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M400E Ozone Analyzer Operator’s Manual Theory of Operation
11.3.7.3. Photometer Temperature
In order to operate at peak efficiency the UV lamp of the M400E’s O
3
photometer is maintained at a constant
58ºC. This is intentionally set at a temperature higher than the ambient temperature of the M400E’s operating
environment to make sure that local changes in temperature do not affect the UV Lamp. If the lamp temperature
falls below 56ºC or rises above 61ºC a warning is issued by the analyzers CPU.
This temperature is controlled as described in the section on the relay PCA (Section 11.3.4.4).
The followi
ng TEST functions report these temperatures and are viewable from the instrument’s front panel:
PHOTO_LAMP - The temperature of the UV Lamp reported in ºC.
SAMPLE _TEMP - The temperature of the Sample gas in the absorption tube reported in ºC.
11.3.7.4. Photometer Gas Pressure and Flow Rate
The sensors mounted to a printed circuit board next to the internal pump (see Figure 3-4) measure the absolute
pressure and the flow rate of gas inside the photometer’s absorption tube. This information is used by the CPU
to calculate the O
3
concentration of the sample gas (See Equation 11-3). Both of these measurements are
made downstream from the absorption tube but upstream of the pump. A critical flow orifice located between
the flow sensor and the pump maintains the gas flow through the photometer at 800 cm
3
/min.
The following TEST functions are viewable from the instrument’s front panel:
SAMPL_FL- The flow rate of gas through the photometer measured in LPM.
PRES – The pressure of the gas inside the absorption tube. This pressure is reported in inches of
mercury-absolute (
in-Hg-A), i.e. referenced to a vacuum (zero absolute pressure). This is not the same
as
PSIG.
NOTE
The M400E displays all pressures in inches of mercury-absolute (in-Hg-A). Absolute pressure is the
reading referenced to a vacuum or zero absolute pressure. This method was chosen so that ambiguities
of pressure relative to ambient pressure can be avoided.
For example, if the vacuum reading is 25" Hg relative to room pressure at sea level the absolute
pressure would be 5" Hg. If the same absolute pressure was observed at 5000 ft altitude where the
atmospheric pressure was 5" lower, the relative pressure would drop to 20" Hg, however the absolute
pressure would remain the same 5" Hg-A.
04315 Rev. C1 213