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.


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%%MDASSML%%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

    1. 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.

        1. 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: