Photometer Layout; Photometer Pneumatic Operation; Figure 9-18. O 3 Photometer Layout - Top Cover Removed - Teledyne T750 User Manual

P
HOTOMETER
The Photometer is where the absorption of UV light by ozone is measured and converted
into a voltage. It consists of several sub-assemblies:
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UV Detector
UV Detector Sample Gas Inlet
Preamp PCA
Figure 9-18. O
P
HOTOMETER
The flow of gas through the photometer is created by an internal pump that pulls air though
the instrument. There are several advantages to this "pull through" configuration. Placing
the pump downstream from the absorption tube avoids problems caused by the pumping
process heating and compressing the sample.
In order to measure accurately the presences of low concentrations of O
it is necessary to establish and maintain a relatively constant and stable volumetric flow of
sample gas through the photometer. The simplest way to accomplish this is by placing a
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L
AYOUT
A mercury-vapor UV lamp. This lamp is coated in a material that optically
screens the UV radiation output to remove the O
radiation. Only light at 254nm is emitted.
An AC power supply that supplies 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 converts 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.
UV Lamp Power
Absorption Tube
Transformer
Power Connector
from
+15 VDC power supply
UV Lamp Power
Supply
(200 VAC @ 30 kHz)
Photometer Layout – Top Cover Removed
3
P O
NEUMATIC PERATION
producing 185nm
3
Sample Gas
UV Lamp Thermistor
Thermistor
(UV Lamp Heater Behind Thermistor)
Sample Gas
Outlet
UV Lamp
in the sample air
3
083730750B DCN8606
UV Lamp Heater
Control PCA