Moderate to high pressure units are usually calibrated with a “dead weight tester.” This is a hydraulic device in which weights are added to one side of a hydraulic circuit to generate a known pressure which is applied to the input of the transducer under calibration. If the transducer is to be calibrated in units of absolute pressure, a barometer should be used to measure atmospheric pressure and an adjustment of the added weights made. The dead weight tester is normally a test bench device. Low pressure units are calibrated with a pneumatic calibrator which supplies a precise air pressure. Since they are easily adjustable, 3-15 psig devices are usually calibrated by this method. Pneumatic calibrators are also test bench devices. Field or process area checks are often made with a hand-held bulb pump and an accurate pressure gauge.