Teledyne 6200T - Sulfides Analyzer - Manual (Page 310)

Principles of Operation Model 6200T Total Sulfur Analyzer
310 Teledyne Analytical Instruments
13.5.4. PMT Cooling System
The performance of the analyzer’s PMT is significantly affected by temperature.
Variations in PMT temperature are directly reflected in the signal output of the PMT.
Also the signal to noise ratio of the PMT output is radically influenced by temperature
as well. The warmer The PMT is, the noisier its signal becomes until the noise renders
the concentration signal useless. To alleviate this problem a special cooling system
exists that maintains the PMT temperature at a stable, low level.
TEC
Control
PCA
PMT Preamp
PCA
Thermistor
outputs temp of
cold block to
preamp PCA
Preamp PCA sends
buffered and
amplified thermistor
signal to TEC PCA
TEC PCA sets
appropriate
drive voltage
for cooler
Heat form PMT is absorbed
by the cold block and
transferred to the heat sink
via the TEC then bled off
into the cool air stream.
PMT
Cold Block
He
at
Sin
k
Cooling Fan
ThermoElectric Cooler (TEC)
Figure 13-19: PMT Cooling System
13.5.4.1. Thermoelectric Cooler (TEC)
The core of the 6200T PMT cooling system is a solid state heat pump called a
thermoelectric cooler (TEC). Thermoelectric coolers transfer heat from a one side of a
special set of semiconductor junctions to the other when a DC current is applied. The
heat is pumped at a rate proportional to the amount of current applied. In the 6200T the
TEC is physically attached to a cold block that absorbs heat directly from the PMT and a
heat sink that is cooled by moving air (refer to Figure 13-19). A Thermocouple
embedded into the cold block generates an analog voltage corresponding to the current
temperature of the PMT. The PMT Preamp PCA conditions and amplifies this signal
then passes it on to the TEC Control PCA.