A Baldwin-Farmer thimble chamber, fitted with a perspex build-up cap was calibrated against an absolute graphite cavity chamber to provide an interim working standard of exposure for 137Cs and 60Co photons. The calibration factor (roentgens per coulomb) at 60Co energy was 2% higher than an NPL. 2MV calibration for this chamber, carried out in the UK by the National Physical Laboratory. The thimble chamber, fitted with an aluminium build-up cap, was also used to make a comparison between the graphite cavity chamber and an aluminium micro-calorimeter which is to be used as an interim absorbed dose standard. Exposure calibration factors for the thimble chamber derived from calorimeter measurements were in good agreement with calibration factors derived from cavity chamber measurements, at both 137Cs and 60Co energies.