Which device is primarily used for pressure calibration?

The piston gage, commonly known as a deadweight tester, is primarily used for pressure calibration due to its design and operational principles. It provides a very accurate and stable reference for pressure measurement. The device operates by balancing a known weight, which creates a specific pressure, against the pressure being measured. This method ensures high precision because the relationship between the applied weight and the resulting pressure is well understood and can be accurately calculated.

Other devices listed do not serve the same role in pressure measurement. RTD sensors and thermocouples are used for temperature measurement, making them unsuitable for pressure calibration. While a barometer does measure atmospheric pressure, it does not provide the same level of accuracy and stability as a deadweight tester when calibrating measurement instruments. Thus, the piston gage is the correct choice for pressure calibration tasks.