The recording of a nitrogen adsorption isotherm employing a known automatically-operating vacuum microbalance is effected by registering the values of the variation in the weight of the specimen under observation with a multi-channel compensation recorder. Different gas pressures in the vacuum micro-balance are then controlled through a manostat by a buoyancy manometer which permits various pressure stages to be adjusted according to a preselected programme. Pressure regulation is effected by means of a minor quantity of gas which is kept constant and admitted to the vacuum micro-balance. Once the desirable nominal pressure value is attained, the gas admitted to the vacuum micro-balance is partially removed by means of a pump until the preselected gas pressure remains constant.
This procedure has serious disadvantages in respect of the following points:
1. Preliminary tests are necessary in order to ascertain how much time is required for adjustment of the equilibrium. Depending upon the specimen used and the gas pressure, between 5 and 200 minutes are needed for adjustment of the equilibrium.
2. A certain safety margin for the time must be allowed in order to be sure that the equilibrium has adjusted.
3. The statement of the variation in weight of the specimen is made in the form of a graph from which the actual weight difference must be laboriously interpolated.
4. For large variations in weight of the specimen, the hundreds and thousands decades must be laboriously deduced from the record strip, because the decade is stepped up or stepped down automatically when the recorder carriage strikes the limit points. The relevant hundreds or thousands decade has to be reconstructed from the number of jumps.
5. The pressure indication can also only be obtained inaccurately, because only the width of the recorder is available for the entire range of pressure.
6. The association of the variation in weight with the corresponding gas pressure is likewise a time-consuming operation.
7. The pressure regulation is effected by a regulated exhaustion of inflowing gas. Due to the slight pressure variations which then occur, the highly sensitive vacuum micro-balance is set into oscillations which lead to inaccuracy in detecting the equilibrium state.