Q - Unless you have a particular reason to believe that it's gone out of calibration, it's probably fine.
A - I have had the unit for a few years, and hunt with it a LOT, so have experience in it's operation and what it is "telling" me when detecting. It has started (in the last couple of months) to tell me a totally different story, for instance a solid quarter hit when in reality a zinc penny is being detected, or a pull tab signal when it detects a quarter. I use these examples as both the quarter and zinc penny signals have been spot on since I purchased the machine, now they are not...so I figured a calibration may be in order. I am in the electronics manufacturing industry and deal daily with analysis equipment that requires calibration on a scheduled basis, and to the experienced user, it is easy to determine when the operation of these types of equipment are in need of attention/calibration/repair. (the "story" they tell changes or is inconsistent.) I figure it could not hurt to send it in for a physical.
Q - Educate me here: what do you mean by calibration? How does one know it NEEDS it?
A - As I said above - the "story" has changed when the detector is in use. This is really the only reason for me to post the question. I may be wrong, but figured it would be a good investment either way...By calibration I mean having the unit tested to "known" inputs, observation of the output, adjustment of the electronic circuitry if the output is out of specification. Usually accomplished via adjustment of potentiometers (adjustable resistors), troubleshooting and repair of suspect components on the circuit board etc...
Hope this helps - Does anyone know how to contact BH customer service?