Pumping tubes conductance

The introduction of AutoAnalyzer equipment has resulted in a considerable degree of standardization in the practice of clinical chemistry laboratories. However, it is well known that when "the same techniques and reagents are used with different AutoAnalyzer modules the results, as evidenced by the standard curves, may vary markedly from one set of equipment to another. From a survey conducted to identify the source of this variation, Northam (N2) concluded that, excluding variation in pump-tube sizes, the most likely cause was the considerable differences found in heating-bath coil volumes he also recommended that a standard practice should be followed when publishing details about AutoAnalyzer methods, including suflBcient information about standard curves in these descriptions to allow other laboratories to compare their own performance directly. 

Pumping speed S and throughput Q were determined from knowledge of the tube conductance Cn. in liters/sec, the pressure drop across the tube (P1-P2) in torrs, the chamber pressure at Ps in torrs, and the chamber pressure at Pi in torrs. Pumping speed was given by... 

The time taken for a system to reach a desired pressure within the molecular flow regime cannot be calculated as simply as it can for viscous flow, because the outgassing of the various materials used in the construction of the system starts to play an important role in terms of the quantities of gas which have to be removed. However, since the product of the pressure in the system and the net pumping speed is equal to the total gas load (measured in units of pressure x volume) it is clear that the net pumping speed (Equation (1.4)) should be kept as large as possible. The molecular flow conductance of a long tube is given by... 

It is important to note that in the molecular flow regime the conductance depends on the cube of the diameter of the tube and the — power of the molecular weight of the gas being pumped, but it is independent of the pressure. 

Fig. 3.23. Conductivity cell with phial magazine Q containing phials and magnetic pusher. The electrode assembly E is that shown in Fig. 3.22. S is a stirrer shaft with a propeller at one end and a glass-enclosed magnet N at the other. The stirrer shaft is held in position by the PTFE bearings Tf Tf, and Tf and the glass tube spacer G Th ha. thermocouple pocket and B a magnetic breaker for the phial P. The propeller, driven by the rotating magnet M, pumps the cell contents around the loop L, so that when P is broken there is very fast mixing.

---