Monitoring function of a heating element

[Ray Diddens]

I am trying to find a way to monitor a heating element once it has reached it maximum operating temperature, however without using a temperature sensor.

Thermocouple type temperature sensors are just too slow in reacting, once the heating element has been turned off. IR sensors work but are too expensive for my application.

I need to know once I am below a certain temperature, as well as understand more or less once I have achieved the heating elements maximum (or stabilized) temperature.

It is not important that I know what the actual temperature is. I am hoping there is something I can monitor within the electrical circuit to identify my acceptable low temperature limit.

I would assume there is something that can be monitored within the electrical circuit while the heating element is warming up, and the this unit of measurement should stabilize once the heating element has reached its maximum operating temperature.

Any ideas on how this can be achieved?

 

[KMoffett]

Thermocouples are too slow? What mechanical type TC and where located...relative to the heating element?

Ken

 

[Ray Diddens]

I am using a hot surface igniter (heating element) for LPG ignition. The heating element reaches about 900 degrees C, in about 35 seconds. The heating element is a small tube, about the size of a Sharpie. I have tried placing the thermocouple probe inside the heating element, as well as just on the outside of the element. Either way, the reading from the thermocouple reacts much slower than the element itself. Heating up is not my problem. Cooling down is my problem. If the heating element powers down for some reason or another, the temperature reading on the thermocouple will read a temperature well above the ignition point of LPG, even when the actual heating element has cooled below this temperature. I will get ignition for about 10 seconds after the heating element has been turned off, however the thermocouple remains at well above 800 C for about 1 minute. I don’t know if this is due to the probe itself taking a while to dissipate its own heat. That is what I am assuming.

 

[KMoffett]

Do you have a link to the TC probe?

Ken

 

[Ray Diddens]

I could give you all the spec's on the TC, however I shop tested it and it just does not work for my application. My goal is to eliminate the TC completely and work directly off the circuit of the heating element.

I have been doing a bit of research on the internet. I am going to try and perform a test with a volt meter to monitor the amperage draw on the circuit as the heating element is warming up. I assume there is a possibility the amperage draw will be higher as the element heats up, and then possibly stabilize itself at a lower amperage once it has reached its full operating temperature. This would give me the ability to understand when it is warming up, and when it has reached its maximum operating temperature. I can monitor this with a current transducer, and integrate it into my system. The heating element runs on 220VAC single phase and draws a maximum of 170 Watts, so possibly if I monitor milliamps, there will be enough of a change between warming up and full temp to identify if I am at full temp. If anyone has a better idea the input would be greatly appreciated. Thanks.

 

[KMoffett]

Most of a TC's speed is based on its mass. If it's a bare TC, two welded wires, then the response is in fractions of a second. If it's encased in a metal sheath or is attached to a larger metal surface, then that stretches the response time.

Most heating elements (nichrome) draw less current as they heat. It has a positive temperature coefficient, as their resistance increases with temperature.

Monitoring the current and watching it plateau should tell you when it has reached maximum temp.

Ken

 

[Ray Diddens]

Thanks Ken, greatly appreciated. Yes, the TC I was using was incased in a stainless steel probe. That must be why my reaction time is so slow. I will run some tests and see is there is enough of a drop in current to safely monitor the heating element. Thanks again.