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XDT User’s Manual
8
2.5 Troubleshooting unexpected readings
If erroneous readings are suspected on a newly acquired instrument, compare the serial number
engraved on the sensor sintered filter, to the label on the instrument. The two should be the same;
if they are not, the instrument may not be calibrated with the installed sensor. To troubleshoot
other problems, identify the unexpected reading category in the following table, and consider the
possible causes and appropriate diagnostic action and remedy.
For non-sensor related problems (e.g. no reading on instrument) refer to section 3.6
Symptom Possible Cause Diagnostic/Remedy
Reading is not
changing
Condensation in sample system.
Condensation will occur if the temperature of the sample system, at any point is below
(colder) the dewpoint temperature of the sample gas. Once having formed, the sample
reaching the sensor will have a dewpoint equal to the temperature of the condensation,
regardless of the dewpoint of the source gas.
Slow Response
1. Water vapor in the system.
2. Flow rate too low.
3. Sample pipe too large and/or too long.
4. Unsuitable sample pipe.
5. Leaks.
6. Hygroscopic materials in sample system
It is usually more satisfactory to bleed a sample gas at atmospheric pressure through
the sensor sampling chamber, and to use 1/8” (3mm) o.d. sample pipe.
See below re sample pipe material, also see section 2.3
Dry Reading SpanCheck™, wrongly set, or faulty Sensor. Verify SpanCheck™, or return sensor for full calibration to your representative.
Wet Reading
Leak in system or use of unsuitable pipe. Cure the leak, or replace unsuitable pipe with copper or stainless steel. Flexible con-
nections should be made with PTFE pipe. NEVER use rubber or plastic pipe.
Comparison of readings with manual cooled-
mirror instrument.
This type of indicator reads about 10ºC dry at about -50ºC dewpoint due to tempera-
ture gradients within the device. The error increases at drier levels.
Display Shows
SAT
Prolonged exposure to wet gas. Dry down the sensor, install sensor in either a known dry gas stream i.e. instrument
quality air or dry nitrogen, or place sensor in a dry can or bottle of desiccant and seal
the container from outside air (the shipping container is designed for this purpose)
also see section 3.4.4.3.1
1. Instrument Failure
Disconnect cable from input terminals, if the instrument still reads
SHR the problem
is with the instrument. However, if the instrument reads
OPN then reconnect the cable
to the input terminals and check possible causes 2 or 3.
Display Shows
SHR
2.Short circuit on sensor cable or connections.
Disconnect cable from sensor and if meter still reads
SHR, cure the short circuit in the
cable or connections or replace cable; otherwise check the sensor.
3.Short circuited sensor.
Disconnect cable from sensor and note that the meter reading returns to
OPN. Use a
new sensor, or apply approximately 20V DC to the sensor MOMENTARILY with the
sensor in a known dry condition. Polarity is not important, but the contact MUST be
very brief or the sensor may be damaged. Consult with your representative for a cir-
cuit diagram.
Display Shows
1. Instrument failure.
Short the SIG and SHIELD contacts of the sensor input terminal, if the instrument
reads
SHR the problem is in the cable or sensor, otherwise return the instrument for
service.
OPN.
2. Open circuit on cable. Disconnect cable from sensor and short center pin of plug to the outer shell. If the dis-
play still shows
OPN, repair cable.
3. Open circuit on sensor. Check sensor connection or replace sensor.