Servomex DF-130E, Operator'S Manual

Page: 42 / 88

38 DF-150E Sample Gas and Plumbing Cautions
8.3 Temperature
Gas temperature should not exceed 50°C (122°F), nor should it fall below 5°C
(40°F). Gas temperatures can be controlled by passing the sample through 5 to
10 feet of metal tubing that is within the recommended temperature range.
8.4 Leaks
Significant measurement error can be caused by leaks in the plumbing. A
simple
Flow Sensitivity Test can be performed to identify oxygen leaks.
1. Observe the oxygen readout at two flow rates: 0.5 and 2.0 scfh. In a tight
plumbing system, only a slight readout change will occur as the flow is
varied. If a leak does exist, an increase in flow, and the resulting dilution of
the sample, will cause a decrease in the oxygen readout. Similarly, a
reduction in gas flow will cause an increased oxygen reading as the
concentration goes up. Readings can vary as much as 25% to 50% in gas
delivery systems with leaks.
2. As the user becomes familiar with the analyzer it can be used to determine
the distance to a leak by noting the relationship between the time it takes
for the analyzer to react to the change in gas flow rate.
3. When flow sensitivity is observed, check the external plumbing for leaks. If
the gas delivery lines are tight, remove the sensor assembly from the
enclosure to check the internal connections. See page 12.
4. Also, carefully check the stem and packing on the sensor inlet flow control
valve (if equipped).
5. If a leak is suspected in the gas inlet line and the sensor is equipped with an
up-stream valve, fully close the valve (clockwise). Pressurize the inlet gas
line from 5 to 10 psig and use a soapy solution such as Snoop
 to identify
leaks.
NOTE
After performing the leak test, open the up-stream valve slowly to
slowly lower the pressure to the sensor.
8.5 Process Up-sets (Oxygen Shock)
If a low range sensor is exposed to high concentrations of oxygen such as
room air, it will enter an over range condition. The sensor may absorb
excessive oxygen and it may take some time before it comes back “on-range”.
If the exposure is prolonged, it may take as long as 12 to 24 hours for the
residual oxygen to fully purge.