Servomex DF-310E, Руководство оператора

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36 DF-310E Sample Gas Preparation and Delivery
6.3.1 Flow Rate Effects on Sensor Performance
Assuming a leak-tight system, higher flow rates may cause O
2
readings to
increase by a few percent of reading above the level that would be displayed if
flow was within the recommended 1.0 to 2.0 scfh range. Lower flow rates
similarly cause O
2
readings to decrease by a few percent of reading. Very low
flow rates (below 0.2 scfh) should be avoided as the sample inside of the
sensor is no longer representative of the actual sample.
The insensitivity to flow rate changes is the basis for the sample system leak
detection described below. The sensor output should be virtually constant for
readings between 0.5 and 2.0 scfh. Therefore, if O
2
readings become higher at
lower flows, then ambient O
2
is leaking into the sample system, or venting
from a dead space (closed pocket with trapped higher O
2
level gas) in the
sample system. A higher flow rate dilutes the O
2
entering the sample system
decreasing the reading. O
2
readings in a leak free sample system should not go
up or down significantly with flow changes between 0.5 and 2.0 scfh.
6.3.2 Checking for Plumbing Leaks using Flow Rate
Effects
Significant measurement error can be caused by leaks in the plumbing system.
A simple test can be performed to identify oxygen intrusion leaks.
Observe the analyzer readout at two flow levels: 0.5 and 2.0 scfh. Only a
slight increase, if any, in readout will occur in a tight system as the flow is
increased. If leakage in the plumbing system exists, then the increased flow
results in a substantial decrease in oxygen readout -- typically dropping by 25
to 50 percent.
When flow sensitivity is observed, check the plumbing system for leaks. Once
proficient with this test, the user can estimate the distance to the leak based on
the response time of the reading changes.
6.3.3 Background Gas Effects on Indicated Flow Rate
If the molecular weight of the background gas is much different from N
2
, the
flowmeter reading is not accurate. The Rotameter type is calibrated for use in
air (or N
2
). Most other gases have molecular weights within ± 25 percent of
air. Since the required flow rate is not extremely critical most gases produces
reasonably correct readings. The exceptions are light gases such as Helium
and Hydrogen whose flow rates should be set to approximately one-third that
of Nitrogen or 0.3 scfh.
6.3.4 Regulator Requirements
If the pressure in the sample line varies, but does not drop below 2.0 psig, use
a regulator to drop the pressure to approximately 1.0 psig. Set final flow rate
with the sensor flow control valve.
If a regulator is not used, the flow rate changes when the pressure at the inlet
of the flow control valve changes. As long as this pressure variation does not
bring the flow rate out of the recommended flow range (1.0 - 2.0 scfh) no
regulator is required. A flow change of ±1.0 scfh may result in a small change
to the oxygen reading.