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I've been using a full wheatstone bridge (4 strain gauges) to read stress on a steel bar, through a circuit mounted on an arduino breadboard. This seemed alright. I later used two strain gauges of similar (but not same) properties and similar size for a half bridge. As soon as I completed the circuit, one of the strain gauges' foil darkened but the other one was fine. Could this happen if I use the wrong resistances in the other half of the bridge?

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The power dissipated in strain gauge elements should always be far less than the power level at which you can do them any harm. Even connecting the "excitation voltage" across a single bridge element should do it no harm, eve though this would not be a useful thing to do. The implications are that you have done something which is fundamentally incorrect. The voltage used must be substantially greater than the rated voltage for the damaged part.

One possible reason for one only gauge being damaged is that the output is grounded or connected to v+ or to some other voltage so that theundamaged gauge "sees" little or no voltage.
What is your half-bridge midpoint connected to?

Do you have a data sheet
What is (or was) the gauge resistance for each part?
What is the applied voltage?
What power dissipation do you expect and what is the rated dissipation?

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Alright, to clarify - I am in fact using an amplifier (INA 125P) and powering the amplifier itself with 5V, which is standard for the amplifier. The gauges' factor is 2. The gauges themselves are aluminium foil, 120 ohms resistance, and about 8 or 9 mm long. They are very standard, and quite cheap. I'm not sure what the voltage/ current works out to be across the gauges. – Adam May 6 '12 at 19:18
Another point to raise is that regardless of the damaged gauge, I still get a signal when I read it on the com. It's only a fraction of the possible signal to be read by the arduino (scaling around 50-100 bits from 1023 available). – Adam May 6 '12 at 19:19
Additionally, when a full bridge was used, the only difference is the presence of two resistors on the arduino breadboard. So I'm assuming that it wasn't the supply that was the problem. – Adam May 6 '12 at 19:55
@Adam - assuming that it was not the supply would be very dangerous. If the colour change was caused by excessive energy dissipation, as seems likely, then either you have an external source or the supply IS the source - which is more likely. Bearing in mind that when you change from 2 resistors in series across Vx to 1 one only resistor across Vx then the dissipation in the single resistor increases by a factor of 4. If dissipation was at the upper acceptable limit this could be fatal. – Russell McMahon May 7 '12 at 7:25

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