Densitometer

You haven't specified the type of densitometer, what it's used for, or any of its specifications.

Using an optical densitometer as an example, it would have a light source, a detector, and electronics to drive the source and to convert the detector output to a meaningful result. This conversion may have to cover a range of inputs over say, 5 decades (that would be e.g. from 1 microampere to 100 milliamperes from the detector).

Each of these items (source, detector and electronics) can be prone to drift over time - more or less depending on the nature of the items used.

The densitometer recalibration period would be specified such that the expected drift in the output result due to changes of each of the system components over that time will ensure that the instrument always operates within specification.

Depending on your actual application, an instrument with longer calibration intervals could probably be made, but would probably be correspondingly more expensive. It probably works out more cost-effective to re-calibrate more frequently.

Thanks Mr John.

It is a NDT Densorapid A Densitometer (Hand held).

Most of what I said above holds for this instrument. The light source is not included, so possibly the re-calibration interval is an attempt to account for variations in light level (which obviously are not withing the control of the instrument manufacturer).

Another point to consider is that with a hand-held instrument, there is a greater risk of physical damage (wear and tear) causing variation of the output readings.The wander probe could also be contaminated (does anyone use it to stir their tea?).

The instrument seems to be supplied with a density step-wedge for calibration. Note that this will itself have a limited life, and should be re-calibrated against a known good standard, or replaced, after the interval (which should be) specified in the documentation.

That clears my doubt Mr John. Thanks.