In an open dewar the cryogenic coolant such as liquid nitrogen and/or argon will evaporate, and will therefore change the level of cryogen around the sample cell stem and consequently the cold zone and warm zone volumes. Therefore, it is crucial that the specific position of the cryogen level on the sample cell stem is kept constant during the measurement. It should be maintained - unless otherwise compensated - at least 20 mm above the sample and constant to within at least 1-2 mm. This can be achieved by moving the dewar up, using a thermistor (sensor) coolant level control, a porous sleeve that surrounds the sample tube stem (to maintain the cryogen level by capillary action), or by periodic replenishing of the cryogen. 

For maximum accuracy, the calibrated volumes and the manifold should be maintained at constant temperature, or alternatively, the temperature may be closely monitored, i.e., the actual manifold temperature need to be taken into account in the calculations of the adsorbed amount.