Hi sayan.cts,
I assume –
(1) target temperature around -18degC.
(2) there is a temperature measuring instrument (X) installed in an approx. warmest location in the room. Hopefully not in the roof.
(3) X has a visible display (typically external to the room).
(4) the location of the active measuring portion of X is (approx.) known and (approx.) accessible.
Some temperature loggers I believe are self-calibrating. I presume you do not have such a device.
Fix a small thermocouple probe, Z, for example of short needle shape, in close proximity/contact to the active measuring point of X. For example use electrical tape. For convenience Z should be matched to its own commercial display unit (it is possible to make one’s own “point” TCs and calibrate them but some expertise is required). The length of connecting lead between Z and its display unit should not be "excessively" long.
The probe Z will require to be previously calibrated either (a) by a company whose calibration equipment is traceable, for example to NIST, or (b) from a “master” unit which itself has been calibrated as per (a). If (b), the procedure can be in-house and duplicate that being currently described. The specific calibration temperature (or temperatures) will depend on your usage. As per (1) above the calibration would be at -18degC, this would likely to be ok for temperatures around -15 to -20 degC (guesstimate).
For routine use, IMEX, an agreement within 1degC is usually acceptable. Otherwise adjust the observed reading accordingly.