It is recommended that all 7250 series calibrators be zeroed once a day to maintain optimal performance. If the calibrator is not being used daily it is only necessary to zero before use. If you are ok with a little additional time to complete your testing, many will zero before running each test or build this into their software automation as it typically only takes a few minutes on gauge mode controllers to complete this process. This further ensure small amounts of sensor drift are removed.
If the calibrator has the Vacuum Reference option it should also be zeroed when switching from vacuum reference mode to gauge mode. In other words, perform a zero if the reference vacuum pump is in use for absolute mode testing and perform another zero if the reference vacuum pump is then turned off and going to be used in standard gauge or negative gauge mode. The Vacuum Reference option includes an internal vacuum sensor and it will be "live" during tests to make corrections for any residual vacuum during absolute mode testing.
If the calibrator is a Permanent Absolute type a hard vacuum will be pulled on the sensor. It is recommended that the sensor be pulled down to a vacuum of less than 200 mTorr (preferably less than 100 mTorr) in order to obtain a proper zero. The Calibrator can be zeroed at higher vacuum levels; however, the uncertainty of vacuum sensors tend to increase at higher vacuum levels, therefore, the uncertainty of the calibrator would also increase if zeroed at higher vacuum levels. A vacuum sensor is also included with this type of controller but it is only used during the zeroing process. It does not make corrections for residual vacuum since it by design has a stable vacuum permanently trapped on the reference side of the sensor.