Strain Gauge and Bridge Measurements with DT50/500/600 Series ...

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Calculation of bridge output and converting to µStrain
on dataTaker DT50/DT500/DT600 range of data loggers
This tech note details the formulas that the dataTaker DT50/DT500/DT600 range of data
logger uses to calculate the output of a bridge measurement. This tech note also explains
how you can convert the output of the bridge from parts per million (ppm) to µStrain.
IMPORTANT: This information ONLY applies to firmware version 7.10 or later (Released
23 rd Oct 2003). This information supersedes the information shown in the User’s Manual
for the DT50, DT500, DT600 Series dataTakers Series 3 – UM-0076-A0.
Bridge output calculation
The following formula is used for all bridge measurements to calculate the output of the
bridge in parts per million. This applies to BGV and BGI channel types.
Tech Note
6
Vout
×10
Bout
=
Vex
Where
B out = The output of the bridge in parts per million (ppm)
V out = The bridge output voltage in volts
V ex = The bridge excitation voltage in volts
For all BGV measurements, Vex is always measured via a reference channel.
For three wire, half and quarter bridge inputs, using the BGI channel type (Wiring Config
13 & 14), Vex is calculated as follows:
Vex = I×
2R
For four wire, full bridge inputs, using the BGI channel type (Wiring Config 15), Vex is
calculated as follows:
Vex = I×
R
Where
I = the constant current selected (either 2.5mA, the default, or 250uA).
R = the resistance of one arm of the bridge in ohms.
Converting to µStrain
The output of the bridge channel types is in units of “ppm”. You can convert this to
µStrain by using the following general formula:
µStrain = k ×Bout
When using a BGI channel type in a three wire configuration (quarter or half bridge input),
k is calculated as:
2
k =
G × N
TN-0022-A1 Page 1 of 2 8 June 2004

When using a BGI channel type in a four wire configuration (full bridge input) or any BGV
channel type configuration, k is calculated as:
4
k =
G × N
where
G = Gauge Factor
N = Number of active gauges
The gauge factor is a measure of the sensitivity of the gauge. Typical foil gauges have a
gauge factor of approximately 2, which means that if they are stretched by 1% their
resistance will change by 2%.
Tech Note
The data logger can do the conversion to µStrain within the logger by applying a
polynomial to the channel definition.
Worked example
For example for a four wire full bridge measurement using constant current excitation
where the Gauge factor is 2.1, the number of active gauges is 2, the k value is calculated
as follows:
k
=
4
= 0.9523809
2.1×
2
The following program fragment demonstrates how to use the k value:
Y1=0,0.9523809"uStrain" 'Poynominal definition
1BGI(4W,Y1)
'Bridge channel –units of uStrain
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Victoria 3178 Australia Fax: +61 3 9764 8997 Web: www.datataker.com
dataTaker is a registered trademark of Datataker Pty Ltd.
TN-0022-A1 Page 2 of 2 8 June 2004