Cathy,
First, I must state I work for a calibration lab. We focus mainly on scales and are accredited with a scope of up to 200,000Lbs. We are not “cheap”, however we are not “outrageous” either. That being said:
You get what you pay for.
Andygr’s last post was dead on.
You truly need to establish your own requirements regarding frequency, tolerance, user range, and the implied “risks” of those requirements.
Part of my job here is to help guide and educate our customers regarding the care and maintenance of their equipment including calibration.
We have customers who need calibration every 15 days and we always find between 3 and 4 scales broken every trip.
We also have customers who perform internal calibration checks, we go in twice a year to issue documentation and find maybe 2 out of 75 scales out of tolerance.
On many occasions we recommend less frequent calibrations depending on the past and present calibration reports of customer equipment, the environment they are used in, etc.
Highly sensitive equipment requires more care, not necessarily more calibration.
Any reputable calibration lab should be willing to work with you.
Fortunately or unfortunately depending on what side of the fence you are on, the trend in the industry is toward accredited calibration even if only once a year. This establishes the competency and objectivity your customers are looking for.
If you are exceedingly through and, competent technicians to maintain your own equipment and can establish the objectivity required, you should be ok however I do recommend accredited documentation at least once a year.
