First, yes you can, and you will have to, demonstrate your competence at performing on site calibration service to the assessor.
The requirements for on site calibrations are the same as for in lab services, with a few special cases:
6.3.5 When the laboratory performs laboratory activities at sites or facilities outside its permanent
control, it shall ensure that the requirements related to facilities and environmental conditions of this document are met.
For this most labs will define the environmental conditions that are acceptable for on site work, and often have a disclaimer that names responsible parties who can authorize on site work if the conditions deviate. This also means recording the pertinent environmental data.
6.4.3 The laboratory shall have a procedure for handling, transport, storage, use and planned
maintenance of equipment in order to ensure proper functioning and to prevent contamination or deterioration.
You will need a procedure for how you handle transporting equipment from the lab to the on site locations.
6.4.4 The laboratory shall verify that equipment conforms to specified requirements before being
placed or returned into service.
You will need to verify that your equipment survived the trip, and is operational. Keep records of what you checked and what equipment you used for the checks. You will need to do this again when you return to the lab.
Most accrediting bodies also have internal policies for on site calibrations. You will definitely check with your accreditation body. I am familiar with A2LA's policies, they are in their R104 Field Calibration Requirements document. Those labs will also have a disclaimer statement on their scope of accreditation, such as:
Field calibration service is available for this calibration and this laboratory meets A2LA R104 – General Requirements: Accreditation of Field Testing and Field Calibration Laboratories for these calibrations. Please note the actual measurement uncertainties achievable on a customer's site can normally be expected to be larger than the CMC found on the A2LA Scope. Allowance must be made for aspects such as the environment at the place of calibration and for other possible adverse effects such as those caused by transportation of the calibration equipment. The usual allowance for the actual uncertainty introduced by the item being calibrated, (e.g. resolution) must also be considered and this, on its own, could result in the actual measurement uncertainty achievable on a customer’s site being larger than the CMC.
