Strain Measurement at Cryogenic Temperature

A question came up recently about how to best approach gaging the inside of a tank that will be filled with liquid nitrogen for a test; following were our recommendations.

The requirements of the application were:

Environment:  Liquid nitrogen, though there is also expected exposure to humidity and water condensation.

Temperature during measurements:  cryogenic, -320 °F (-196 °C)

Tank material:  steel

Mode of testing:  static measurements

Term:  short-term testing, long-term between tests

Since the strain gage pattern and resistance had already been determined, there remained four parts to consider for the installation:  the gage series, adhesive selection, signal lead wire installation, and environmental protection. 

We suggested a WK-Series strain gage, as it is rated for -452 degrees F (-269 degrees C) at its low end.  It is also fully encapsulated and has high endurance lead wires.  M-Bond AE-10 or M-Bond AE-15 can be used to bond the gage and bondable terminal to the internal tank shell.  Alternative adhesives that are available are M-Bond 600 or 610. 

361A-20R solder was recommended for attaching the signal wires.  However, 361A-20R contains lead so 450-20R solder would be the choice if a lead-free alternative is required.  Note that 450-20R does require a higher melt temperature and can be more brittle at low temperatures than the361A-20R.  Cable of silver-plated copper with Teflon insulation (330-FTE) was recommended Signal wires back to the instrumentation.  The cable will pass from inside the tank through a sealed gland, and then out to instrumentation. 

The final consideration was a proper protective coating.  If the strain gage installation were only to be exposed to liquid nitrogen, then no protective coating would be required.  However, the tank undergoing testing is to be cycled continually from full, and then vented to atmosphere, so there is likelihood of moisture accumulating on the strain gage and exposed solder connections which will affect the readings.  As a protective coating, our recommendation was to apply M-Bond AE-10 epoxy over the exposed area and then reinforce with a layer of FGC-1 woven fiberglass cloth saturated with M-Bond AE-10, over the base coat of epoxy.  A body putty paddle is used to squeegee out the excess adhesive, effectively embedding the cloth into the epoxy.  The fiberglass cloth serves to stabilize the epoxy during thermal cycling, preventing separation of the coating and cracking at the cold temperatures. 

As with this example, when you have a question about best practices or how to solve a problem using strain measurement, please contact Micro-Measurements at mm@vpgsensors.com.  An Applications Engineer, Technical Sales Manager, or independent Sales Representative will be ready to assist you.