Measuring Bridges and Strain Gauges
The dataTaker provides comprehensive support for Wheatstone bridge circuits in full, half and quarter bridge configurations.
Wheatstone bridge circuits are extensively used for measuring the output of strain gauges, and for measuring the output of other sensors where a relatively small change in resistance must be detected.
Bridge circuits have the advantage of high measurement sensitivity, and also provide a significant degree of temperature compensation.
The dataTaker supports two methods of Wheatstone bridge measurement
ï excitation of sensors is by a constant current
ï excitation of sensors is by a voltage
Both of these methods of bridge support can have a number of options, depending the number of active arms in the bridge and the number of wires used to connect bridges to the dataTaker.
Constant Current Excitation of Bridges
The constant current excitation method of bridge measurement has a constant current of 2.500 mA or 250.0 µA flowing in the bridge circuit for excitation.
The bridge sensitivity and zero is independent of the length of leads used to connect the bridge to the dataTaker.
In some cases the bridge output can have greater linearity and reduced temperature sensitivity for constant current excitation, than for voltage excitation.
The Bridge Excitation Current
The bridge excitation current is supplied by the Excite terminal of the analog input channel during measurement. The bridge excitation current is 2.500 mA by default, but may be set for 250.0 µA if required.
If the 250.0 µA excitation current is required, then this is specified as a channel option in the channel specification.
Using DeTransfer, 250.0 µA excitation current is specified as follows
where the channel option I specifies that a 250.0 µA excitation current is output from the Excite terminal during bridge measurement.
The default excitation current of 2.500 mA is equivalent to
where the channel option II specifies that a 2.500 mA excitation current is output from the Excite terminal during bridge measurement.
Using DeLogger, the excitation current can be selected in the Channel Properties dialog box of the Program Builder. When the bridge channel has been created, right click on the Data Use icon and select PropertiesÖ Click on the Excitation tab, and select the excitation current required.
Both the 250.0 µA and 2.500 mA excitation currents can be used within the same application, for different bridges connected to different channels.
The bridge excitation current is supplied from the Excite terminal for a period of 30 mS during measurement of the bridge circuit.
The Arm Resistance
The data that is returned by the dataTaker from bridge circuits excited by constant current is the ratio of the change in arm resistance to the nominated arm resistance. The data is returned in units of ppm.
The arm resistance for the bridge being measured must be known by the dataTaker, and is specified as a channel option in the channel specification. The default arm resistance is 350 Ohm, which is typical for many types of strain gauges.
Using DeTransfer, the arm resistance is specified as follows
In this example the bridge circuit connected to analog channel 5 has an arm resistance of 120.5 Ohm. The arm resistance is specified in Ohms.
Using DeLogger, the arm resistance for constant current excited bridges can be defined in the Resistance Wiring Configurations dialog of the Program Builder, which opens when you select the Bridge input channel type.
Full Bridge with Constant Current Excitation
The dataTaker can provide excitation and measure the output from a full bridge of devices such as strain gauges, pressure cells, load cells, etc. This configuration is a 4 wire input, and supports 1, 2 or 4 active arms. Any of the bridge arms can be active arms. The configuration also provides compensation for cable wire resistance, allowing long cable wires to be used.
Bridge arms which are not active must have bridge completion resistances. These can be an inactive device of the same type as the devices on the active arms, or these can be a resistor with the same resistance value as the active devices at rest, and ideally have a temperature coefficient that is similar to that of the active devices.
The entire bridge circuit is external to the dataTaker ñ the logger does not provide any bridge completion for partial bridges.
The full bridge configuration with constant current excitation is connected to the dataTaker as a 4 wire input as follows
Figure 100 ñFull Bridge with Constant Current Excitation
where the Excite terminal provides current excitation of 250.0 µA and 2.500 mA, which returns via the Analog Return terminal. The bridge output is read between the +ve and ñ ve terminals.
Full bridges with constant current excitation are sampled, and the data is returned when a Schedule containing the channel is executed.
Using DeTransfer, full bridges with constant current excitation are measured by the command for example
BEGIN
RA5M
1BGI(4W,120) 2BGI(4W,120)
END
which instructs the dataTaker to measure the output from full bridges connected to analog input channels 1 and 2. The full bridge circuits have an arm resistance of 120.0 Ohm.
The BGI specifies that the signals applied to these channels are from constant current excited bridge circuits. The 4W channel option indicates that the bridge is connected in a 4 wire configuration, where the excite terminal provides excitation. This option must be specified for all four wire bridge connections.
The excitation current channel option is not specified, and so the default 2.500 mA excitation current is used.
Using DeLogger, full bridges with constant current excitation can be measured by the following Program Builder program. The 4 wire connection is selected from the Bridge Wiring Configurations dialog which opens when you have selected the analog input channel.
The dataTaker will read the inputs every 5 minutes, and readings are stopped by entering a H (Halt) command.
Interpreting the Data from a Full Bridge with Current Excitation
The data returned from full bridges is the ratio of change in measured resistance to the arm resistance, expressed in parts per million as follows
where
∆R is the sum of changes in arm resistances taking into account sign of the changes
Rarm is the nominated arm resistance, default is 350 Ohm
Calculating Microstrain for Full Strain Gauge Bridges
When using stain gauges in full bridges, it may be desirable to convert the returned data from units of ppm to units of Microstrain. This can be done by the following formula
This full bridge method of strain gauge measurement has a measurement resolution of approximately 0.2 Microstrain.
Using DeTransfer, the output from a full bridge with constant current excitation can be calculated to units of microstrain by the program for example
BEGIN
RA5M
5BGI(4W,120,=1CV,W)
2CV(ìMicrostrain =ì)=(4/(4*2.0))*1CV
END
which instructs the dataTaker to firstly read the bridge output in ppm, and save this in Channel Variable 1 (1CV), then calculate microstrain from the reading using the formula above.
Here it is assumed that the gauge factor is 2.0 for the strain gauges used (check your strain gauge supplier or manufacturer for details of the gauge factor).
The use of calculations in the dataTaker are discussed in detail in Section III ñ Channel Variables and Calculations.
Using DeLogger, the calculation can be entered in the Program Builder as follows
Refer to your DeLogger Manual for details of using calculations in the Program Builder.
Half Bridge with Constant Current Excitation
The dataTaker can also provide excitation and measure the output from a half bridge configuration of strain gauges, pressure cells, etc. This configuration is a 3 wire input, and supports 2 active arms.
This configuration compensates for cable wire resistance and temperature difference.
The half bridge configuration with constant current excitation is connected to the dataTaker as a 3 wire input as follows
Figure 101 ñ Half Bridge with Constant Current Excitation and Two Active Arms
When using stain gauges in half bridges and current, it may be desirable to convert the returned data from units of ppm to units of Microstrain. This can be done by the following formula
This bridge configuration can be used over a wide variation of resistance.
The half bridge configuration can be used to measure the position of the wiper of a potentiometer (<5 KOhm) - the ends of the potentiometer are connected between Excite/+ve terminals and Analog Return, and the wiper is connected to the ñ ve terminal. The arm resistance is set to the total resistance of the potentiometer.
Using DeTransfer, half bridges with constant current excitation can be measured by the command for example
BEGIN
RA10M
1BGI(250) 3BGI(250)
END
which instructs the dataTaker to measure the half bridges connected to analog input channels 1 and 3. These half bridge circuits all have an arm resistance of 250.0 Ohm.
The BGI specifies that the signals applied to these channels are from constant current excited bridge circuits. The default configuration for this type of bridge input is a 3 wire connection, and so no connection needs to be specified.
Using DeLogger, half bridges with constant current excitation can be measured by the following Program Builder program. The 3 wire connection is selected from the Bridge Wiring Configurations dialog which opens when you select the analog input channel.
The dataTaker will read the inputs every 10 minutes, and readings are stopped by entering a H (Halt) command.
Interpreting the Data from a Half Bridge with Current Excitation
The data returned from half bridges is the ratio of change in measured resistance to the arm resistance, expressed in parts per million as follows
where
∆R is the sum of changes in arm resistances taking into account sign of the changes
Rarm is the nominated arm resistance, default is 350 Ohm
Calculating Microstrain for Half Strain Gauge Bridges
When using stain gauges in half bridges, it may be desirable to convert the data from units of ppm to units of Microstrain. This can be done using the standard formula
or
This half bridge method of strain gauge measurement has a measurement resolution of approximately 0.2 Microstrain.
Using DeTransfer, the output from a half bridge with constant current excitation can be calculated to units of microstrain by the program for example
BEGIN
RA5M
7BGI(=10CV,W)
15CV(ìMicrostrain =ì)=(4/(2*2.0))*10CV
END
which instructs the dataTaker to read the bridge output in ppm, save this in Channel Variable 10 (10CV), then calculate microstrain from the reading using the formula.
Here it is assumed that the gauge factor is 2.0 for the strain gauges used (check your strain gauge supplier or manufacturer for details of the gauge factor).
The use of calculations in the dataTaker are discussed in detail in Section III ñ Channel Variables and Calculations.
Using DeLogger, the calculation can be entered in the Program Builder as follows
Refer to your DeLogger Manual for details of using calculations in the Program Builder.
Quarter Bridge with Constant Current Excitation
The quarter bridge configuration for measuring bridges is a variation of the half bridge configuration, where there is one active device such as a strain gauge, and a bridge completion resistance to balance the bridge.
The bridge completion resistance can be an inactive device of the same type as the active device, or can be a resistor with the same resistance value as the active device, and ideally has a temperature coefficient similar to that of the active device.
The entire bridge circuit is external to the dataTaker ñ the logger does not provide any bridge completion for partial bridges.
Quarter bridges with constant current excitation are connected to the dataTaker as follows
Figure 102 ñ Quarter Bridge with Constant Current Excitation
This 3 wire configuration provides compensation for cable wire resistance, allowing long cable runs to be used.
The basic quarter bridge configuration can be used for multiple quarter bridges, with a shared bridge completion resistor. The shared bridge completion resistor should be adjacent to the dataTaker to ensure accurate lead wire compensation.
Figure 103 ñ Multiple Quarter Bridge with Shared Bridge Completion
The quarter bridge configuration with constant current excitation is sampled, and the data is returned when a Schedule containing the channel is executed.
Using DeTransfer, quarter bridges with constant current excitation can be measured by the command for example
BEGIN
RA30S
1..3BGI(120)
END
which instructs the dataTaker to measure the output from quarter bridges connected to the analog input channels 1, 2 and 3. These quarter bridge active arms all have a resistance of 120.0 Ohm.
The BGI specifies that the signals applied to these channels are from constant current excited bridge circuits. The default configuration for this type of bridge input is a 3 wire connection, and so no connection needs to be specified.
The excitation current channel option is not specified, and so the default 2.500 mA excitation current is used.
Using DeLogger, quarter bridges with constant current excitation can be measured by the following Program Builder program.
The 3 wire connection is selected from the Bridge Wiring Configurations dialog which opens when you select the analog input channel.
The dataTaker will read the inputs every 30 seconds, and readings are stopped by entering a H (Halt) command.
Interpreting the Data from a Quarter Bridge with Current Excitation
Quarter bridge data is returned in units of ppm, and is ratio of the change in measured resistance to the arm resistance as follows
or
where
Ract is the active arm resistance
Rc is the bridge completion resistance
DR is the change in bridge resistance
Rarm is the nominated arm resistance, defaults to 350 Ohm
The bridge completion resistor Rcmust have a resistance equal to that of the active device at rest, for the bridge to be properly balanced.
Calculating Microstrain for Quarter Strain Gauge Bridges
When using stain gauges in quarter bridges, it may be desirable to convert data from units of ppm to units of Microstrain. This can be done by the standard formula
or
This quarter bridge method of strain gauge measurement has a resolution of approximately 0.2 Microstrain.
Using DeTransfer, the output from a quarter bridge with constant current excitation can be calculated to units of microstrain by the program for example
BEGIN
RA5M
2BGI(=5CV,W)
8CV(ìMicrostrain =ì)=(4/2.0))*5CV
END
which instructs the dataTaker to firstly read the bridge output in ppm, and save this in Channel Variable 5 (5CV), then calculate microstrain from the reading using the formula above.
Here it is assumed that the gauge factor is 2.0 for the strain gauges used (check your strain gauge supplier or manufacturer for details of the gauge factor).
The use of calculations in the dataTaker are discussed in detail in Section III ñ Channel Variables and Calculations.
Using DeLogger, the calculation can be entered in the Program Builder as follows
Refer to your DeLogger Manual for details of using calculations in the Program Builder.
Voltage Excitation of Bridges
The alternative method for measuring bridge circuits with the dataTaker is the voltage excitation with ratiometric measurement. The principal of the method is that the bridge is excited by a constant voltage source, and the bridge output voltage is measured as a ratio of the measured excitation voltage.
In practice, resistance of the cable wires connecting the bridge to the logger reduces the excitation voltage that is actually applied to the bridge, which in turn results in a proportionate loss of output signal voltage from the bridge.
To correct for this error the actual voltage applied across the bridge is measured using a second channel.
The Bridge Excitation Voltage Source
The bridge excitation voltage, also often referred to as the bridge power supply, can be supplied from a number of sources
the Excite terminal of the analog channel, which can output a nominal 5 Volts (actually nearer 4.5 Volts)
the Excite terminal of the analog channel, which can output a 2.500 mA or
250.0 µA precision current
the switched 5 Volt sensor power supply terminal of the dataTaker, which is limited to 100 mA total current draw
an external voltage source
The bridge excitation voltage must be switched on during the period of measurement
if excitation from the dataTaker is used, then excitation can be switched on by the logger at appropriate times
if an external voltage is used for excitation, the bridges can be either permanently powered or can be powered only during measurement by using a digital output channel to control a relay which switches power to the bridges
The default bridge excitation voltage is the 5 Volt supply from the Excite terminal, and is automatically selected when bridge inputs with voltage excitation are specified.
However if the bridges are powered from external sources, then the Excite terminal voltage should be disabled.
Using DeTransfer, this is done by the command for example
where BGV specifies that a bridge voltage is to be measured, and the N channel option specifies no voltage excitation from the Excite terminal.
Alternatively if the bridge is to be excited by either of the Excite terminal current sources then channel option I for 250.0 µA current, or channel option II for 2.500 mA current, should be used.
Using DeTransfer, this is done by the command for example
where BGV specifies that a bridge voltage is to be measured, and the I and II channel options specify current excitation from the Excite terminal.
DeLogger does not directly support the measurement of bridges that are excited by a voltage. However bridge measurements, including controlling the method of powering, can be programmed into thedataTaker via the User channel type (DeLogger Ver 4.2.15 or later) in the Program Builder. This procedure is illustrated in the following topics.
Measuring the Bridge Excitation Voltage
In practice the resistance of the cable wires connecting bridges to the dataTaker reduce the excitation voltage that is actually applied to the bridge.
This results in a proportionate loss of output signal from the bridge. To correct for this error, the actual excitation voltage across the bridges is also measured.
The bridge excitation voltage is connected as a differential or single ended voltage input to any analog input channel, and must be measured immediately before the output of any bridge is measured.
This measurement is referred to as the ‘bridge reference voltageí, and is measured on the bridge reference channel that is identified to the dataTaker by the BR channel option for the particular channel.
Using DeTransfer, the command for example
identifies that the bridge reference voltage is to be measured as a differential voltage connected to analog input channel 1.
dataTaker 50,500,600 series loggers : The bridge reference channel has a maximum input voltage of 2.5 Volts for dataTaker 50,500,600 series loggers. Therefore if the bridge excitation voltage is greater than 2.5 Volts, then this must be externally attenuated (see Section II ñ Measuring High Level Voltages) before input to the bridge reference channel on these loggers.
Whenever the bridge excitation voltage must be attenuated, the attenuation factor is also declared as a channel option to the bridge reference voltage channel as follows
which declares an attenuation of 2.0:1, that is appropriate for bridges powered by an external 5 Volt supply.
dataTaker 505,605 series loggers : If a dataTaker 505,605 series logger is being used, then the bridge excitation voltage can be measured directly as a High Level Voltage (see Section II ñ Measuring High Level Voltages) on the bridge reference channel, for example
The bridge reference channel does not return any data when it is scanned. The data is retained for subsequent use in bridge measurements and calculations.
However if you want to include the bridge power supply or excitation measurements in your data, then the bridge reference voltage can be returned by a second command for example
where the channel 1V will return the bridge reference voltage.
Note : The bridge reference channel must precede the bridge measurement channel(s) in the dataTaker program, because the bridge reference voltage is used to calculate the bridge data for the subsequent bridge measurement channels.
Note : If bridge measurements are included in more than one Schedule, then the bridge reference channel(s) must be declared in each Schedule.
If a bridge reference channel is not declared, then the bridge reference voltage defaults to 5 Volts. This is based on the assumption that most voltage excited bridges will be powered from the dataTaker 5 Volt sensor power supply.
DeLogger does not directly support the measurement of bridges that are excited by a voltage. However the bridge reference channel, and bridge measurements channels can be programmed into thedataTaker via the User channel type (DeLogger Ver 4.2.15 or later) in the Program Builder. This procedure is illustrated in the following topics.
Full Bridge with Voltage Excitation
The full bridge with voltage excitation configuration is the more traditional method for the measurement of bridge outputs. However a full implementation of requires more resources than any of the constant current methods, requiring
two channels for the each bridge, if each bridge has a separate bridge excitation
two channels for the first bridge, and one channel for each additional bridge that is excited by the same bridge power supply. This configuration is only appropriate if all cable wires are the same length, such that all bridges receive the same voltage excitation as measured for the first bridge
This configuration supports 1, 2 or 4 active arms. Any of the bridge arms can be active arms.
Bridge arms which do not have active devices must have bridge completion resistances to balance the bridge. These can be inactive devices of the same type as the active devices, or can be a resistor with the same resistance value as the active devices at rest, and ideally have a temperature coefficient that is similar to that of the active devices.
Where the bridge power supply and bridge output are measured for each bridge, this is referred to as a six wire connection as illustrated below
Figure 104 ñ Full Bridge with Voltage Excitation
The entire bridge circuit is external to the dataTaker ñ the logger does not provide any bridge completion for partial bridges.
Full bridges with voltage excitation are sampled, and the data is returned when a Schedule containing the channel is executed.
Using DeTransfer, full bridges with voltage excitation are measured by the commands for example
BEGIN
RA5M
1V(BR) ëbridge reference channel
2BGV(4W,N) ëbridge measurement channel
END
which instructs the dataTaker to measure the bridge excitation voltage connected to analog input channel 1 (bridge reference channel), and the bridge output connected to the analog input channel 2.
The BR indicates which analog channel the bridge excitation voltage is connected to for measurement. Note that the bridge reference channel is measured before the bridge output channel is measured.
The BGV specifies that the signal applied to this channel is from a bridge that is excited by a voltage. The bridge output data is returned in units of ppm.
The 4W channel option indicates that the 4 wire measurement method is to be used. This option must be specified for all full bridge inputs.
The bridge excitation voltage is supplied from an external source in this example, and so the Excite terminal is disabled by the N channel option.
DeLogger does not directly support full bridges with voltage excitation. However, full bridges with voltage excitation can still be measured with DeLogger (Ver 4.2.15 or later) by using the User channel as illustrated by the following Program Builder program.
The bridge reference channel could also be entered as a low level voltage channel, and the Channel Properties set to Bridge excitation voltage channel in the Reference tab as follows
The dataTaker will read the inputs every 5 minutes, and readings are stopped by entering a H (Halt) command.
Various compromises are possible with full bridges with voltage excitation as follows
the number of cable wires can be reduced from six to four by measuring the bridge excitation voltage at the dataTaker, rather than at the bridge. However this does not take into account reduction of the excitation voltage at the bridge due to cable resistance.
the bridge reference channel can be shared by a number of bridge measurement channels. No errors will be introduced provided all of the bridges have the same excitation voltage. This can be done by close proximity of the bridges to a shared supply, or the use of cable wires of the same type and length.
Interpreting the Data from a Full Bridge with Voltage Excitation
Data returned from full bridges with voltage excitation is calculated as the ratio of the change in bridge output voltage to bridge excitation voltage, expressed in parts per million as follows
where
∆V is the change in bridge output voltage
Vexcite is the bridge excitation voltage, measured by the bridge reference channel
Calculating Microstrain for Full Strain Gauge Bridges
When using stain gauges in full bridges, it may be desirable to convert the returned data from units of ppm to units of Microstrain.
This can be done by the following formula
This full bridge method of strain gauge measurement has a resolution of approximately 0.2 Microstrain.
Using DeTransfer, output from a full bridge with voltage excitation can be calculated to units of microstrain by the program for example
BEGIN
RA5M
1V(BR)
2BGV(4W,N,=1CV,W)
2CV(ìMicrostrain =ì)=(4/(4*2.0))*1CV
END
which instructs the dataTaker to read the bridge excitation and bridge output voltages, calculate the ratio in ppm and save in Channel Variable 1 (1CV), and calculate the microstrain from the reading in ppm using the formula above.
Here it is assumed that the gauge factor is 2.0 for the strain gauges used (check your strain gauge supplier or manufacturer for details of the gauge factor).
The use of calculations in the dataTaker are discussed in detail in Section II ñ Channel Variables and Calculations.
Using DeLogger (Ver 4.2.15 or later), the calculation can be entered into the Program Builder as follows
The bridge reference channel could alternatively be entered as a low level voltage channel, and the Channel Properties set to Bridge excitation voltage channel in the Reference tab as illustrated on the previous page.
Refer to your DeLogger Manual for details of using calculations in the Program Builder.
Half Bridge with Voltage Excitation
Half bridges with two active arms and voltage excitation are commonly used when a large number of bridges need to be located in close proximity.
The dataTaker supports this configuration by using single ended inputs and the single ended reference SE Ref.
Half bridges with two active arms require two bridge completion resistances to balance the bridge. The two bridge completion resistances can be either inactive devices of the same type as the active device, or can be a resistor with the same resistance value as the active devices, and ideally have a temperature coefficient similar to that of the active devices.
This half bridge configuration with voltage excitation can be used to measure a single half bridge, or to measure a number of half bridges which share the same bridge excitation voltage supply, and share the same set of bridge completion resistors.
Multiple half bridges that are excited from a single excitation voltage source and share bridge completion resistors are illustrated below in Figure 105. The configuration for a single half bridge is that for the innermost half bridge.
Figure 105 ñ Half Bridges with Voltage Excitation
dataTaker 50,500,600 series loggers : If this bridge configuration is connected to a dataTaker 50,500,600 series logger, then the bridge completion resistors must provide a 2:1 attenuation of the 5 Volt bridge excitation voltage, to a reduce the signal suitable for input to the bridge reference channel.
dataTaker 505,605 series loggers : If this bridge configuration is connected to a dataTaker 505,605 series logger, then the bridge excitation voltage can be measured as a High Level Voltage (see Section II ñ Measuring High Level Voltages) on the bridge reference channel.
The half bridges should preferably all be in close proximity to the bridge completion resistors. However if this is not possible, then the bridge completion resistors can be located at the dataTaker, and each half bridge connected by three leads. This will provide lead compensation for zero, but no scale compensation.
Half bridges with voltage excitation are sampled, and the data is returned when a Schedule containing the channel is executed.
Using DeTransfer, half bridges with voltage excitation are measured by the commands for example
BEGIN
RA10M
1V(BR,5.0)
2*BGV(N,X)
2+BGV(N,X)
2-BGV(N,X)
END
which instructs the logger to measure the bridge reference voltage that is differentially connected to analog channel 1, and measure the output from half bridges connected as single ended inputs to analog channels 2* through 2ñ .
Assuming that the excitation voltage is supplied from an external 10 Volts source, the default Excite terminal voltage output is disabled by the N channel option.
The BR channel option indicates the analog channel to which the bridge excitation voltage is connected for measurement, and is attenuated by a factor of 5.0:1 to reduce the 10 Volt excitation voltage into range for a dataTaker 50,500,600 series logger. The reference channel must be read before the measurement channel.
The BGV specifies that the signals applied to these channels come from voltage excited half bridges. The X channel option indicates that the single ended inputs are to be measured with reference to SE REF terminal.
DeLogger does not directly support half bridges with voltage excitation. However, half bridges with voltage excitation can still be measured with DeLogger (Ver 4.2.15 or later) using the User channel type as illustrated by the following Program Builder program.
The dataTaker will read the inputs every 10 minutes, and readings are stopped by entering a H (Halt) command.
Interpreting the Data from a Half Bridge with Voltage Excitation
Data returned from half bridges with voltage excitation is calculated as the ratio of the change in bridge output voltage to bridge excitation voltage, expressed in parts per million as follows
where
∆V is the change in bridge output voltage
Vexcite is the bridge excitation voltage, measured by the bridge reference channel
Calculating Microstrain for Half Strain Gauge Bridges
When using stain gauges in half bridges, it may be desirable to convert the data from units of ppm to units of Microstrain. This can be done by the standard formula
or
This full bridge method of strain gauge measurement has a measurement resolution of approximately 0.2 Microstrain.
Using DeTransfer, output from a half bridge with voltage excitation can be calculated to units of microstrain by the program for example
BEGIN
RA10M
1V(BR,5.0)
2*BGV(N,X,=1CV,W)
2CV(ìMicrostrain 1 =ì)=(4/(2*2.0))*1CV
2+BGV(N,X,=3CV,W)
4CV(ìMicrostrain 2 =ì)=(4/(2*2.0))*3CV
2-BGV(N,X,=5CV,W)
6CV(ìMicrostrain 3 =ì)=(4/(2*2.0))*5CV
END
which instructs the dataTaker to
read the bridge excitation and bridge output voltages
calculate the ratio in ppm and save in Channel Variables
calculate the microstrain from the readings in ppm using the formula above
Here it is assumed that the gauge factor is 2.0 for the strain gauges used (check your strain gauge supplier or manufacturer for details of the gauge factor).
The use of calculations in the dataTaker are discussed in detail in Section II ñ Channel Variables and Calculations.
Using DeLogger (Ver 4.2.15 or later), the calculation can be entered into the Program Builder as follows
etc, etc
Refer to your DeLogger Manual for details of using calculations in the Program Builder.
Converting Bridge Outputs to Engineering Units
This chapter has provided methods to convert measured bridge output in ppm to units of Microstrain for the various bridge configurations. However units of Microstrain apply to strain gauge bridges which are measuring deformation.
Many sensors available today employ a bridge circuit to sense the parameter they are designed to measure. For example some pressure cells, load cells, micro-displacement transducers, etc. in fact contain a diaphragm or similar structure which has a full strain gauge bridge bonded to one surface. The diaphragm is mechanically distorted by the pressure or load, which is measured by the strain gauge bridge. This distortion is calibrated to units of pressure, or load, etc. by the manufacturer.
Supporting these types of sensors with the dataTaker is quite simple, as shown by the following examples.
Pressure Transducer
A pressure transducer that is constructed as a full bridge device with a 4 wire connection, is connected to the dataTaker as a full bridge with constant current excitation (type BGI) as illustrated in Figure 100.
The transducer has an output of 0.05 V full scale at 10 VDC excitation. The dataTaker will measure
and
Therefore 1ppm = 100 kPa/5000 ppm = 0.02 kPa. This transducer calibration can be used in a dataTaker program to return the data in units of kPa.
Using DeTransfer, the program will be similar to
BEGIN
Y1=0,0.02îkPaî
RA1S
1BGI(Y1)
END
Using DeLogger, the calibration for the pressure transducer must be entered as a polynomial into the Polynomials dialog under the Settings tab of the Program Builder as follows
The polynomial is then attached to the bridge input channel in the program to convert the raw data to units of lbs as follows
For further discussion of polynomials, see Section III ñ Polynomials and Spans of this manual, and the DeLogger Users Manual.
Load Cell
A load cell that is constructed as a full bridge device with a 4 wire connection, is connected to the dataTaker as a full bridge with constant current excitation (type BGI) as illustrated in Figure 100.
The load cell measures a load of 100 lbs full scale, and has an output of 2.0006 mv/V at full scale. The dataTaker will measure
and
Therefore 1ppm = 100 lbs / 2000.6 ppm = 0.049985 lbs. This transducer calibration can be used in a dataTaker program to return the data in units of lbs.
Using DeTransfer, the program will be similar to
BEGIN
Y1=0,0.049985îlbsî
RA1S
1BGI(Y1)
END
Using DeLogger, the calibration for the load cell must be entered as a polynomial into the Polynomials dialog under the Settings tab of the Program Builder as follows
The polynomial is then attached to the bridge input channel in the program to convert the raw data to units of lbs as follows
For further discussion of polynomials, see Section III ñ Polynomials and Spans of this manual, and the DeLogger Users Manual.
Measurement Ranges and Accuracy
The dataTaker measures all bridge inputs as a low level voltage, with a resolution of 1 µV, and a nominal accuracy of 0.1%.
The accuracy for particular applications can be calculated from this information, and the excitation current or voltage used.
Error Messages
There are no specific error messages for bridge inputs. However input voltage signals which fall outside the voltage range of the dataTaker will produce an over-range reading of ñ99999.9 ppm or +99999.9 ppm.
The dataTaker also reports the error condition with the error message ëE11ñinput(s) out of rangeí if the Messages Switch /M is enabled.
