Oven temp is screwy :)

[Davek0974]

Just got a new oven and have a problem (yes i'm waiting to hear from the supplier)

The temp controller was off, i always like to know my temps are ok especially with a new oven and use a couple of digital probes to check.

I wanted 160c so set that on the controller, warmed it up and it went way past and stayed there so i thought id check the probe type was set right, type K was supposed to befitted and that was set ok.

I managed to get the temp right but it took an offset of some 40c in the controller to get her to run at 160 when set at 160.

The thing now is that it starts at 55c when you turn on and it should read 15c (today) but its ok at 160c, what i am worried about is will it be right at say 240c or not. I can test it but i've never seen this with a controller before unless the probe fitted and set did not match.

Any thoughts?

I'll throw it back to supplier for help but the thing is too big to return so I don't mind fiddling myself but any ideas? Does anyone check their oven temps?

 

[daleyd]

Domestic kitchen oven or an industrial type?

What controller is fitted? I work for a company that specialises in temperature control and on our controllers you can have any one of a number of different types of sensors fitted, but if you select the wrong one in the configuration (eg type k instead of type s for example) you can be miles out.

 

[Davek0974]

Its a semi-industrial style oven, 4.2kW 750x750x1200 got from a guy that makes them to order on eBay.

Its built really well, thing is massive and weighs a ton which is why i'm not overly bothered in getting returns etc as the value is in the build, for me at least.

The controller is a chinese Pid style square one, usual clone of well known makes, SSR control.

I asked the guy and he promised it was a K style thermocouple and in the setting at shows K style selected, I might try J style as that is a very popular probe, see if the readings align better.

If not I'll pop the side off and look at the probe wire colours, they 'usually' are coded.

 

[daleyd]

Also make sure they’ve used the correct compensating cable between the probe and the controller - you can’t (or at least shouldn’t) use standard cable as it introduces errors when measuring thermocouples.

If you’ve got an accurate (millivolt resolution) multimeter you can measure the voltage at the terminals which should correspond to a certain temperature, which will tell you if the probe is reading at least in the ballpark. Link to a calculator from fluke below, at 15 degrees c you should read something like 0.6 mv whereas at 55 it’ll be more like 2.3 mv

https://eu.flukecal.com/Thermocouple-Table-Voltage-Calculator

 

[Davek0974]

Great, thanks

 

[piman]

Hello Dave,

when checking the colours make sure the polarity is correct at the controller, as this will give a completely wrong reading if crossed.

Alec.

 

[Davek0974]

Hello Dave,

when checking the colours make sure the polarity is correct at the controller, as this will give a completely wrong reading if crossed.

Alec.

Yeah,

I have replaced the thermocouple with a new one, K type as marked on packet, same style tip etc.

Results are just as bad so far, seems about 30-35 deg discrepancy between controller and a probe after soaking for 30 minutes to stabilise.

The place he has mounted the probe in the oven gives a very slow response - its a very short tip and protrudes into the box about 10mm in the front/top/right corner, i think it measures the oven wall as much as the air inside - trouble is i have no idea how this relates to powder coating.

When you open the door, the hot air escapes and my probe (in free air) will drop rapidly too, the oven temp however does not as the walls have mass so cool slower.

But when it heats up, the air inside will get very hot due to the lag in the walls getting hot - with the controller on 100c, from cold, the air will hit 250c before the display hits 100c, then the air temp will drop as the heater turns off and things will slowly stabilise.

So, is it best to measure the air - probe well into the cavity OR measure the box - probe near the walls?

I've done a fair bit of process control but not relating to powder coat.

My summary on the discrepancy is that the controller is a POS and i'll probably throw an Omron or Watlow in there soon.

 

[daleyd]

I’d measure the air, unless there is a need to protect the heating element from getting too hot, as this is where whatever your heating is going to sit.

For critical applications like heat treatment (particularly aerospace etc) we will have an oven probe and also have probes attached to the parts being heated (load probes) to ensure they receive the correct amount of heat. Things like soak times are only started when all load probes are within spec.

 

[Davek0974]

Ok, ta

I'll move the probe further into the cavity and apply a small mass to it just help stabilise things a little.

I haven't checked temp gradient yet, it does have two circulating fans though so should be ok.

 

[daleyd]

I’d double check it’s not located where is for a reason, usually to protect the element, but otherwise it makes more sense to me to measure the air.

If it has a second channel could you add another probe for indication? I’ve done systems in the past with load control but a limit on the element to protect it, though that’s probably overkill for a simple oven

 

[Davek0974]

Yeah its nowhere near the elements, way up top.

I've moved it into the box a couple of inches, mounted on a small bracket, responds a bit faster now but not silly fast.

Just running a test on it then i'll do an auto-tune, see what happens.

 

[Davek0974]

Seems ok at present, after moving the probe it behaves better, maybe the area the probe was mounted was a cold spot?

Controller calibration adjustment is now only 10 degrees.

There is no temperature gradient either, maybe 1-2 degrees top to bottom, i think thats pretty good.

 

[hotponyshoes]

The Chinese pid temp controllers I have used have all been pretty good out of the box but I think most of them have a calibration routine you can run (involves ice water and boiling water) might be worth finding the instructions for yours in case it has been run with it already connected or something?

Another option is wireless thermal probes, you can then just chuck them in the racks with the parts and monitor where ever in the oven you want, cheap version on eBay, search "wireless bbq thermometer "