Teledyne 6400EH - Sulfur Dioxide Analyzer - Manual (Page 52)

Theory of Operation Model 6400EH
Teledyne Analytical Instruments 40
9.2.2 Pneumatic Sensors
The 6400EH uses two pneumatic sensors to verify gas flow. These
sensors are located on a printed circuit assembly, called the pneumatic
pressure/flow sensor board. This PCA is attached to a manifold
containing the critical flow orifice that sets the instrument flow rate.
9.2.2.1 S AMPLE P RESSURE S ENSOR
An absolute pressure transducer plumbed to the input of the
analyzer’s sample chamber is used to measure the pressure of the sample
gas before it passes through the critical flow orifice. This is used to
validate the critical flow condition (2:1 pressure ratio) through the
instrument’s critical flow orifice.
The actual sample gas pressure measurement is viewable through
the analyzer’s front panel display as the test function
PRES
.
9.2.2.2 V ACUUM P RESSURE S ENSOR
An absolute pressure transducer measures the pressure on the
vacuum side of the critical flow orifice and is used to measure the
sample gas pressure in the reaction cell. If the vacuum pressure is not in
the correct range, a warning will be displayed by the software. Also, if
the temperature/pressure compensation (TPC) feature is turned on, the
output of this sensor is also used to supply pressure data for that
calculation.
The actual pressure of the gas downstream from the critical flow
orifice (including the gas inside the reaction cell) viewable through the
analyzer’s front panel display as the test function
VAC
.
9.2.2.3 S AMPLE F LOW C ALCULATION
Unlike the 6400E, which uses a thermal-mass flow sensor to
directly measure the gas flow though the instrument, the 6400EH
calculates the gas as follows.
 The ratio of the two pressures is measured and used to validate
critical flow. If the ratio is not correct (< 2:1) the
SAMPLE
FLOW WARN message is activated. Also, the value of the
SAMP FL test function is set to XXXX .
If the pressure ratio between the two sensors is valid
(≥ 2:1),
the
instrument calculates the flow based on sample gas pressure level
(PRES)
and is viewable via the front panel as the
SAMP FL
test function.