Rule of thumb for resistance of heater - when is it dead? 1

[MetalworkerMike]

In the plastic industry we use a lot of heater bands and cartridge heaters and other simple resistance heaters. It's easy to do the numbers to figure out what the resistance of a heater should be, given voltage and wattage, but is there a rule of thumb for how far away from this value the heater can go before it's 'on the way out'? When several heaters are running in parallel on the same controller it gets tricky to efficiently figure out if they're all working properly.

Mike

 

[itsmoked]

Yep you're right hard to tell. Usually it comes down to they open circuit. I don't think any other parameter will work very well. If you can see the current then detecting a bad heater in parallel is not hard.

There are little clip on current idiot-lights you can get that would show at a glance a blown heater.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

Most resistance heaters tend to be either OK or open circuit as Keith says, unless you have a heater which is either so hot that it is oxidising or one that is consumed by the process. Both the latter will experience a resistance increase due to metal loss. Unless it is factored in to the original design, metal loss usually leads to fairly rapid failure.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[jraef]

Well, if you use an Ohms Wheel you can determine it easy enough. If you know the rated wattage of the heater and you know the applied voltage and you measure the current flow, you will know if the heater has the correct resistance value or not. In other words, say you have a heater you know to be rated 100W at 250V. The resistance then should be E²/P or (120 x 120)/100 or 144Ohms. Then I = E/R so I = 120/144 = .83A. If the measured current is lower, the resistance must be higher, indicating a material loss as ScottyUK said.

 

[VEBill]

If it's a heater that reaches high temperatures, then the cold resistance will probably be quite different than the operating resistance. Makes the simple Ohms Law to power approach less than useful.

 

[btrueblood]

We used a rule of thumb that 10% resistance increase over the as-built condition was a near end-of-life condition; this was for tungsten filaments operating at a constant voltage in vacuum (radiatively coupled). The math for this is surprisingly easy, if you assume a uniform evaporation of the filament along its length. As the wire loses mass, the diameter drops and the surface area of the filament decreases, which causes the temperature of the filament to climb, which increases the rate of mass loss, and ... well you can see how this leads to a "runaway" failure condition.

For conductively-coupled elements, there is less likely to be a uniform loss of mass along the length of the wire/ribbon, and more likely that a failure occurs at a local imperfection which forms a "hot spot". Also, the mass loss of the element has more to do with chemical reactions (oxididation) and diffusion of the metal into its surrounding insulation, which are more complex phenomena. Still, the failure mode should have a runaway curve just like the evaporating filament. Logging the voltage/current on a regular basis may help give you some empirical data to help find a good "endpoint" at which you should replace the heater.

 

[jraef]

Oh yeah VE1BLL, didn't think that one through.

But, you could still apply the same approach to monitoring resistance trending against known values, which could probably be used to determine deterioration.

 

[MetalworkerMike]

I respect the notion that these heaters are either good or bad. However, I have had situations where a pair of heaters on one controller kept blowing the breaker, then when I replaced the higher-resistance heater everything went back to normal. I don't have a solid explanation for why this should be the case, but I do know that it is what happened. Since I have had odd situations occur with these heaters before, I was hoping for a 'magic bullet' solution to deciding if they were okay or not. If no such solution exists then I will continue on as I have been.
I thank all respondents for their kind attention.

Mike

 

[OperaHouse]

I suspect with blowing the breaker, the problem was a short to ground. I believe firmly that a change in resistance will indicate nothing. Most failures occur from necking and generate a hot spot in only one place. Failure is very quick. An overall increase in resistance is self protecting.

 

[MetalworkerMike]

Interesting point... the short to ground. I didn't check that, and the heater in question is cast aluminum so it could easily short to ground. Thanks for the tip!

Mike

 

[analogkid2digitalman]

I agree with Operhouse, I have had a unit fail in a likewise manner that kept blowing fuses and verified it with a meter- element was shorted to ground.

When is it dead? When Bones comes along declares: "He's dead, Jim" (Star Trek original series)

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"It's the questions that drive us"
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[itsmoked]

When it stops moving?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MetalworkerMike]

I will start being diligent in testing the element to ground (or at least testing the element to its casing, since almost none of these units are grounded). In fact, megger testing might be the magic bullet that I've been looking for... Hmmm... checking the insulation resistance against 500V might give me the answer that I need... or it might blow the heater, but at least that would sort out the sheep from the goats, so to speak. The heaters run on 240V normally, so 500V would be a reasonable voltage for a megger test, I think. It wouldn't blow anything that wasn't ripe for the blowing, so to speak. I _could_ test them at 250V if anyone thinks that 500V would be too destructive, but IEEE standards use 500V to test 110V motors, so I would expect 500V would be okay for 240V resistance heaters. Any thoughts on that?

 

[ScottyUK]

500V is the standard test for line-neutral on a UK electrical installation so it sounds reasonable to me. Also, don't forget that 240V AC has a peak of 340V, so a 250V DC Megger test wouldn't really prove much.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[IRstuff]

Don't forget that most resistance heaters are made from copper and have a tendency to work harden from flexing, which usually shows up as an increased resistance. Even relatively gentle bending, if repeated enough can cause resistance increases or outright breakage.

TTFN

FAQ731-376

 

[ScottyUK]

Copper? Most heater elements are Nichrome or something similar over here. Do you mean copper sheaths on the elements, or are you thinking of something else like an induction heater?


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[IRstuff]

I was thinking of flex heaters, so "most" was inappropriately broad.

TTFN

FAQ731-376

 

[stevenal]

I think the meggar test may not tell you what you need to know. When the breaker opens, you know you've had a short circuit at that particular point in the heating cycle. A megger test on the same heater after it has cooled may falsely indicate there's no problem.

Suggestions: Use a better heater. Some use a higher voltage (and wattage) rated heater for increased longevity. Or protect each heater separately.

 

[btrueblood]

Oh, yeah. A good measure of the life capability of a conductively-coupled heater is the "watt density" of the filament, i.e. the number of watts divided by the surface area of the wire element. This is a backwards but effective way of knowing the wire surface temperature in operation, obviously hotter wire will degrade more rapidly, so a lower watt density is better. Of course, you can't get this data from a McMaster catalog; you have to either get it from the manufacturer or cut apart some heaters and measure it for yourself (all you really need to know is the diameter of the conductors and alloy used, if you know the resistance and wattage in operation, you can calculate the length of wire required, its operating temperature and resistance).

 

[MetalworkerMike]

I'm hoping the megger testing will tell me if a heater is _going_ to fail, not that it already has. I'm looking to add a predictive element to our preventative maintenance. Right now it amounts to replacing heaters as they fail, but that could be hours or even days after the failure. I don't always have spares, since the heater bands are of different diameters, different widths, and have different hole patterns. Keeping spares for _all_ of them would be quite an undertaking.
I suppose the only other option is to pull the wire boxes apart and try to get amperage readings on each heater and see if one seems to be drawing fewer amps than it should.

Mike