LVDT sensors for corrosive environments

LVDT (Linear Variable Differential Transformer) is an essential tool for accurate strain measurement at the specimens test section. LVDT sensor converts a linear displacement from a mechanical reference zero position into a proportional electrical signal containing phase for direction and amplitude for distance information. LVDT operation is based on electromagnetic coupling.

Cormet offers LVDT sensors for corrosive environments. We have two LVDT sensor types. One for low temperature tests made from plastics and another one for high temperature and pressure use made of Hastelloy C-276 (N10276). These LVDT sensors are popular among users doing testing in wet H2S environments. For less aggressive HT/HP water environments, for example nuclear material studies, we have also Stainless Steel AISI 316 LVDT sensor.

LVDT location is critical for For meaningful results. LVDT sensor needs to be mounted near the samples test section. Rigid mounting is essential especially in HP/HT applications. LVDT measurements of the pullrod movements should not be mixed with LVDT measurements at the sample. Cormet uses micrometer and stepper-motor information to record pullrod movement.

LVDT sensor can be mounted to round, flat and CT specimens. With round and flat specimen it’s used for strain measurement. With CT specimen crack opening displacement (COD) measurement. On compression applications multiple LVDT sensors might be justified, as can be seen on the picture below.

Once installed LVDT usage is very simple and the position values are stored to our data acquisition software. Metallic version of the LVDT sensor for elevated temperatures needs to be always calibrated at the use temperature. LVDT output varies depending on the temperature, pressure doesn’t have an effect for the reading. For temperature calibration we provide calibration holders for the LVDT sensor.

The picture on the right side shows our 25kN SSRT autoclave lid opened.  LVDT calibration specimen holders are installed in place. Pictured model is for both tension and compression. Therefore it has the relatively large sample holders that clamp the specimen eliminating slack.

Most of the time LVDT is used for stain measurement, but it can also be the input for the movement control as described in ASTM E606 strain controlled fatigue tests.