Densitometers, like any measuring device, need to have their calibration verified from time-to-time. The ones produced today are high precision electronic instruments. All components are solid state so the reliability of these instruments is exceptional. In most instances the calibration will hold for weeks, if not months. Even so, over time, the measurement accuracy can drift as the components in the instrument age. Two of the components that effect this the most are the age of the filters and the age of the lamp.
Most of this variation can be drastically minimized if not eliminated by re-calibrating the instrument. This should be done at regular intervals, depending on the instrument's usage.
In general practice, it is always a good idea to check calibration at least daily. Here, the operator is simply measuring the calibration reference to verify the instrument has remained in calibration. The tolerance acceptable for both reflection and transmission densitometers is the same, +/- 0.02 Density units. If the measured values of the calibration reference are within this tolerance, the instrument does not need to be re-calibrated.
During times of heavy usage or if more than one shift uses the same instrument, calibration should be checked more frequently. For shift work, once at the beginning of each shift is recommended. In this way should the instrument fall out of calibration due to improper handling or unit failure, the defect will be detected before it is used to monitor the quality of work being produced. For instances of heavy usage, the instrument should have its calibration checked at least twice, but preferably three times per day.
To assure calibration is maintained at its highest level, the calibration reference must remain in excellent condition. For the specific calibration procedures for your particular instrument, please refer to the operator’s manual.