VW Strain Gauge Q&A

Question: What checks can I perform on VW strain gauges.?

Answer: You can perform standard wire checks as follows:

1. Read the device with the appropriate VW indicator and check its frequency of oscillation against expectations. Note that some gauges will not perform unless they are welded to a surface.

2. Read the temperature device embedded in the unit which should typically yield a reading within a couple of degrees centigrade. Most VW sensors now have a thermistor.

3. Check for short circuits between cable leads and the metallic body on the gauge, and between circuits per the table below. This can be done with a digital multi-meter with ohms range of 20 megaohm. Do not use a high-voltage megger device as this can damage the gauge by breaking down insulation.

Question: How can I convert Hz readings of the spot-weldable strain gauge to microstrain?

Answer: This is covered in the manual for our spot-weldable strain gauge. You’ll need acrobat reader to view or print this manual. You can also use the formula below:

microstrain value = 0.0007576 x SQ(Reading in Hz) + (-2030.1)

Note that these numbers work only with the VW spot-weldable strain gauge. They are not suitable for the arc-weldable or embedment strain gauges.

Question: For some time, our technician was recording readings from the VW spot-weldable strain gauge using the VWP Indicator’s “microstrainVS” setting (rather than the correct “microstrainVW” setting. How can I convert these VS readings to the correct microstrainVW readings?

Answer: The data can be converted without too much difficulty. The main problem is that the VS readings used a
2nd order equation, consequently you will need to solve a quadratic equation using the following formula:

Hz = SQRT(711.09 * VS + 6783) – 82.4     (where VS is the microstrainVS reading)

Then, to convert the result to the correct microstrainVW reading, use the formula below:

uStrain = (Hz^2 / 1320) – 2030.1

For your convenience, we’ve made an Excell worksheet that provides these equations. The file is called VS2VWmicrostrain. Note that is equation is just for use with the VWP Indicator. You’ll need an UnZip utility to access the file.

Question: I am not using a Slope Indicator readout. How can a obtain a reading in degrees C from the RTD used in your sensors?

Answer: Our published instructions assume that users are reading these 2K ohm RTDs using Slope Indicator readouts or the Campbell Scientific CR10X with AVW1 or AVW100 vibrating wire interface. These instructions will not work with other devices.

If you are using another device, obtain a reading in K ohms. Then apply either of the following polynomial coefficients to obtain degrees C:

Solution A: For a range of -10 to +30 C.

Ax2 + Bx + C, where

A= -23.50833439
B = 227.625007
C = -341.217356415

Solution B: For a range of -50 to 120 C.

Ax5 + Bx4 + Cx3 + Dx2 + Ex + F

A = 0.95659
B = -12.65962
C = 70.30009
D = -216.32308
E = 483.85218
F = -472.87637