What are the trouble shooting steps for a LDO sensor that is more than 0.5 ppm difference from calculated sample value?

Document ID

Document ID TE4165

Version

Version 1.0

Status

Status Published

Published Date

Published Date 12/05/2016
Question
What are the trouble shooting steps for a LDO sensor that is more than 0.5 ppm difference from calculated sample value?
Summary
These are the trouble shooting steps if the LDO probe reading is different from the calculated value by more than 0.5 ppm.
Answer
If the probe’s reading is different from the calculated value by more than 0.5 ppm, the following items should be checked:
  1. Is the temperature that the probe is reading correct?  The DO reading is very strongly influenced by temperature.  The DO value will change by about .25 ppm per degree centigrade.  The LDO probe monitors temperature using a thermistor that is located on the end of the probe underneath the sensor cap.  Since the thermistor is potted into the probe, the probe will have to be replaced if the temperature reading is incorrect.
  2. Remove the sensor cap and see if the blue and red LED’s are flashing. If either of the LED’s is not flashing, the readings will be incorrect.  Verify that the sc controller’s display is lit up.  If it isn’t then there is a problem with the sc controller.  If the controller was OK, verify that the probe’s connector is screwed all the way into the controller's connector.  If the connection is OK the probe will have to be replaced if the LED’s are not flashing.
  3. Check the red and blue LED signal amplitudes under SENSOR SETUP; DIAG/TEST; AMPL DIAG.  The amplitudes should both be larger than 0.08.  Make sure the sensor cap is screwed on properly.  Check to see if the sensor paint has been scraped off the cap.  If it was scraped off, then replace the cap.  If these were all OK, then there is a problem with the LED’s and the probe will have to be replaced.
  4. Check the values for GAIN CORRECTION and OFFSET CORRECTION under the DIAG/TEST menu.  If the gain and offset values were incorrect, the probe will give erroneous readings.  If the gain has been changed from 1.0 or if the offset has been changed from 0.0, go to the CALIBRATION menu and select SET CAL DEFAULT to reset the probe to the default 1.0 and 0.0 values.  If the readings are still wrong, replace the probe.
Verify that the sensor code that was entered into the sc100, sc200, and sc1000 matches the sensor code on the sensor cap.  Also try replacing the sensor cap with a new cap and see if the readings are corrected

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