Every mechanically activated pressure instrument relies on an elastic element to bend in response to a pressurized medium. Most often, the element is a metallic “C” shaped or coiled Bourdon tube that attempts to straighten under pressure. Specifiers need to be especially careful that the chemical composition of the excitation medium does not react unfavorably with the instrument’s wetted materials. Measuring the pressure of compressed air or water seldom results in an incompatibility. However, when oil and gas first come out of the ground, the story can be quite different.