Basics of testing flame detectors: Part 2

Editor's note: This is the second of a two-part series on how to test and troubleshoot various types of industrial flame detectors. The first part appeared in the September issue. Most older flame combustion controls (except CAD cells) incorporate a flame signal jack on the amplifier or on the control housing.

By George Allen October 10, 2004

Most older flame combustion controls (except CAD cells) incorporate a flame signal jack on the amplifier or on the control housing. The flame detector’s current signal can be measured through this jack using a digital multimeter (DMM) with a

During the burner run cycle, the minimum acceptable flame current typically is 1.5—2

On newer microprocessor-based combustion controls, the flame signal is a dc voltage, which can be measured at the combustion controller. Typically, positive and negative test jacks are available somewhere on the combustion control. The exact location of these jacks depends on the design of the control.

Using a volt-ohm-milliammeter (VOM) or DMM, connect the red meter lead to the positive controller or amplifier test jack and the black meter lead to the negative controller or amplifier test jack. The flame signal for the pilot alone, the main burner flame alone, and both together must be steady and a minimum of 1.25 Vdc.

If the flame signal is unsteady, or less than the minimum acceptable voltage, check the flame detector installation and circuitry in the following procedure:

  1. Compare the readings to acceptable minimum and maximum flame signal voltage strengths according to the manufacturer’s specifications for each flame detector and controller type.
  2. A low flame signal reading (current or voltage) indicates the flame detector is not receiving sufficient visible flame radiation, which can be the result of an improperly positioned sight pipe, restricted field of view, contaminated protective window or focusing lens, or a defective detector.
  3. For maximum detector life expectancy, the flame signal should be no more than 5

Testing the CAD Cell

To field test CAD cell operation, use the following procedure:

  1. Shut off the furnace and locate the two wires that connect the CAD cell to the controller. Start the burner, observing applicable safety procedures. Shortly after the burner starts, place a temporary jumper between the terminals on the controller where the CAD cell was wired. Connect an ohmmeter across the CAD cell lead wires to measure its resistance, which should be less than 1600 {Ω} while the flame is present.
  2. Stop burner and remove temporary jumper.
  3. With burner off, check the resistance of the dark CAD cell by placing the meter leads across the lead wires. The resistance should be greater than 20K {Ω}. If the CAD cell resistances are different than specified, recheck wiring and location of the cell. If necessary, replace the CAD cell socket
  4. Reconnect the CAD cell lead wires to the controller. Check the burner sequencing relay control according to the manufacturer’s instructions to verify proper operation.
  5. Remove and inspect the CAD cell. It should be clean for this test. Cover the cad cell with a piece of black electrical tape. The resistance should be approximately 100K {Ω}. If the cell does not respond to a change in light intensity, replace it. If the resistance varies between 1500 and 100K {Ω} as light is added or removed, replace the CAD cell.
  6. Ensure tape, jumpers, and test leads are removed and the CAD cell is properly mounted.
  7. Inspect the entire furnace control circuit.
  8. Place the combustion system back into normal operation.
  9. Flame detector troubleshooting

The flame detector supplies a signal proportional to the flame intensity to the burner controller. The signal must be steady and of sufficient magnitude. However, if the signal is unsteady, or less than the minimum acceptable level, check the flame detector installation and circuitry according to the schematics and documentation provided by the manufacturer. When maintaining and troubleshooting burner controls and flame safeguards, ensure that:

  • The manufacturer’s documentation is available for each type of equipment
  • Wiring connections are correct and all terminal screws and electrical connections are tight
  • The proper flame-failure response time is selected on the controller
  • The amplifier is securely mounted on the controller relay module
  • Flame detectors are properly positioned and cleaned according to the manufacturer’s instructions
  • The correct combination of amplifier and flame detector is used.

There are differences among flame safeguard and combustion controls. Refer to the documentation supplied by the manufacturer for specific maintenance, troubleshooting, and safety procedures. However, you can use the following troubleshooting tips (applicable to most systems) as a general reference:

  1. Check the supply voltage at the incoming ac supply terminals. Ensure that the master switch is closed, connections are correct, and the ac supply voltage is correct.
  2. Check the detector wiring for defects, damage, or deterioration.
  3. For flame rod systems ensure that:
  4. There is enough ground area
  5. The flame rod is properly located in the flame
  6. Temperature at the flame rod insulator is no greater than 500 F.
  7. For visible light detector systems, with the burner on, check the temperature at the detector. If it exceeds 165 F, do one or more of the following:
  8. Use a heat block
  9. Add additional insulation between the wall of the combustion chamber and the detector
  10. Add a shield or screen to reflect radiated heat away from the detector.
  11. For all optical detectors, clean the detector-viewing window, lens, and inside of the sight pipe as applicable.

With the burner on, check the temperature at the detector. If it exceeds the detector’s maximum rated temperature, do one or more of the following:

  1. Add a heat block to stop conducted heat traveling up the sight pipe
  2. Add a shield or screen to reflect radiated heat
  3. Add cooling (refer to sight pipe ventilation in the detector instructions).
  4. Ensure that the flame adjustment is not too lean.
  5. Ensure that the optical detector is properly sighting the flame. If necessary, reposition the detector.
  6. If you cannot obtain proper operation, replace the plug-in amplifier.
  7. If you still cannot obtain proper operation, replace the flame detector.

When working with flame safeguards or any other piece of equipment, it is important to ensure that the plant maintenance staff is properly trained and has thorough understanding of the equipment to be maintained. This includes knowledge of and experience with electrical circuits and the tools used to measure and service them.

Author Information
George Allen is the Marketing Manager for Meterman Test Tools. He can be reached by phone at 206-550-3627 or by e-mail at .