Thread: incorrect temperature reading

I have just made a pipe half of which is copper and the other half is acrylic tube (soft). If I fill it with hot water the copper feels hotter than the acrylic, but if I put my temperature reader up to it the acrylic reads as being hotter - copper = 27 deg.c acrylic = 32 deg. c.

This cannot be correct! I am not able to check if the readings are correct with another instrument.

How can it be explained - does it look wrong to anyone else? Could there be different wavelengths that my instrument (cheap off ebay) is reacting to (say shorter from the copper but longer wavelengths from the acrylic)?

Originally Posted by fatman57

I have just made a pipe half of which is copper and the other half is acrylic tube (soft). If I fill it with hot water the copper feels hotter than the acrylic, but if I put my temperature reader up to it the acrylic reads as being hotter - copper = 27 deg.c acrylic = 32 deg. c.

This cannot be correct! I am not able to check if the readings are correct with another instrument.

How can it be explained - does it look wrong to anyone else? Could there be different wavelengths that my instrument (cheap off ebay) is reacting to (say shorter from the copper but longer wavelengths from the acrylic)?

It depends on the principle how the thermometer works. Is it a metal probe? Then it probably contains a heat dependent resistor that produces a voltage that is proportional to the heat that is transferred. If it is a bolometer, it might also measure the radiative heat from the hot water inside instead of only the current heat through the material of the container. Are you measuring the water (inside) or the container (outside)? Note that temperature does not only depend on the heat (energy) but also on the heat capacity of the material. Glass has a higher heat capacity than copper, you need more heat to reach the same temperature increase.

Is it an infrared thermometer that doesn't contact the surface? If so you are measuring the radiative flux which will be higher for a black object (plastic pipe) than for a shiny metallic object (copper pipe) at the same temperature. You have to correct the reading for the surface emissivity.

The temperature readings are probably correct.

Something is hot to the touch because it is transferring heat to the skin. A metal object will transfer heat to the skin faster than a plastic one, even if the metal is at a slightly lower temperature.

Interesting observation.

i found it an interesting find.

it is an infrared thermometer. what I want to measure is the temperature of the surface as an indication of how much heat it is giving out.

I expect the acrylic to give out less heat than the copper.

If I fill the pipes, wait for them to warm up then dump the water out and take a reading do you think I will get a better result?

Why will the Radiative flux be higher for a black object rater than a metal one?

Originally Posted by fatman57

Why will the Radiative flux be higher for a black object rater than a metal one?

For the same reason as a black object will absorb more heat sitting in the sun than a reflective one. A good absorber is a good radiator.

fatman57,

What do you mean by how much heat an object is giving out?

In physics we have temperature and heat transfer. They are two different things.

If you have an instrument called a thermometer it gives you the temperature.

If you touch something with your hand you are experiencing heat transfer.

It is perfectly appropriate they don’t always agree.

I have had similar experience with insulated pipes - the measured temperature of the well insulated part being higher than the less well insulated. I am inclined to think about it along the lines suggested by mikelizzi but there are still puzzling aspects to it. It could be that the thermometer is locally cooling part of the copper pipe and that this happens more with copper than acrylic because copper is a better heat conductor than acrylic. Most thermometers would be better conductors than your finger. However, I wouldn't expect a thermometer to conduct away enough heat to make such an effect noticeable.

(In my case, there wasn't any inconsistency between the measured temperature and what the pipe felt like to touch, but the warmer section of pipe was the more insulated section - opposite to what one would expect. I did wonder if the water in the better insulated portion was actually at a higher temperature because of the insulation. However, as the water was flowing through the pipe continuously, this seems unlikely. My measurements were made with a thermometer specifically intended for surface temperature measurements).

i see the sense in what Janus says and it does explain the problem - only thing is by 'black' object i was referring to a use of the term by Bunbury - in my case it isnt black (transparent) but is a form of plastic (acrylic).

The point of me choosing acrylic was so it would conduct less heat. yes temperature and heat transfer are 2 different things but i would have expected the insulating material to not let as much heat through.

Is it likely the case then that the acrylic actually is warmer but is not giving out as much heat transfer as the copper?

Does your infrared thermometer have an adjustment for the emissivity of the surface that you are reading, and did you make the adjustment? The instrument reading is highly dependent on emissivity, with that of polished copper being much less than opaque plastic.

Originally Posted by Harold14370

Does your infrared thermometer have an adjustment for the emissivity of the surface that you are reading, and did you make the adjustment? The instrument reading is highly dependent on emissivity, with that of polished copper being much less than opaque plastic.

nope - it is a cheap item from ebay.

is the emissivity high simply because the acrylic is transparent?

If you look at this table you'll see the emissivity of polished copper is 0.05 while that of plastics is 0.91. Your copper probably isn't polished so its emissivity will be higher than 0.05 but not as high as for plastics. You could try to correct for the difference in emissivity by painting both surfaces matt black, if there isn't an adjustment you can make on the instrument.

http://www.engineeringtoolbox.com/em...nts-d_447.html

Originally Posted by fatman57

Originally Posted by Harold14370

Does your infrared thermometer have an adjustment for the emissivity of the surface that you are reading, and did you make the adjustment? The instrument reading is highly dependent on emissivity, with that of polished copper being much less than opaque plastic.

nope - it is a cheap item from ebay.

is the emissivity high simply because the acrylic is transparent?

No, the emissivity of the plastic is different than the copper because the copper is shiny and reflects more light. Get a hold of the operating manual for your meter and read it.

Originally Posted by Bunbury

If you look at this table you'll see the emissivity of polished copper is 0.05 while that of plastics is 0.91. Your copper probably isn't polished so its emissivity will be higher than 0.05 but not as high as for plastics. You could try to correct for the difference in emissivity by painting both surfaces matt black, if there isn't an adjustment you can make on the instrument.

http://www.engineeringtoolbox.com/em...nts-d_447.html

Edit: this is a better table of emissivities: http://www.infrared-thermography.com/material-1.htm

awesome, thanks.

i would have said more mass would have decreased emissivity as the radiation has to get past more mass before reaching a surface - wiki has it that the thinner the material the more reduced will be the emissivity.

Originally Posted by fatman57

wiki has it that the thinner the material the more reduced will be the emissivity.

That's interesting. In dealing with solids such as radiation from steel pipes and refractory concrete in furnaces it's assumed that emissivity is just a surface property, not affected by thickness. When considering gases the geometry of the enclosure has to be taken into account so the radiation is averaged over different depths or thicknesses using a variable called the mean beam length, but this is a multiplier applied to the gas emissivity - the emissivity per se doesn't change. Wiki's statement may be correct but it seems at odds with what we use in furnace design. Maybe it's a question of scientific correctness versus engineering prgamatism.

Originally Posted by Bunbury

Originally Posted by fatman57

wiki has it that the thinner the material the more reduced will be the emissivity.

That's interesting. In dealing with solids such as radiation from steel pipes and refractory concrete in furnaces it's assumed that emissivity is just a surface property, not affected by thickness. When considering gases the geometry of the enclosure has to be taken into account so the radiation is averaged over different depths or thicknesses using a variable called the mean beam length, but this is a multiplier applied to the gas emissivity - the emissivity per se doesn't change. Wiki's statement may be correct but it seems at odds with what we use in furnace design. Maybe it's a question of scientific correctness versus engineering prgamatism.

I have seen many lectures where the physicist proves engineers wrong with his science..........but I would hope that science and engineering are MOSTLY walking hand in hand!

I hope I fully assumed correctly that the surface makes a difference due to resonance much like the surface of the sun. It is very interesting that it acts as a more effective insulating layer this way - so in that respect the most efficient type (one with a very low emissivity) would be a thin but very reflective layer.

In the National Ignition Facility they use thick Glass with Neodymium to store the laser energy and build it up until they release it - so a thinner layer will store less light energy.

Even so - I find it contradictory that a thinner layer will reduce the emissivity as (excluding the surface layer effect) it will have a greater optical depth with a resultant increase in resistance to radiation/light trying to penetrate it.

Annoying as I still have no direct answer - I have learnt lots since but in my current application I have transparent acrylic pipes which are supposed to be more insulative than the copper ones they lead to. Hot water will be fed through and I want most of the heat to travel through the plastic then on to the copper where it is dispersed into the environment.

I will make the copper pipes dull to reduce their surface effect but would still like to know for sure if the acrylic is actually transferring more heat to the environment than the copper - even though it doesn't feel like this to the touch.

Originally Posted by Bunbury

Originally Posted by fatman57

wiki has it that the thinner the material the more reduced will be the emissivity.

That's interesting. In dealing with solids such as radiation from steel pipes and refractory concrete in furnaces it's assumed that emissivity is just a surface property, not affected by thickness.

Since we are talking about an optical phenomenon, I would think that if the surface looks the same, it will emit the same. I'm thinking it would be like the thickness of a paint film. With a thin film of paint, you can see through it to the layer beneath. After a few coats, it doesn't make a difference any more, though.

Originally Posted by fatman57

I will make the copper pipes dull to reduce their surface effect but would still like to know for sure if the acrylic is actually transferring more heat to the environment than the copper - even though it doesn't feel like this to the touch.

The acrylic will transfer less heat by conduction, more by radiation. So even if you had an accurate surface temperature measurement, you still wouldn't have an answer to your question, without some additional data, like if the pipes are outdoors, is it day or night, how much air moverment there is, etc. If you want to keep the heat in, why not insulate the plastic pipe?

thanks harold.

your description about the optical phenomenon sounds right - but still contradicts what is on wikipedia...........what people are saying in the 'classical' sense is that in terms of mass more is more and less is less..........is wiki wrong on this issue?

makes sense about conduction and radiation. I wanna keep the plastic pipe contents visible so it might be something I have to live with............funny thing is that the pipe is made out of an insulating material (lets leave that one there ey!).

Last thing that gets me - conduction is only achieved via the radiation from the source, but it has to be in close contact for it to work - so in this respect radiation and conduction are the same but radiation can happen over longer distances (is this correct?). Would it be correct to state that you would say something is heated via conduction if the effect is greater than by 'radiation' alone?

Originally Posted by fatman57

Last thing that gets me - conduction is only achieved via the radiation from the source, but it has to be in close contact for it to work - so in this respect radiation and conduction are the same but radiation can happen over longer distances (is this correct?). Would it be correct to state that you would say something is heated via conduction if the effect is greater than by 'radiation' alone?

No, it wouldn't be correct at all. Conduction and radiation are two completely different heat transfer mechanisms. The rate of conduction will be proportional to the difference in temperature, or pretty close to it. Radiation is proportional to the fourth power of the absolute temperature.
http://en.wikipedia.org/wiki/Stefan%...3Boltzmann_law

Edit - Wikipedia is not wrong about the optical thickness, but for your case it is pretty much irrelevant. Once the material is thick enough to be opaque, there is no practical difference in the emissivity of a thin or thick walled pipe.

thanks - thats a lot of radiation!

Its my fault for needing transparent pipes really where i need low emissivity...........

as far as I am concerned the issue is solved - but for the record why do I feel less heat from the acrylic transparent pipe than I do from the copper one even though the acrylic is technically more emissive? is it to do with the wavelengths?

Originally Posted by fatman57

thanks - thats a lot of radiation!

Its my fault for needing transparent pipes really where i need low emissivity...........

as far as I am concerned the issue is solved - but for the record why do I feel less heat from the acrylic transparent pipe than I do from the copper one even though the acrylic is technically more emissive? is it to do with the wavelengths?

Heat transfer from the inside of the pipe to the surface is mainly by conduction. Heat transfer from the surface to the surroundings is a combination of conduction, convection and radiation. With the transparent pipe, there will be a little bit of radiation from the water in the pipe, but it's not much, I don't think. It wouldn't heat the surface of the pipe. The radiation is the part you would feel by putting your hand near it without touching it. Now if it was glowing red hot, you'd feel a lot of radiation. Radiation is what you feel when you are standing in front of a fire.

The acrylic is a better insulator than the metal, so there is less heat transfer by conduction to the surface. Furthermore, even if it was the same temperature as the metal pipe before you touch it, it would feel cooler because the conduction to your hand is slower, so your hand is able to cool the surface off a bit.

Originally Posted by Harold14370

Originally Posted by fatman57

thanks - thats a lot of radiation!

Its my fault for needing transparent pipes really where i need low emissivity...........

as far as I am concerned the issue is solved - but for the record why do I feel less heat from the acrylic transparent pipe than I do from the copper one even though the acrylic is technically more emissive? is it to do with the wavelengths?

Heat transfer from the inside of the pipe to the surface is mainly by conduction. Heat transfer from the surface to the surroundings is a combination of conduction, convection and radiation. With the transparent pipe, there will be a little bit of radiation from the water in the pipe, but it's not much, I don't think. It wouldn't heat the surface of the pipe. The radiation is the part you would feel by putting your hand near it without touching it. Now if it was glowing red hot, you'd feel a lot of radiation. Radiation is what you feel when you are standing in front of a fire.

The acrylic is a better insulator than the metal, so there is less heat transfer by conduction to the surface. Furthermore, even if it was the same temperature as the metal pipe before you touch it, it would feel cooler because the conduction to your hand is slower, so your hand is able to cool the surface off a bit.

thanks - that does explain it.