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Theory Of Operation Teledyne API M100E Analyzer Operation Manual
240
11.4.4. 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
Heat Sink
Cooling Fan
ThermoElectric Cooler
Figure 11-16: PMT Cooling System
11.4.4.1. THERMOELECTRIC COOLER (TEC)
The core of the M100E 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
M100E 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 11-16). A Thermocouple embedded into the cold block generates an
analo
g
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.
11.4.4.2. TEC CONTROL BOARD
The TEC control printed circuit assembly is located ion the sensor housing assembly, under the slanted shroud,
next to the cooling fins and directly above the cooling fan. Using the amplified PMT temperature signal from the
PMT preamplifier board (refer to Section 11.4.5), it sets the drive voltage for the thermoelectric cooler. The
warmer the P
M
T gets, the more current is passed through the TEC causing it to pump more heat to the heat
sink.
A red LED located on the top edge of this circuit board indicates that the control circuit is receiving power. Four
test points are also located at the top of this assembly. For the definitions and acceptable signal levels of these
test points refer to Chapter 12.
04515F DCN6048