Teledyne 356WA - page 28

Teledyne Analytical Instruments5-6

5 Maintenance & 5 Maintenance &

5 Maintenance & TT

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Model 356WModel 356W

Model 356WAA

flowmeter column or humidifier column.

5.9 Leak Detection5.9 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 Procedure5.9.1 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/min150 cc/min

150 cc/min150 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/min150 cc/min

150 cc/min150 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.

5.9.2 Cell Leak5.9.2 Cell Leak

5.9.2 Cell Leak