Subject: Strain Guage Proposal
From: Kevan Hashemi
Date: Tue, 13 Mar 2007 12:29:54 -0400
To: Friedrich Lackner
CC: Werner Riegler
Dear Friedrich,
Suppose you want to measure strain in one direction on one of your tubes. I think you need only one 120-Ohm strain guage per tube, plus a 120-Ohm RTD temperature sensor.
You know that your measurement system will not heat up the sensors, so the tube, the strain guage, and the RTD will all be at the same temperature. So, you warm up the tube somehow, under no strain, and measure the strain guage resistance versus temperature as it cools down. You cool it down and do the same as it warms up. Now you have a graph of strain guage resistance versus temperature at zero strain.
You can read out both the RTD and the strain guage with the same circuit, the A2053S. The trick is to make a 120-Ohm RTD. Most are 100-Ohm, and the ones we have are 1000-Ohm. So we take a 100-Ohme RTD, which I believe you have already in your lab, and we put a 20-Ohm resistor in series with it. I'm buying some 1% axial-lead resistors. You can solder these in series with one of the RTD leads. The 20-Ohm resistor is stable to 0.002 Ohm/C, while the RTD varies by 0.4 Ohm/C, so the variation in the RTD will dominate.
Now you have both resistive sensors being read out by the same LWDAQ device. You run their cables to the same place. In software, you compensate for the strain guage temperature dependence.
Yours, Kevan
--
Kevan Hashemi, Electrical Engineer
Physics Department, Brandeis University
http://alignment.hep.brandeis.edu/