Science Buddies: "Ask an Expert"

Hi

The scenario for one of my physics projects is that aircraft designers need to know the forces affecting different parts of an aircraft during flight. One way of obtaining this information is to attach a strain gauge to the aircraft. One of the properties of a strain gauge is that its electrical resistance changes when its length changes.

I am told that, for the strain gauge I will be using, the resistance with no force applied is 350 Ohms. If the length of the gauge increases by 1%, the resistance of the gauge increases by about 10 Ohms. I am trying to determine the best method for calibrating the gauge in terms of the change of resistance produced by different applied forces, and how the measurements would be made and used.

I have tried connecting a variable resistor (in place of the strain gauge) in series with an ammeter and in parallel with a voltmeter, but the voltage did not remain constant when I varied the resistance. I am not sure what the reason for this is, since in parallel, voltage is to remain the same throughout. I have also considered a whetstone bridge circuit design after seeing a strain gauge connected in a bridge circuit diagram online, but I am not sure how that works.

I'd appreciate any advice I can get for this topic.

Thank you

Darien

Darien - There may be a problem with your setup or observation. From your description I sketched the simple circuit diagram. I added a series resistance just for current limiting. If you change the value of the variable resistor, the measured voltage across it will certainly change. Using the current measurement and voltage measurement across the variable resistor (strain gauge) you can calculate the resistance of the gauge. Could you simply use an inexpensive multimeter in a resistance range to make the measurement? No other components would be needed. The multimeter would give you 1 ohm resolution which might be enough for your purposes. If you need to get a very precise calibration (tenths of ohms change in a 350 ohm strain gauge) you might be best to set up the Wheatstone bridge. You can find plenty of references for that on the web.

Rick Marz

Small strain gauges tend to be very sensitive to current in terms of thermal heating. You really should use a Whetstone Bridge circuit with something like two matched 10K resistors in the top legs and a precision matching resistor, in your case 350 ohms in the oposite bottom leg. The easiest way to precisely match the 350 ohms would be to use something like a 470 ohm resister in parallel with a 10 turn 1K potentiometer.

You use a microamp meter to measure current between the two sides of the bridge.

With the strain guage in what you consider the neutral stress condition, you adjust the 10 turn pot to null out the cross current. You can also use 10 turn 10K potentiometer in series with the bridge and power supply to adjust the strain guage to meter reading factor so you can simply multiply the current reading by nice number in your head instead of having to multiply or divide a three digit number.

Thank you for your advice!