Hydraulic Testers Explore Ashcroft hydraulic deadweight tester and gauge comparators for your specific application. If you rely on accurate readings from your pressure measuring instruments, periodic calibration checks with precise calibration equipment will be necessary. Ashcroft hydraulic deadweight testers are primary pressure standards that derive their accuracy directly from principles of physics and maintain it for an extended period of time. Pressure gauge comparators are secondary standards that use the same hydraulic test pump to generate pressure, but compare to high accuracy “master” test gauges for fast and convenient calibration.