What instrument is used to measure the pressure of gases?

Manometer is one of the earliest pressure measuring instruments is still in wide use today because of its inherent accuracy and simplicity of operation. It's the U-tube manometer, which is a U-shaped glass tube partially filled with liquid. This manometer has no moving parts and requires no calibration. Manometry measurements are functions of gravity and the liquid's density, both physical properties that make the U-tube manometer a NIST standard for accuracy.

Higher accuracy digital manometers are used to calibrate pressure transmitters and other pressure instrumentation in the field. Digital calibrators are faster and simpler as they require no boxes, gas cylinders, regulators, or weights to set up and have no special platforms or critical leveling requirements.