ZR22 Detector Troubleshooting

Meter Checks:

  • Heater: 57-85Ω (typical is 62-67Ω)
    • Typical failure is Open or shorted
  • TC: if the probe is at room temperature -  less than 3Ω, in the stack and hot less than 100Ω (it is a type K Thermocouple)
    • Typical failure is open or shorted to the case.
  • Cell negative wire to the case. It should be a direct short to the case. If resistance is above 5Ω it can cause intermittent/erratic readings.

Cold junctions:

  • RTD type (red/black or red/white wire)
    • Resistance approx. 1100Ω at room temperature, no polarity (2 wire PT1000 RTD). Typical failure is Open or shorted.
  • Transistor type
    • Checks out like a diode. One direction is high resistance (typically in the MΩ) then the other direction will be open. Typical failure will have high resistance both directions or low resistance in both directions.

High Voltage Tests:

Use a 500Vdc Insulation Meter Checks (Aka: Megger). Be sure there are no wires attached to probe before doing this test.

  • TC + to the case. Should be >2MΩ
  • TC – to the case. Should be >2MΩ
  • Cold Junction + to the case. Should be >20MΩ
  • Cold Junction – to the case. Should be >20MΩ
  • Both sides of the heater to the case. Should be >2MΩ
  • If the O2 cell is removed you can also check the Cell positive to the case. It should be >2MΩ

Testing O2 cell:

  1. Perform a 2 point calibration. Check the cell resistance (under the magnifying glass button). Consider replacing the O2 cell if it is higher than 1000Ω. Higher resistance means the response time is reduced and can cause calibration errors if the response is very slow. 
  2. Remove the cell + & - wires from the channel card.
  3. Place a short between the Cell + & - on the channel card.
  4. Turn power off and on with the channel card power under maintenance and allow the channel to come up to temperature
  5. Note: Channel should read ~21% and the reading should be stable. If the unit comes up with a cell failure or the reading is erratic the problem is in the channel card.
  6. Apply Zero and/or Span gas (500 cc/Min). Measure the loose wires at the electronics.
  7. Span Gas (instrument Air) should read close to 0 mV (+/- 5 mV)
  8. Zero Gas Should read as follows:

        0.4% O2        84 mV’s (75-90 mV’s)
           1% O2        67 mV (55-70 mV’s)
           2% O2        52 mV’s (45-55 mV’s)


For a multichannel converter (AV550):

  1. Be sure that the heater and signal cables are landed on the correct channel card.  If the heaters are crossed it can cause this. Try swapping a working channel card with the channel in question. If a different card works properly it is a bad channel card.
  2. Swap probe to a working channel.  If it works on a new channel and not on the old channel the power unit is damaged.

Temperature low or not heating up:

  1. Reset the channel.
  2. If the probe fails to warm up turn off the channel.
  3. Unwire the Heater, TC, and CJ and perform the ohm meter check on the Heater, TC , and CJ.
  4. If you find a failure in one of the above, go to the back of the probe and check them inside the probe housing.
  5. If there is not failure in the probe housing, but there is one at the electronics the problem is in the cable.
  6. If all the values check out in the probe then you need to check out the electronics.
  7. Reconnect the wires from the heater, tc, and cj.
  8. Reset the analyzer/channel, measure the voltage going to the heater. It should be 60-120Vac (Voltage is a modulated voltage but can be measured as AC)
  9. If power is not present check the fuses.
  10. On the AV550 they are on the power unit (just above where the heater wires up). There are 2 fuses per channel.

High Oxygen Readings:

  1. Check for a leak in in the calibration system or cal line.
  2. Remove the fitting from the probe where the calibration gas is connected. Replace it with a pipe plug with Teflon tape. If the problem goes away the problem is a leak in the cal gas line to the probe or in the autocal system.
  3. If the reading is still high it could be a leak inside the probe between the cal gas inlet and the end of the probe by the cell. Remove O2 cell. Plug end of cal line (You can flip the o2 cell over, place a small piece of rubber in between the cell and flange and install the cell to plug off the line). Apply a approximately 1 PSI to the cal line, shut the pressure off and see if it holds pressure.
  4. Bad O2 cell
  5. Run calibration. If cell resistance is above 200Ω replace the cell.
  6. If you get a cal error replace the cell.
  7. Cracked O2 cell
  8. Run Calibration. Apply zero gas at 500 cc/min max. If reading is not stable or it is reading high it is likely the cell is cracked.

Low Oxygen Readings:

  1. Leak in calibration system (AutoCal system or Manual Calibration Panel). 
  2. Plug off cal line at the back of probe. If problem goes away find the leak in the zero cal gas side of the cal system. Zero Gas may be leaking past a valve into the cell.
  3. Bad O2 cell
  4. Run calibration. If cell resistance is above 200Ω replace the cell.
  5. If you get a cal error replace the cell.
  6. Excess hydrocarbons (fuel)
  7. O2 probes control at 750°C, If the process has excessive hydrocarbons (un-burnt fuel) they can burn on the tip of the probe consuming the oxygen at the tip of the probe and making the reading go low.
  8. Excess moisture in process
  9. If excessive moisture is in the process it can come in contact with the O2 probe (750°C) and cause steam right at the O2 cell. If you have excessive steam at the O2 cell it will read low, as well as possibly damage the O2 cell.

Erratic Reading:

  1. Be sure heater and signal wires are in separate conduits.
  2. Flow zero gas at 300-500 cc/min on probe.
    1. If reading continually drifts up the O2 cell is cracked.
  3. Perform a calibration
    1. If readings are unstable and calibration fails see cell troubleshooting.

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