Filter
Top Products
Troubleshooting & Repair Teledyne API - Model 200EH/EM Operation Manual
248
11.3.5. INABILITY TO ZERO - NO ZERO KEY
In general, the M200EH/EM will not display certain keyboard choices whenever the actual value of a parameter
is outside of the expected range for that parameter. If the calibration menu does not show a ZERO key when
carrying out a zero calibration, the actual gas concentration must be significantly different from the actual zero
point (as per last calibration), which can have several reasons.
Confirm that there is a good source of zero air. If the IZS option is installed, compare the zero reading
from the IZS zero air source to a zero air source using NO
X
-free air. Check any zero air scrubber for
performance. It may need to be replaced (Section 9.3.5).
Check to make sure that there is no ambi
ent air leaking into zero air line. Check for leaks in the
pneumatic systems as described in Section 11.5.
11.3.6. NON-LINEAR RESPONSE
The M200EH/EM was factory calibrated to a high level of NO and should be linear to within 1% of full scale.
Common causes for non-linearity are:
Leaks in the pneumatic system. Leaks can add a constant of ambient air, zero air or span gas to the
current sample gas stream, which may be changing in concentrations as the linearity test is performed.
Check for leaks as described in Section 11.5.
The calibration device is in error. Chec
k flow rates and concentrations, particularly when using low
concentrations. If a mass flow calibrator is used and the flow is less than 10% of the full scale flow on
either flow controller, you may need to purchase lower concentration standards.
The standard gases may be mislabeled as to type or concentration. Labeled concentrations may be
outside the certified tolerance.
The sample delivery system may be contaminated. Check for dirt in the sample lines or reaction cell.
Calibration gas source may be contaminated (NO
2
in NO gas is common).
Dilution air contains sample or span gas.
Ozone concentration too low because of wet air in the generator. Generator system needs to be
cleaned and dried with dry supply air. Check the Perma Pure dryer for leaks. This mostly affects
linearity at the low end.
Sample inlet may be contaminated with NO
X
exhaust from this or other analyzers. Verify proper venting
of the pump exhaust.
Span gas overflow is not properly vented and creates a back-pressure on the sample inlet port. Also, if
the span gas is not vented at all and does not supply enough sample gas, the analyzer may be
evacuating the sample line. Make sure to create and properly vent excess span gas.
Diffusion of oxygen into Teflon-type tubing over long distances.
5PTFE or related materials can act as
permeation devices. In fact, the permeable membrane of NO
2
permeation tubes is made of 5PTFE.
When using very long supply lines (> 1 m) between high concentrations span gases and the dilution
system, oxygen from ambient air can diffuse into the line and react with NO to form NO
2
. This reaction
is dependent on NO concentration and accelerates with increasing NO concentration, hence, affects
linearity only at high NO levels. Using stainless steel for long span gas supply lines avoids this problem.
04521C (DCN5731)