You can usually find the serial number on the back of the measuring instrument, shown after S/N.

Are you sending in the measuring instrument for periodic calibration? To be sure the sensors are working properly? Preventive maintenance, in other words.

Or is the meter not working properly and does a sensor or another part need replacing? 

Compare calibration with a roadworthiness test, and repair with fixing a fault. Naturally your measuring instrument is also calibrated after a repair.

If you already know that a sensor is not working or that there is another fault, it is helpful for our service technicians if you indicate this here in advance.