This article is intended to address the condition where the incubator effectively maintains the upper air temperature and product temperature but is unable to maintain the lower air temperature to within 0.2 degC of the upper air temperature. This criteria assumes the door ajar switch is operating properly.
Warning: This troubleshooting procedure may involve working in close proximity to live line (mains) voltage parts. Don’t attempt this unless you understand the risks of working with line voltage and methods to keep yourself safe. Your safety is your responsibility.
Theory of Operation
The Norlake large capacity incubator (NSHI331 line) has three temperature sensors: an upper air temperature sensor, a lower air temperature sensor, and a product temperature sensor. The product temperature sensor should be submerged in a tube of glycerin or similar to simulate the specific heat property of the warmed product.
The heater in the incubator is controlled by the upper air temperature. The fan moves warm air from the top of the incubator down to the bottom of the incubator. The circulating fan is controlled by the difference between the upper and lower air temperatures as read by the sensors or runs constantly if configured to do so. When operating properly, the upper air temperature sensor and the heater should work together to maintain the target temperature set point and the fan should work to keep the lower temperature within the configured difference of the upper temperature.
The conditional of low lower air temperature must be caused by incorrect fan configuration or by malfunction of one of the components in the fan control path.
The control path for the fan is as follows:
- The controller detects a condition which requires the fan to run. This condition is: The door is closed AND EITHER the fan is set to run constantly OR the difference between temperatures read by the upper and lower air temperature sensors is greater than the “Fan On Delta” setting.
- The controller turns on output N01 which passes the control signal for the fan. The fan control signal is line (mains) voltage and also provides power to the fan.
- The fan control signal (and power) enters the common terminal of relay #2.
- The fan control signal passes through relay #2 and enters the cabinet to power the fan. The relay is controlled by the fan speed signal from the controller output N02. If the fan is on low speed the fan power will leave the relay through the normally closed terminal. If the fan should be high speed, the fan power will leave the relay through the normally open terminal.
- The fan runs until the “Fan Off Delta” is reached or continuously if so configured.
Troubleshooting
Follow the troubleshooting steps in order. If you make a correction at any step, test the incubator operation before continuing further.
- Verify fan settings. There are 4 different settings that affect fan operation. These are:
- Fan On Delta: When (upper air temperature) – (lower air temperature) > (fan on delta), the fan runs. A reasonable default value for this setting is 0.4
- Fan Off Delta: When (upper air temperature) – (lower air temperature) < (fan off delta), the fan will stop running. A reasonable default value is 0.2
- Cont On: If (upper air temperature) > (cont on temperature) the fan will run at normal speed. Norlake doesn’t recommend setting this below 45 degC. This is at least partially because the heat generated by the fan motor can potentially heat the incubator above the intended setpoint. For example, I have seen the constant on fan maintain the temperature in the incubator at 37 degC when the setpoint was 35 degC.
- High Speed: If (upper air temperature) > (high speed temperature) the fan will run at high speed. This is only used for very high incubator temperatures.
- Verify there is a clear path for air to move down to the bottom of the cabinet. The fan pushes air down the back wall of the incubator so there should be space between the back of the shelves and the back wall. When installed properly, the shelves should stop short of the back when pushed all the way in. Remove any obstructions.
- Verify the sensor calibration. Use a reference thermometer to verify that the incubator temperature sensors are reading an accurate temperature value. You can see the temperature sensor values by pushing the down arrow on the control panel.
- Verify the fan control signal. Follow these steps to verify the fan control signal:
- Open the control enclosure at the top of the incubator. This enclosure contains live electrical parts when the incubator is plugged in, so take appropriate precautions.
- Remove the black and white wires from relay 2. The white wire is the electrical neutral and the black wire is the fan control/power signal from controller output N01. The black wire will carry line (mains) voltage if the fan should be on.
Fan Control Relay
- Connect a multimeter across the black and white wires and set the meter to measure VAC.
- Change the fan Cont On setting to 1 degC (or something much below the current upper air temperature). Now the incubator should be in a state where the fan should turn on when the door is closed and off when the door is open.
- Open and close the door and observe the reading on the multimeter. When the door is open the multimeter should read close to 0 Volts (a few 10ths is ok). When the door is closed, the multimeter should read line (mains) voltage (120 VAC in my case). Repeat this test several times. You should get the appropriate voltages every time. If you get something else (like 0V when you should be getting line voltage or some voltage other than 0 or full line voltage, or something that bounces around), you probably have a bad controller. Contact Norlake to get a new one.
- Verify the operation of relay 2.
- Reconnect the white wire to relay 2.
- Remove the blue and gray wires from relay 2.
- Connect your multimeter across the COM (where the black wire was connected) and NC (where the gray wire was connected) terminals of relay 2 and set the multimeter to test connectivity.
- Adjust the fan High Speed setting so that it is well above the current upper air temperature (if it isn’t already).
- With the door closed, observe the connectivity test on the multimeter. The multimeter should indicate connectivity between COM And NC.
- Adjust the fan High Speed setting so that it is well below the current upper air temperature.
- With the door closed, observe the connectivity test on the miltimeter. The multimeter should indicate no connectivity between COM and NC.
- Repeat steps 4-7 until you are confident the NC terminal is being properly disconnected and reconnected. If the NC terminal is not being properly disconnected and reconnected, contact Norlake to acquire a new relay.
- Move your multimeter probe from the NC terminal to the NO (where the blue wire was connected) terminal of relay 2.
- Adjust the fan High Speed setting so that it is well above the current upper air temperature (if it isn’t already).
- With the door closed, observe the connectivity test on the multimeter. The multimeter should indicate no connectivity between COM And NO.
- Adjust the fan High Speed setting so that it is well below the current upper air temperature.
- With the door closed, observe the connectivity test on the multimeter. The multimeter should indicate connectivity between COM And NO.
- Repeat steps 4-7 until you are confident the NO terminal is being properly connected and disconnected. If the NO terminal is not being properly connected and disconnected, contact Norlake to acquire a new relay.
- Reconnect all the wires to relay 2.
- Verify current draw by fan/check fan motor resistance. If you have gotten this far without finding a problem, you probably need a new fan. Just to make sure check the fan motor.
- Disconnect the white, gray, and blue wires from relay 2.
- Set your multimeter to measure resistance and connect the meter between the white and gray wire. The resistance measured should be between X and Y Ohms (X and Y are something like 160 and 300, I’m working on getting the exact values for each motor type from Norlake). If the resistance is very high, you probably need a new fan.
- Now connect the meter between the white and blue wires. The resistance measured should be between X and Y Ohms (X and Y are something like 160 and 300, I’m working on getting the exact values for each motor type). If the resistance is very high, you probably need a new fan.
- Reconnect all wires to relay 2.
- Place an amp probe around the black wire and set your multimeter to measure AC current.
- Set the fan Cont On setting to something well below the current upper air temperature and the fan High Speed setting to something well above the current upper air temperature.
- With the door closed, the fan should be running at low speed. The current draw by the fan should be something between X and Y amps (X and Y are something like .5 and 1, I’m working on getting the exact values). If the current is 0 Amps, you need a new fan.
- Set the fan High Speed setting to something between Cont On and the current upper air temperature.
- With the door closed, the fan should be running at high speed. The current draw by the fan should be something between X and Y amps (X and Y are something like .5 and 1, I’m working on getting the exact values). If the current is 0 Amps, you need a new fan.
If you get all the way to the end of this troubleshooting guide without identifying your problem, you should do 2 things.
- Contact Norlake and let them know what is going on. Norlake’s incubator technical support is very good and they can most likely help you.
- Let me know what you found out either in the comments or @jasondoolittle on Twitter so I can improve this guide and help others.