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Theory of Operation T400 Ozone Analyzer Operator’s Manual
298
13.3.7. PHOTOMETER LAYOUT AND OPERATION
The Photometer is the component where the absorption of UV light by ozone is
measured and converted into a voltage. It consists of several sub-assemblies:
A mercury-vapor UV lamp. This lamp is coated in a material that optically screens
the UV radiation output to remove the O
3
producing 185nm radiation. Only light at
254nm is emitted.
An AC power supply to supply the current for starting and maintaining the plasma
arc of the mercury vapor lamp.
A thermistor and DC heater attached to the UV lamp to maintain the lamp at an
optimum operating temperature.
42 cm long quartz absorption tube.
A thermistor attached to the quartz tube for measuring sample gas temperature.
Gas inlet and outlet mounting blocks that rout sample gas into and out of the
photometer.
The vacuum diode, UV detector that converts UV light to a DC current.
A preamplifier assembly, which convert the Detector’s current output into a DC
Voltage then amplifies it to a level readable by the A to D converter circuitry of the
instrument’s motherboard
Absorption Tube
UV Lamp Heater
Control PCA
Sample Gas
Outlet
UV Lamp Power
Transformer
Power Connector
from
+15 VDC power supply
UV Lamp Power
Supply
(200 VAC @ 30 kHz)
Sam
p
le Gas Inlet UV Detector
Preamp PCA
UV Detector
Sample Gas
Thermistor
UV Lamp
UV Lamp Thermistor
(UV Lamp Heater Behind Thermistor)
Figure 13-17: O
3
Photometer Layout – Top Cover Removed
CAUTION – UV Radiation Risk
Do not look directly at the light of the UV lamp.
06870C DCN6332