Filter
Top Products
Teledyne API - Model T200H/T200M Operation Manual Principles of Operation
271
8.2.3. AUTO ZERO
Inherent in the operation of any PMT is a certain amount of noise. This is due to a
variety of factors such as black body infrared radiation given off by the metal
components of the reaction cell, unit to unit variations in the PMT units and even the
constant universal background radiation that surrounds us at all times. In order to
reduce this amount of noise and offset, the PMT is kept at a constant 7° C (45° F) by a
thermo-electric cooler (TEC).
While this intrinsic noise and offset is significantly reduced by cooling the PMT, it is
not eradicated. To determine how much noise remains, the T200H/M diverts the sample
gas flow directly to the exhaust manifold without passing the reaction cell once every
minute for about 5 seconds (Figure 8-4). During this time, only O
3
is present in the
reaction cell, effectively turning off the chemiluminescence reaction. Once the chamber
is completely dark, the T200H/M records the output of the PMT and keeps a running
average of these AZERO values. This average offset value is subtracted from the raw
PMT readings while the instrument is measuring NO and NO
X
to arrive at a auto-zero
corrected reading.
Figure 8-4: Reaction Cell During the AutoZero Cycle
07270B DCN6512