Its cheaper to buy a new calibrated meter than pay the postage to and from for your meter, before you have even paid for it to be calibrated. Farnell sell a calibrated meter with cert for around £40.00. TBF calibration is a PITA you have to get all the meters in to check them all, we give out a loan meter while the engineers is being calibrated and trying to organise getting them swapped around is like hearding squirrels sometimes. You have to get the loan back before you can do the next meter. To calibrate you need something to check current, resistance, voltage, ect, against a known calibrated meter. There is no other easy way to check calibration to my knowledge you cant just put it on a machine that does all that for you, you have to change the settings on the meter for starters. + or- 10% is acceptable