Contents
GETTING STARTED ............................................................................................4
DESCRIPTION.......................................................................................................4
Key Features........................................................................................................................4
Application..........................................................................................................................5
Mechanical Dimensions .......................................................................................................5
DIN Rail Mounting & Removal..............................................................................................6
ELECTRICAL CONNECTIONS............................................................................7
SENSOR INPUT CONNECTIONS .......................................................................7
Digital Input Connections (Tare)...........................................................................................9
Output Connections.............................................................................................................9
Power Connections............................................................................................................11
Optional Bus Power Connections .......................................................................................12
Earth Ground Connections .................................................................................................13
USB Connections................................................................................................................14
EMI Filter Installation.........................................................................................................15
INTRODUCTION TO STRAIN......................................................................... 16
Quick Overview of Strain Measurement.............................................................................16
The Wheatstone Bridge .....................................................................................................18
Strain Bridge Equations......................................................................................................21
Resolving Strain or Load.....................................................................................................26
How the TT351 Measures Strain and Load..........................................................................29
Example Calculation for a Strain Gauge..............................................................................30
Example Calculation for a Load Cell....................................................................................31
CONFIGURATION SOFTWARE...................................................................... 33
Quick Overview –Android .................................................................................................33
Quick Overview –Windows ...............................................................................................34
TECHNICAL REFERENCE................................................................................ 37
OPERATION STEP-BY-STEP.......................................................................... 37
Wired Connections ............................................................................................................37
1 Communication Setup.....................................................................................................39
2 I/O Configuration............................................................................................................40
3 Sensor Setup...................................................................................................................41
4 Scaling/Computation ......................................................................................................44
5 Input Calibration.............................................................................................................45
6 Output Calibration..........................................................................................................48
7 Diagnostics .....................................................................................................................49
8 Shunt Calibration ............................................................................................................50
9 Load Cell Calibration .......................................................................................................53
BLOCK DIAGRAM ............................................................................................. 54

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 3 -
http://www.acromag.com
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HOW IT WORKS................................................................................................ 54
TROUBLESHOOTING....................................................................................... 56
Diagnostics Table...............................................................................................................56
Service & Repair Assistance ...............................................................................................58
ACCESSORIES .................................................................................................... 59
Software Interface Package ...............................................................................................59
USB Isolator ......................................................................................................................59
USB A-B Cable ...................................................................................................................59
USB A-mini B Cable ...........................................................................................................59
USB OTG Cable .................................................................................................................60
DIN Bus Connector Kit........................................................................................................60
End Stops...........................................................................................................................60
SPECIFICATIONS .............................................................................................. 61
Model Number ..................................................................................................................61
Input .................................................................................................................................61
Output...............................................................................................................................66
Power................................................................................................................................67
USB Interface.....................................................................................................................68
Enclosure & Physical..........................................................................................................68
Environmental...................................................................................................................69
Agency Approvals ..............................................................................................................69
Reliability Prediction..........................................................................................................70
Configuration Controls.......................................................................................................70
REVISION HISTORY......................................................................................... 71
All trademarks are the property of their respective owners.
IMPORTANT SAFETY CONSIDERATIONS
You must consider the possible negative effects of power, wiring, component, sensor, or software failure in the design of
any type of control or monitoring system. This is very important where property loss or human life is involved. It is
important that you perform satisfactory overall system design and it is agreed between you and Acromag, that this is your
responsibility.
The information of this manual may change without notice. Acromag makes no warranty of any kind regarding this
material, including, but not limited to, the implied warranties of merchantability and fitness for its intended purpose.
Further, Acromag assumes no responsibility for any errors that may appear in this manual and makes no commitment
to update, or keep current, the information contained in this manual. No part of this manual may be copied or
reproduced in any form without the prior written consent of Acromag, Inc.
This manual covers the TT351-0700 Strain Gauge Transmitter which converts a 4 or 6 wire strain gauge bridge or load cell
sensor signal (or millivoltage signal) to an isolated voltage or current output signal. Acromag offers a complete line of
other TT modules targeted to input signals that include current, voltage, thermocouples, frequency, thermistors, and
resistance temperature detectors.

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 4 -
http://www.acromag.com
- 4 -
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GETTING STARTED
DESCRIPTION
Symbols on equipment:
Means “Refer to User’s Manual
(this manual) for additional
information”.
The TT351-0700 is an ANSI/ISA Type IV (4-Wire) transmitter with an input that
mates to a 4 or 6-wire strain gauge sensor or load cell wired as a Wheatstone
bridge. It measures the ratio-metric bridge output Vo and bridge excitation Vex to
resolve indicated strain or load of the sensor and linearly extrapolate an isolated DC
voltage or current output signal. The ADC ratio-metrically converts the bridge signal
relative to a reference derived from bridge excitation. Its input bipolar range varies
with the product of gauge rated output (RO), excitation (Vex), and user-specified
over-range limit. The unit is setup, calibrated, and scaled with USB configuration
software and a connection to a Windows-based PC’s (Windows 7 and later versions
only), or using a USB-OTG cable to an Android smartphone or tablet running our
Agility mobile app. This model provides an adjustable and scalable input and output
stage, null offset and automatic tare controls, plus drive capability for isolated
voltage or current output, full 3-way isolation, variable excitation, and variable input
filtering.
Key Features
•USB configured and calibrated using Windows software, or a wired USB-OTG
connection to Android smartphones or tablets using the Acromag Agility app.
•Thin 17.5mm wide enclosure for high-density mounting.
•Isolated Input/Exc, Output, & Power –increases safety & noise immunity.
•SG Bridge, Load Cell, or mV Input w/support of sensor rated outputs from 1-
10mV/V and 4-11V of excitation. Measures both sensor output Vo (ratio-
metrically) and sensor excitation Vex to resolve corresponding strain or load
(may optionally be used for ±4mV to ±165mV differential input signals).
•Digitally Adjustable Internal Sensor Excitation from 4-11V in ~111mV
increments up to 120mA, turned OFF for use with external excitation.
•Remote Sense of Bridge Excitation Vex –Input samples coincident excitation
via its ±SNS terminals allowing it to boost its internal excitation if needed to
overcome voltage drop due to load, temperature, or excessive lead resistance.
•Ratio-Metric Input Digitization of Ratio-Metric Sensor Vo simultaneous and
relative to an ADC reference derived from sensor excitation Vex for increased
accuracy and immunity to changes in excitation and requisite noise.
•Optional Auto Null-Compensation/Zero-Balance of Bridge Output Vo –Initial
bridge offsets can be automatically removed from sensor Vo measurements.
•Optional Automatic Tare Removal –Tare may be automatically removed from
sensor Vo measurements and can be set via software control or remotely via an
isolated digital input on the module.
•Optional Half or Quarter Bridge Completion Built-In –precision resistor pair
ratio-matched to ±0.01% can be jump wired to IN± to accomplish half or
quarter to full-bridge completion in either polarity (also useful to bias a millivolt
input signal to keep it from floating).
•Optional Custom Input Linearization –can linearize a non-linear input signal
(strain bridge, load cell, or millivolt signal) with up to 25 user-defined signal
breakpoints for 24 piece-wise linear segments.
•Separate voltage & current output terminals –choice of nominal ±10V, ±5V, 0-
10V, 0-5V, or 0-20mA/4-20mA isolated output or scaled sub-ranges of same.
!

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 5 -
http://www.acromag.com
- 5 -
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Key Features...
•Flexible I/O Scaling with independently adjustable & scalable input and output.
•Input Zoom Feature for Load Cells that allows the sensor over-range limit
specification to increase gain over a smaller portion of a load cell’s rated range,
potentially improving resolution & accuracy for some applications where a wide-
input load sensor is used over a smaller application range.
•Variable digital input filter (none, low, medium, or high) plus input averaging.
•Transparent Continuous ADC Background Self-Calibration helps compensate the
ADC for gain and offset error and drive a nearly flat output span with minimal drift
over time and temperature.
•Self-Diagnostics are built-in and operate upon power-up to provide reliable
service, easy maintenance, and trouble-shooting.
•Normal or Reverse Acting Transmitter Output Signal.
•Wide Signal Range supports sensor rated outputs from 1mV/V*4V to 10mV/V*11V
with up to 50% of over-range (i.e. ±4mV to ±165mV).
•High measurement accuracy & linearity with 24-bit ADC & 16-bit DAC conversion.
•LED Indication of power (green) and output fault (orange).
•Wide-range DC power input from 9-32V is bus/redundant power ready.
•Wide ambient temperature operation from -40°C to +70°C.
•Thoroughly tested and hardened for harsh environments.
•CE Approved.
•FCC Conformity Class A.
•Model TT351-0700 cULus Listed - Ordinary Location.
•Model TT351-0710 cULus Listed Class I/Division 2 - Haz. Loc., ATEX, & IECEx.
Application
For additional information on
these devices and related
topics, please visit our web site
at www.acromag.com.Also see
whitepaper 8500-699:
Introduction to Strain
Measurement.
This model isolates and transmits a grounded or ungrounded strain gauge or load cell
sensor wired to it as a Wheatstone bridge. It can optionally supply variable sensor
excitation and will measure both sensor output Vo and coincident excitation Vex,
which it uses to compute requisite strain or load and linearly drive an isolated sourcing
output that supports 0-20mA or 4-20mA current, or ±10V, ±5V, 0-10V, 0-5V voltage
ranges. It supports high-density mounting on T-type DIN rail, allowing units to be
mounted side-by-side on 0.7-inch (17.5mm) centers with support for 9-32V DC power
via terminals on the unit, or optionally wired to a DIN-rail bus connector.
Mechanical Dimensions
Units may be mounted to
35mm “T” type DIN rail (35mm,
type EN50022), providing
isolated I/O channels on 0.7-
inch centers.
WARNING: IEC Safety
Standards may require that this
device be mounted within an
approved metal enclosure or
sub-system, particularly for
applications with exposure to
voltages greater than or equal
to 75VDC or 50VAC

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 6 -
http://www.acromag.com
- 6 -
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DIN Rail Mounting & Removal
NOTE: It is recommended that this
unit be mounted upright on a DIN
rail allowing free air flow intake
from the bottom vent to flow
through the unit and out the top
vent. This will allow the unit to
run cooler, perform better, and
help to extend the life of the
electronics.
Refer to the figure below for attaching and removing a unit from the DIN rail. A
spring-loaded DIN clip is located on the input side bottom. The opposite rounded
edge at the bottom of the output side allows you to tilt the unit upward to lift it
from the rail while prying the spring clip back with a screwdriver. To attach the
module to T-type DIN rail, angle the top of the unit towards the rail and place the
top groove of the module over the upper lip of the DIN rail. Firmly push the unit
downward towards the rail until it snaps into place. To remove it from the DIN rail,
first separate the input terminal blocks from the bottom side of the module to
create a clearance to the DIN mounting area. You can use a screwdriver to pry the
pluggable terminals out of their sockets. Next, while holding the unit in place from
above, insert a screwdriver into the lower path of the bottom of the module to the
DIN rail clip and use it as a lever to force the DIN rail spring clip down while pulling
the bottom of the module outward until it disengages from the rail, and lift it from
the rail.

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 7 -
http://www.acromag.com
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ELECTRICAL CONNECTIONS
WARNING –EXPLOSION HAZARD –Do not disconnect equipment unless power has
been removed or the area is known to be non-hazardous.
WARNING –EXPLOSION HAZARD –Substitution of any components may impair
suitability for Class I, Division 2.
WARNING –EXPLOSION HAZARD –The area must be known to be non-hazardous
before servicing/replacing the unit and before installing.
Wire terminals accommodate 14–28 AWG (2.08–0.081mm2) solid or stranded wire
with a minimum temperature rating of 90oC. Input wiring may be shielded or
unshielded type. Ideally, output wires should be twisted pair or shielded twisted
pair. Use insulated wire to keep circuits isolated. Terminals are pluggable and can be
removed from their sockets by prying outward from the top with a flat-head
screwdriver blade. The TT351 models allows bridge sensors to be wired to TB1, TB2
and TB3. Strip back wire insulation 0.25-inch on each lead and insert the wire ends
into the cage clamp connector of the terminal block. Use a screwdriver to tighten the
screw by turning it in a clockwise direction to secure the wire (0.5-0.6Nm torque).
Use adequate wire insulation and follow proper wiring practices, as common mode
voltages can exist on signal wiring. As a rule, output wires are normally separated
from input wiring for safety and low noise pickup.
Sensor Input Connections
Input is wired as a Wheatstone
Bridge directly to terminals TB1,
TB2, and TB3 at the bottom of
the module (spring-loaded DIN
clip side). Follow proper polarity
when making input connections.
•Sensor Excitation is isolated from unit power, the remote tare trigger, and the
transmitter output. Excitation may be supplied internally or externally.
•You must disable internal excitation before connecting sensor to external
excitation (use the config software to set Excitation to Ext).
•Complete input connections prior to power-up to avoid propagating input offset.
!

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 8 -
http://www.acromag.com
- 8 -
https://www.acromag.com
Sensor Input Connections...

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 9 -
http://www.acromag.com
- 9 -
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Sensor Input Connections...
Digital Input Connections (Tare)
Optionally used for remotely
triggering a Tare measurement.
This unit can be setup to automatically remove or cancel “tare”weight in its
measurement of sensor output Vo. Tare refers to a defined common weight
between measurements that can be discounted from subsequent measurements
(like the weight of a container or shipping materials). Do not confuse tare with offset
null or zero balance, which is another control to correct Vo, but meant for smaller
non-zero imbalances in the sensor application typically present when no load or
strain is applied. Tare is established by triggering its measurement under software
control, or remotely when the TARE digital input is asserted (asserted via a high 15-
30V DC input, 200ms min pulse). Any application that requires frequent tare
adjustment during operation should make provisions for wiring to the digital input
TARE and COM as part of the installation. Note that a new Tare offset will take effect
immediately but is only stored to non-volatile memory after 10 seconds of TARE
trigger inactivity. If power is lost during a tare measurement, the tare offset may be
lost and will have to be repeated. While this write delay may seem inconvenient, it is
done to help preserve the life of non-volatile memory in the module if changing tare
is triggered too frequently.
Output Connections
(To DC Current or
Voltage Terminals)
Note: Your analog output is
based on your Indicated
measurement scaled using your
Instrument Gauge Factor and
Software Gain in place of the
sensor Gauge Factor and a
software gain of 1.
The transmitter output is modeled after an ANSI/ISA Type 4 transmitter (4-Wire) with
unit power separate from the input and output.
•Sourcing/Active Output connections are polarized. The sourcing transmitter
current and voltage outputs at the TB4 terminals share a common return (RTN).
Current is sourced from I Out+ and returned to RTN. Voltage is sourced positive
at V Out+ with respect to RTN. Only one output terminal (V or I) may be loaded
at one time.
•Variations in output load resistance has negligible effect on output accuracy
when load limits are respected with respect to output type (see below).

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 10 -
http://www.acromag.com
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Output Connections...
Observe proper polarity. Shielded twisted-pair wiring is recommended for best
results to connect the longest distance between the field output and its remote load
as shown above. An output connection to earth ground at the return terminal will
help protect the isolated output circuit from damage in noisy environments.
WARNING: For compliance to applicable safety and performance standards, the use
of twisted pair output wiring is recommended. Failure to adhere to sound wiring and
grounding practices as instructed may compromise safety, performance, and possibly
damage the unit.
TIP - Ripple & Noise: Place additional capacitance at the load to help reduce the
60Hz/120Hz ripple sometimes present in industrial applications. For large 60Hz
ripple, connect an external 1uF or larger capacitor directly across the load to reduce
excess ripple. For sensitive applications with high-speed acquisition at the load, high
frequency noise may be reduced significantly by placing a 0.1uF capacitor directly
across the load, and as close to the load as possible.

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 11 -
http://www.acromag.com
- 11 -
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Power Connections
IMPORTANT –External Fuse: If
this unit is powered from a
supply capable of delivering more
than 3A to the unit, it is
recommended that potential
fault current be limited via a high
surge tolerant fuse rated for a
maximum current less than 3A
(for example, see Bel Fuse MJS or
RJS fuse types).
The unit is powered from 9-32V DC (36V DC peak) by connecting power as shown
below. This transmitter can be optionally powered (or redundantly powered) via the
DIN rail bus when coupled to an optional DIN rail bus connector (Acromag Model
1005-063) with a bus terminal block (Acromag 1005-220 or 1005-221). This optional
power connection method can allow several modules to share a single power supply
connection without wiring power to each unit’s power terminals.
•Power connections are isolated from the input and output. The supply voltage
should be from 9-32V DC. This voltage must never exceed 36V DC peak, or
damage to the unit may result.
•Variations in power supply voltage between the minimum required and 32V
maximum, has negligible effect on transmitter accuracy.
•Note the placement of earth ground at power. The power cable shield and DC-
should ideally be grounded closest to the module.
CAUTION: Risk of Electric Shock –More than one disconnect switch may be required
to de-energize this equipment before servicing.

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 12 -
http://www.acromag.com
- 12 -
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Optional Bus Power Connections
Power is normally wired to the TB5 terminals of the unit as shown on the previous
page. However, this device is equipped to be optionally or redundantly powered via
a DIN rail bus connector (Acromag 1005-063) mated to an optional plug-in terminal
block (Acromag 1005-220 or 1005-221, depending on left side or right-side wire
entry). Any power input via the bus connector is diode-coupled to the same point in
the circuit as unit power connected at its power terminal TB5. You could power
multiple units by snapping them together along the DIN rail bus using connector
1005-063, then connecting a mating terminal block (select a left side or right-side
connector, see figure below). While the intent of the bus power connector is to
allow several units to conveniently share a single supply, you could also use the bus
power connector to redundantly power units (with local power also applied at TB5),
allowing a backup supply to maintain power to the units should the main supply at
TB5 fail.
Acromag TTBUS-KIT connector kit contains bus connector 1005-063, plus left-side
terminal 1005-220, and right-side terminal 1005-221, allowing units to snap
together, side-by-side, along the DIN rail and share the power connection.
Important –End Stops: If this module uses the optionally powered (or redundantly
powered) via the DIN rail bus for hazardous location installations (Class I, Division 2
or ATEX/IECEx Zone 2) it must use two end stops (Acromag 1027-222) to secure the
terminal block and module (not shown).
!

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 13 -
http://www.acromag.com
- 13 -
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Optional Bus Power Connections…
The figure below shows how to wire power to the optional bus terminal block when
mated to the bus connector. Note power is wired to the rightmost bus terminals on
the right, or the left-most terminals on the left. Observe proper polarity.
Earth Ground Connections
(To Protect Your Equipment, Lower System Noise, and Reduce Emissions)
This Transmitter’s input, output, and power circuits are electrically isolated from one another, allowing each of these
circuits to be individually earth-grounded as indicated in their connection drawings (its housing is plastic and does not
require an earth ground connection). Earth Grounding isolated circuits as shown is recommended for best results and to
help protect the unit and its isolated circuits by giving each a low impedance path to ground for shunting destructive
transient energy away from sensitive module circuitry. If the transmitter is mounted in a metal housing, a ground wire
connection is usually required for the metal housing and you should connect that enclosure’s ground terminal (green
screw) to earth ground using suitable wire per applicable codes. See the Electrical Connections Drawings for Input,
Output, and Power, and note the position of earth ground for each isolated entity.
•Avoid inadvertent/extra connections to earth ground at other points in any isolated circuit than those indicated, as
this could drive ground loops and negatively affect operation.
•A USB isolator is recommended when configuring or calibrating this unit to avoid the ground loop that occurs if your
input is also earth grounded (A PC commonly earth grounds its USB port contacting both the USB signal and shield
ground which are held in common to the input circuit ground of this transmitter).

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 14 -
http://www.acromag.com
- 14 -
https://www.acromag.com
USB Connections
This transmitter is configured and calibrated via configuration software that runs on
a Windows-based PC connected to the unit via USB (Windows 7 or later required),
or via a USB-OTG connection to an Android smartphone or tablet using the Acromag
Agility mobile app. Refer to the drawing below to connect your PC or laptop to the
transmitter to reconfigure or calibrate it using this software.
WARNING: The intent of mating
USB with this transmitter is so that
it can be conveniently set up and
calibrated in a safe area, then
installed in the field which may be
in a hazardous area. Do not
attempt to connect a PC or laptop
to this unit while installed in a
hazardous area, as USB energy
levels could ignite explosive gases
or particles in the air.
USB Signal Isolation is recommended as shown above and required when connected
to a grounded input. The bridge input and USB connections are isolated from the
output and power of this model. USB Isolation is recommended for safety and
noise suppression and is required when the input bridge sensor or its excitation
signal happens to already be earth grounded. You may use Acromag model USB-
ISOLATOR to isolate your USB port, or you can optionally use another USB signal
isolator that supports USB Full Speed operation (12Mbps).
IMPORTANT: USB logic signals to the transmitter are referenced to the potential of
the transmitter’s input circuit ground. Input ground is held in common with USB
ground and USB cable shield ground. Thus, an isolator is required when the input
signal is earth grounded and the unit is connected to the USB port of an earth-
grounded PC. You could avoid the use of an isolator if a battery powered laptop
was instead used to connect to the transmitter, and the laptop had no other earth
ground connection, either directly or indirectly via a connected peripheral.
!
Note: Output/Power to Transmitter must
be applied before USB connection
(See Output/Power Connections).

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 15 -
http://www.acromag.com
- 15 -
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EMI Filter Installation
For low CE-rated radiated emissions, the use of one or two split/snap-on ferrite cores on
I/O cables or harnesses to/from the device as shown in the drawing below is helpful.
You can increase filtering by looping the cable one time through the ferrite as shown.
Ferrites are also helpful for cables connected to the Host USB port. Use Laird 28A3851-
0A or similar for inputs/outputs and power, and Laird 28A2025-0A or similar for USB
cable. Locate ferrites by clamping them outside of I/O cables or wiring harnesses
to/from the module (input, USB, output, DC power, remote tare, and remote shunt
calibration connection groups), as close to the module as possible. While the use of
these ferrites is helpful to obtain low CE-rated emissions, it may not be required for your
application. Note that individual cables may share a ferrite, but it is not good practice to
combine isolated circuits inside the same ferrite, but rather separate isolated circuits for
safety and greater noise immunity.

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 16 -
http://www.acromag.com
- 16 -
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INTRODUCTION TO STRAIN
Quick Overview of Strain Measurement
An “active” gauge is oriented in a
direction to measure the effect of
an applied force on a material
(usually in the same direction as
the force or lateral to it).
micro-strain (µ  ) =  L/L * 10-6.
Strain gauges change their resistance in proportion to applied force that may result
from loading, torque, pressure, acceleration, and vibration. Because their change in
resistance to force is very small, they often connect in a Wheatstone Bridge of four
elements. You can wire a bridge with one active strain gauge and 3 resistors
(quarter bridge), two active strain gauges and two resistors (half-bridge), or 4 active
strain gauges (full-bridge). The number of active gauges in a bridge and how they
are oriented relative to applied force will determine the bridge type, its application,
and relative strain computation.
The load cell is a simpler form of a Wheatstone Bridge based sensor, one whose
output Vo is relative to percent of rated load and will correspond to the pressure or
weight applied. Load cells are much simpler than strain gauge bridges because their
relationship to load does not require additional detail of how its internal bridge
elements are arranged, its Gauge Factor, or the Poisson’s ratio of the material it is
applied to. The only important considerations for resolving the load cell are its
rated output (mV/V), excitation voltage, and rated capacity.
The output voltage of a Wheatstone Bridge is directly proportional to bridge
excitation and very sensitive to the small mechanical deformations that drive
resistance imbalances in one or more legs of a bridge. Because the mechanical
deformation resulting from an applied force is a very small percentage, the
corresponding strain it represents is often expressed as a multiple of 10-6 or micro-
strain (micro-strain=L/L * 10-6). Stress applied to an active bridge element will
drive an unbalanced bridge condition that produces an offset in the bridge output
voltage that can be related to the magnitude of the applied force. The electrical
sensitivity of a bridge network or load cell is how much the bridge output voltage
changes relative to 1V of bridge excitation and expressed in millivolts of Vo per volt
of excitation Vex (mV/V). This is usually referred to as the gauge or cell Rated
Output and denoted by Vr=Vo/Vex. The full-scale output of a strain gauge bridge or
load cell sensor at its full rated load (100%) is the product of the gauge Rated
Output (mV/V) and applied excitation voltage Vex.
= L / L
In most real-world applications,
strain measurements are rarely
encountered larger than a few
milli-strain (  =0.003 or about
3000  ).
Strain () is a measure of the mechanical deformation of a material and computed
as a fractional change in dimension (length, width, or height) resulting from a force
along that dimension (= L / L). Strain may be positive (tensile +), or negative
(compressive -) and the magnitude of deformation it represents is a very small
percentage such that it is often expressed as an integer multiple of 10-6, or micro-
strain ().
There are normally two sensor measurements used to resolve strain or load: the
sensor bridge output signal Vo and the sensor bridge excitation Vex. Using sensor
specifications and these two measurements, the applied strain or load can be
resolved from the bridge output signal.

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Strain Measurement...
GF = (R/R)/(L/L) = (R/R)/
R = R*GF*
Poisson’s Ratio () = -T / .
Poisson’s Strain (T) = -.
-1.0 ≤ ≤ +0.5
The effect of Poisson’s strain is
often ignored in strain calculations
< 5000µ  where it is small.
Example Calculation: If you
measured 3000  with a 120Ω
metallic strain gauge (GF=2.00), its
corresponding fractional change in
resistance  R/R = 2.00*3000*10-6
= 0.006, implying that this 120Ω
gauge changed its resistance by
only 0.6% or 0.72Ω.
Like the sensitivity of a bridge reflects how much the magnitude its full-scale output
changes per volt of excitation, the sensitivity of a strain gauge or Gauge Factor GF is
how much its resistance changes in proportion to applied strain (its ratio of
fractional change in resistance to applied strain ). For common metallic strain
gauges, the Gauge Factor is typically around 2.0, meaning its resistance change is
about 2x its dimension change to an applied force (R/R = 2x L/L). However, GF
varies slightly for most applications and this affects relative strain. For many
instruments, sensor Gauge Factor will be used to compute ideal strain, but
Instrument Gauge Factor will be used to compute indicated strain.
A material subject to a tensile or compressive uniaxial force in one direction will
coincide with a lateral force referred to as Poisson’s Strain. Most materials that
undergo a tensile force or positive strain (when stretched or elongated) will
contract slightly due to coincident negative strain in the lateral/transverse
dimension (when its Poisson’s ratio = -T / is a positive number). Less common,
there are some materials with a negative Poisson’s ratio that expand in the
transverse direction (+T) in response to being stretched (+) in the longitudinal
direction. In general, the proportion of contraction or expansion is indicated by the
application material’s Poisson’s Ratio. The Poisson’s Ratio () is the negative ratio of
the simultaneous transverse strain that occurs in the perpendicular direction to the
main strain parallel to the applied force.
Strain gauge sensors typically consist of a very fine foil or wire grid that is bonded to
an application material surface in the direction of an applied force (uniaxial) or
lateral to it (bending force). These are referred to as a bonded metallic or
resistance strain gauges. They are designed to change their resistance slightly in
proportion to stress. Most strain gauges have nominal resistance values that vary
from 30 to 3000, with 120, 350, and 1000being the most common. Their
cross-sectional area is minimized by design to reduce the negative effect of the
corresponding shear or Poisson’s Strain coincident to applied strain.
The bonded metallic strain gauge has a foil grid attached to a thin backing material
or carrier strip which is directly attached to the application material to help
facilitate an efficient transfer of strain on the body to the foil grid of the gauge and
allow it to respond with a linear or nearly-linear change in electrical resistance.
Ideally, the strain gauge resistance should only change in response to applied strain,
but unfortunately in practice, this is somewhat of an inexact science and it is
difficult to make both the gauge material and application material expand and
contract equally over temperature. As you can surmise, properly mounting the
gauge is critical to ensure the application material strain is accurately transferred
through the bonding adhesive and backing material to the gauge foil.
To curb potential problems caused by mismatched expansion and contraction rates
between the gauge and application material, gauge manufacturers try to minimize
sensitivity to temperature by selecting specific gauge materials for specific
application materials. While this helps to minimize strain error, temperature
remains a source of potential error and additional compensation is usually required.
Adverse effects like this are the reason strain instruments provide additional
parametric controls for rescaling their measurement as required, like utilizing
Instrument Gauge Factor and Software Gain to help overcome application-induced
skew in strain measurement.

Model TT351-07x0
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R = R*GF*implies that at
3000,  R/R ~ 2.00*3000*10-6 =
0.006 or 0.6%
We stated that because strain measurement requires the detection of very small
resistance changes resulting from very small mechanical deformations, we wire
strain gauges as elements of a Wheatstone bridge which converts the signal to a
bipolar output voltage that can be conveniently measured in proportion to the
applied stress and its direction. Note that the magnitude of most strain
measurement in stress applications is commonly between 2000 and 10000,
and rarely larger than about 3000 or ~0.6%. The Wheatstone Bridge offers an
accurate method for measuring these very small changes in resistance and even
provides an ability to compensate for the inherent sensitivity of the strain gauge
relative to temperature.
The Wheatstone Bridge is comprised of four resistive
arms or bridge elements arranged in the
configuration of a diamond as shown at left. An
excitation voltage Vex is applied vertically across the
diamond or bridge input and an output voltage Vo
can be measured horizontally across the diamond as
shown in Figure 1.
From Kirchhoff’s Voltage Law & Ohm’s Law, the bridge output voltage Vo is the
difference between two divided voltages with Vo = [R3/(R3+R4) –R2/(R2+R1)] *
Vex. If the adjacent resistors of each divider match their ratios such that R1/R2 =
R4/R3, then Vo=0 and the bridge is balanced. Note it is not required that R1=R4 and
R2=R3 to achieve balance, just that the ratios of adjacent resistors R1:R2 and R4:R3
be equal (this also allows you to use half bridge completion resistors of a different
value than the nominal gauge resistance). But for simplicity, if all four of the
resistances in each leg of the bridge are equal, the two divider pairs will have equal
ratios and the bridge will be balanced with a bridge output voltage Vo=0. Any
change in resistance in any leg of the bridge will unbalance the bridge and produce
Vo≠0. Remember that the same Vo offset can be obtained from adjacent pairs of
different resistor values, if the ratios of the adjacent pairs are kept equal (R1/R2 =
R4/R3).
As stated, if R1/R2 = R4/R3, then Vo=0 and the bridge is balanced. Examine the two
Vex voltage dividers Vo- =R2/(R1+R2)*Vex and Vo+ =R3/(R4+R3)*Vex. Note a
decrease in R4 or R2 increases Vo by increasing the +node voltage and decreasing
the -node voltage. Likewise, a decrease in R1 or R3 will decrease Vo by increasing
the -node voltage and reducing the +node voltage. The strain due to a decrease in
resistance R4 or R2 will be negative and increase Vo (convention is that negative
strain is compressive and positive strain is tensile). Thus, bridge Vo will grow
positive for a compressive stress on R4 driving negative strain and decreasing its
resistance. This is the convention used throughout this manual.
The Wheatstone Bridge

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Wheatstone Bridge...
If you replace R4 in the bridge with an active
strain gauge (Rg), then any change in the
strain gauge resistance (R) will unbalance
the bridge and produce an offset in Vo
proportional to the change in resistance R.
Using Gauge Factor, the change in resistance
due to the applied strain is R = Rg * GF * .
From our divider equation Vo=Vex*[R3/(R4+R3) –R2/(R1+R2)], substituting R1=R2,
R3=Rg, and R4=Rg+R yields: Vo/Vex = - GF * / 4 * [1 / (1 + GF*/ 2)]. This is
the strain computation term Vr=Vo/Vex in mV/V and represents the sensitivity of
the bridge network in mV/V (not to be confused with strain gauge sensitivity or
Gauge Factor). The presence of the extra term 1/(1+GF*/2) is representative of a
small non-linearity in the output of the quarter bridge network with respect to
strain. But for quarter-bridge strain levels below ~5000 micro-strain, the effect of
this non-linearity is small and can be ignored in most applications.
Strain sensors formed from a bridge network may employ multiple strain gauges in
their construction with 1, 2, or 4 active gauges in a bridge possible. Note that one
active strain gauge (Rg) may occupy one leg of a four element Wheatstone Bridge
(Quarter-Bridge), two legs of a bridge (Half-Bridge), or four legs of a bridge (Full-
Bridge). Any remaining legs of a quarter or half bridge network are occupied by
fixed resistors or "dummy" gauges, but it is the number of active gauges in the
bridge that determines whether the bridge is a quarter (1), half (2), or full bridge (4)
type.
Refer back to Figure 2 and note that tensile (positive) strain drives negative Vo for
elements 1 and 3, and elements 4 and 2. Compressive (negative) strain drives Vo
positive for 1 and 3, and elements 2 and 4. A change of resistance in the same
direction for adjacent bridge resistors is subtractive and tends to cancel each other
out, but a change of resistances in opposite directions for adjacent cells is additive
and reinforces their effect on the output Vo. Similarly, resistance changes between
diagonally opposite resistors are additive in same direction and reinforce their
effect on output Vo, but subtractive in opposite directions and tend to cancel each
other out.
Because strain gauges are inherently sensitive to temperature, we can additionally
arrange them in a bridge to compensate its Vo for temperature by making sure the
adjacent element pair ratios R1:R2 and R3:R4 remain equal over temperature.
Because opposite changes in resistance of adjacent bridge resistors R1 & R2 and R3
& R4 are numerically additive and reinforce each other, but numerically subtractive
if in the same direction, then placing similar gauges and lead-wires in adjacent arms
and exposing them to the same temperature will allow them to act together to null
the net thermal effect on the bridge output Vo and effectively cancel temperature
induced strain error.
That is, for an active strain gauge in one arm, use an identical strain gauge in the
adjacent arm, subject to the same temperature but not the stress force. Then its
temperature resistance change will track that of the primary gauge, and the ratio
between them will stay the same, having no unbalancing effect on the bridge
output Vo signal.

Model TT351-07x0
Strain Gauge Input Transmitter w/USB
Acromag, Inc. Tel: [phone] - 20 -
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Wheatstone Bridge...
Returning to the prior example where we substituted R3=Rg and R4=Rg+R in the
bridge, their net effect with temperature on Vo was avoided by carefully mounting
both gauges such that they are subject to the same temperature, but the “dummy”
gauge R3=Rg mounted transverse to the axial strain (perpendicular to applied
strain) such that strain on R4=Rg+R has little or no effect on R3=Rg. Because the
ambient temperature affects both gauges the same but not the force, they keep the
same ratios and any temperature resistance changes cancel each other and Vo is
not affected by temperature.
Refer to Figure 4 and note opposite adjacent resistance changes reinforce each
other. If you make the adjacent second gauge active by mounting it in the same
axis as the applied strain, but of the opposite sign (e.g. one active gauge in tension,
one active gauge in compression), you form a half-bridge configuration that doubles
the sensitivity of the bridge to strain. That is, the output voltage Vo of the half-
bridge is linear and approximately double the output of the quarter-bridge Vo for
the same excitation Vex.
Consider one half of the balance beam bridge application in Figure 5. Solving for
the sensitivity Vr in a half bridge application of two adjacent active gauges would
yield: Vr = Vo/Vex = - GF*/2. In Figure 5, note that adjacent arrows are opposite
to depict the two elements are mounted such that one is in compression, and the
other in tension, for the same applied strain. Now this bridge sensitivity can be
further increased by making all four arms of the bridge active strain gauges, with
diagonally opposite legs combined such that two diagonal legs are in compression,
and two diagonal legs in tension. This forms a full-bridge circuit that has double the
sensitivity of the half-bridge circuit, and four times the sensitivity of the quarter
bridge circuit. Solving for the sensitivity of a full-bridge balance beam application a
shown, we get: Vr = Vo/Vex = - GF*, effectively twice that of the half-bridge
circuit.
All the equations thus far have been simplified because they assume an initially
balanced bridge with Vo=0 with no strain applied. This is rarely achieved in practice
where resistance tolerance and application strain errors usually result in Vo≠0 with
no strain applied. The equations also failed to account for lead wire resistance Rl in
the measurement leads and connections to excitation. The equations that follow
will be embellished to account for unloaded offset, sensor lead resistance, and
material Poisson’s ratio where applicable using permutations of the three basic
bridge configurations: quarter, half, and full, where applicable.