Rapid-response variable-temperature thermostat bath employing analog-digital control circuitry

Abstract

expected rates calculated from the equation ( 4 ) DMP A s = ~ ( i ) l n h RTwhere S is the diffusion rate grams sec-', D is the diffusion coefficient cm2 sec-I, M is the molecular weight of the vapor, P is the total pressure dynes m r 2 , R is the gas constant, erg K-' molecl, T is the operating temperature K, A is the cross-sectional area of the capillary tube cm2, L is the length (cm) of the diffusion path, X = ~ where P and p , the p -P vapor pressure of the diffusion component at temperature T, can be expressed in any convenient units.The diffusion coefficients used were 9.32 X lo-* and (4) J. Stefan, W e n Ber., 63,63 (2) (1871).15.20 X lo-? cm? sec-1 for benzene and methanol, respectivelyWith the fixed reservoir-type cell used, the gravimetric results showed a steady decline in the diffusion rate with time; these results with benzene were confirmed by the flame ionization detector. It is not possible to maintain steady conditions due to the concentration gradient in the space above the liquid level changing as the level falls. (5).With the constant level diffusion cell, the parameter remains fixed at its chosen initial value and concentration gradient effects are eliminated. The observed diffusion rates are found to be better than 95 of the theoretical values for continuous operating periods which are limited only by the capacity of the reservoir.