Critical Flow Orifice; Sample Pressure Sensor - Teledyne 300E Instruction Manual

THEORY OF OPERATION
By placing the pump down stream from the sample chamber several problems are
avoided.
First the pumping process heats and compresses the sample air
q
complicating the measurement process.
Additionally, certain physical parts of the pump itself are made of materials
q
that might chemically react with the sample gas.
Finally, in certain applications where the concentration of the target gas
q
might be high enough to be hazardous, maintaining a negative gas pressure
relative to ambient means that should a minor leak occur, no sample gas
will be pumped into the atmosphere surrounding analyzer.

10.2.2. Critical Flow Orifice

In order to accurately measure the presences of low concentrations of CO in the
Sample Air it is necessary to establish and maintain a relatively constant and stable
volumetric flow of sample gas through the instrument. The simplest way to
accomplish this is by placing a Critical Flow Orifice directly upstream of the pump
but down stream from the Sample Chamber.
As the pump pulls against the restricted airway of the orifice a pressure differential
is created. By keeping a sufficiently large the pressure differential across the orifice
(approximately 2:1), a simple and effective control mechanism is established for
maintaining an even flow rate. This has several other benefits:
Fluctuations in the flow rate of the sample gas due to hysteresis in the
q
pump's operation are smoothed out.
Pressure wave created by the pump's action are filtered out.
q
The flow rate of gas through the sample chamber will be the same
q
regardless of whether the analyzer is at the bottom of Death Valley or on
top of Pikes Peak.
The flow rate of the gas is also unaffected by degradations in pump
q
efficiency due to age.

10.2.3. Sample Pressure Sensor

An absolute value pressure transducer plumbed to the outlet of the Sample
Chamber is used to measure sample pressure. The output of the sensor is used to
compensate the concentration measurement for changes in air pressure. This
sensor is mounted to a printed circuit board with the Sample flow sensor on the
sample chamber; see following section and Figure 3.6.
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Model 300E Carbon Monoxide Analyzer
Instruction Manual
P/N 04288 Rev: A