Teledyne 306WA - Analog trace oxygen analyzer - Manual (Page 32)

TELEDYNE ELECTRONIC TECHNOLOGIES
Analytical Instruments
5-8
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5.9 Leak Detection 5.9 Leak Detection 5.9 Leak Detection 5.9 Leak Detection 5.9 Leak Detection
The most frequent cause of trouble in trace measurement is leakage.
Tiny leaks which may be unnoticeable can cause serious errors in trace
measurements. One of the principal problems is that air can diffuse into a
gas line through a small leak, even though the gas pressure in the line may
greatly exceed atmospheric pressure.
When a leak occurs in a system where the mass flow velocity is less
than the molecular velocity, gas molecules move in both directions through
the leak. The net flow of a particular gas, e.g. oxygen, will depend on the
relative partial pressure of that gas on each side of the leak. In a sample
having only a few parts-per-million oxygen, there will be a net flow of
oxygen inward unless the sample pressure is many thousands of pounds.
5.9.1 Leak Detection Procedure 5.9.1 Leak Detection Procedure 5.9.1 Leak Detection Procedure 5.9.1 Leak Detection Procedure 5.9.1 Leak Detection Procedure
The procedure outlined here is based on the premise that the leak rate
is independent of sample flow rate.
a) Stop the sample flow to permit oxygen to accumulate at the
point of the leak.
b) After approximately one minute, restart the sample flow. It is
advisable to practice establishing the flow rate to 150 cc/min 150 cc/min 150 cc/min 150 cc/min 150 cc/min.,
the reference flow indication on the flowmeter, with one quick
turn of the throttle valve.
c) Simultaneous to restarting the flow, start a stopwatch to
measure the time required for the recorder/meter to respond to
the accumulated oxygen.
d) The following are approximate times for the accumulated
oxygen to reach the cell from various points in the sample path
through the analyzer at 150 cc/min
150 cc/min 150 cc/min 150 cc/min 150 cc/min.
1.5 to 2 seconds Calibrator
3.5 to 3.75 seconds Base of humidifier column*
5.5 seconds Flowmeter
6.5 seconds Metering valve
7.5 seconds Gas connection at rear of analyzer
> than 7.5 seconds Sample connection lines leading to
analyzer
* A leak at this location may indicate a leak in the column or in
the reservoir system.