2. The Thermal Conductivity Detector (TCD) - Eawag-Empa Library

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It is obvious that the time constant will increase with increasing participation of
diffusion in the transport process. The time constant of a semi-diffusion cell can be
expressed by the relationship
1
z = 0.632t,,, -
2-- V
111
where trl is the gas flow-rate through the measuring branch (see Fig. 2.9). The time
constants of the cell types discussed are listed in Table 2.5. In addition to the time
constant of the measuring cell, the sensor time constant, determined by its mass and
heat capacity, must also be considered. Thus the sensor requires a certain time to
record a change. The time constant of a heated filament is given by the relationship
As the heat capacity of the filament is proportional to its volume, i.e., mC, - $1, the
requirement of high TCD sensitivity (see equations (2.9) and (2.19)) leads to an
increase in the sensor time constant. The heat capacity of the thermistor approxima-
tely equals the material constant, 6, and varies around 1 sec for most sensors of this
type. In a provisional arrangement, the transistor sensor had a large time constant
of 10 sec [31], which the authors felt could be decreased to 0.6 sec.
In some designs, the heated filament is sealed in glass in order to decrease the
catalytic action of the heated filament on the thermal decomposition of substances
and to prevent corrosion of the sensor. Similarly, materials other than glass have also
been employed, e.g., fluorinated plastic [5]. These modifications lead to an increase
in the lifetime of the sensor and make its use at higher temperatures possible, but the
TABLE 2.6
THE TIME CONSTANT OF THE THERMAL CONDUCTIVITY DETECTOR
Cell
Sensor
heated filament
filament in thermistor transistor
Length of cell
- sensor [cm]
2 10 2 0.2 0.2
Diameter of cell
- sensor [cm]
0.5 0.002 0.1 0.2 0.2
Gas flow-rate [ml/min] 30
Time constant [sec] 0.45 0.01 4 1 9.6