The ion chambers used in measuring the adsorbed dose are generally vented ion chambers. Thus the mass of the air  inside the sensitive volume of the chamber varies with respect to the density(𝝆) and volume(V) i.e. m = 𝝆V,  As the volume of the chamber remains constant, the density varies with respect to pressure and temperature

As the pressure increases the density increases(i.e.more air molecules) {Boyel’s law} , hence more interaction occurs. This results in more number of charge collection. The dose measured in this case is overestimated from that measured in standard condition

If the temperature increases the density decreases{Charle’s Law}. In this case, the number of interactions are less and the dose measured is under estimated from the measured dose under standard conditions.

To compensate this , the temperature-pressure correction factor(K_TP) was introduced

Po and To are reference values of pressure(KPa) and temperature(℃) according to SSDL. P and T are air pressure and temperature at the time of measurement

The dosimeters what we use in clinics are relative dosimeters, i.e. they are calibrated under laboratory conditions. These laboratory conditions may change between SSDL’s, one might have did the calibration with temperature as 20 ℃  and other might have done with 21℃ , so always its a best practice to refer calibration certificate for reference temperature and pressure.