Measurement Computing DataShuttle - Manual (Page 104)

13-4 Analog Input & Output 11-12-01 DataShuttle and DynaRes
Strain Gages
Strain gages are made of a wire or resistor that changes resistance when stretched or compressed. They are
normally bonded to the material in which the strain is being measured. They can be connected as a
resistance type sensor, but they are usually used in bridge configurations. Load cells consist of a strain
gage bridge arranged so it is temperature compensated and mounted on a load bearing support. Any
analog input board with differential inputs can measure strain gage signals.
Connections: Figure 13-2a shows a bridge connection in which opposing pairs of strain gages are mounted
so that one pair is in tension while the other is in compression. This gives the maximum sensitivity of any
bridge configurations as well as providing temperature compensation
Figure 13-2b is a bridge configuration that uses fewer strain gages but requires R4 and R3 to be precision
stable resistors. The bridge configuration of Figure 13-2c uses only one strain gage and has three precision
stable resistors. The resistors may be mounted in the auxiliary component locations in the terminal panel.
Often unmounted strain gages are used in place of resistors because they match the active strain gages in
resistance and in temperature coefficient.
All strain gage circuits require a stable supply voltage, since the signal is proportional to the supply. The
stable 6.6-6.9 voltage supply available on our data acquisition boards can provide the necessary power for
strain gage circuits that draw current of 20mA or less. Due to the 20mA maximum current, the circuit must
provide at least 345 Ohms of resistance. To determine the amount of resistance in your circuit, use the
following Ohms formulas:
For a pair of resistors wired in series:
Total Ohms = R1 + R2
For a pair of resistors wired in parallel:
Total Ohms = [R1 x R2]
÷ [R1 + R2]
For circuits that have less than 345 Ohms of resistance, another power supply, preferably 5V or less,
providing ample current, must be used.
Keep the common mode voltage to the analog board within the common mode range of the board for
proper operation (±7.5 volts).
Current from other sensors may create a voltage drop in the wires between the analog board and the
terminal panel. Be careful that this does not reduce your accuracy.