Hi guys,

How to find the emissivity of a bus bar if we have electric tape attached on it?

Have a nice day!!

Hi Jenish,
You need to find out the emissivity of your tape, but this is usually around 0.95. Set this along with the reflected temperature in the image properties to get the true temperature. Next, put a spot meter on the metal. Keeping the reflected temperature the same, lower the emissivity until you get the same true temperature as on the tape. This is the emissivity of the metal.

SG is (nearly) correct but the truth is for any surface with a low-emissivity, this methodology will result in a high degree of error.

Just as it is difficult to measure temperatures of low-emissivity surfaces accurately, it is also difficult to measure their emissivity! And for the same reasons: (1) the emit little relative to their temperature and (2) the reflect their surroundings.

If you really must know the bus bar emissivity, you need to heat it (or a similar, de-energized one)to approximately 100C and test it several times while also controlling the background. Use the average value you obtain. BUT, when you use that value to correct the temperature of the bus bar, expect to have some degree of error, probably substantial.

You are FAR better off to put a high-emissivity "target," such as a loop of electrical tape, on the bus bar near all joints. Measure temperatures at that point and they will be much more accurate.

Life is not as simple as the camera manufacturers would lead us to believe: look up a value, push a button and measure away! For field measurements of any surface below 0.6 (i.e. ALL unpainted metals) expect unacceptably high errors.

thats true.
i just would like to share my exp.
Normally i am looking for cable termination in cable raceways. cable ends are mostly wound with Insulation tapes. normally i take 0.95 emmisivity for cable insulation and consider that is max ( or closest) temp of the cable conductor.
Is that much more difference of emmisivity for cable insulation and PVC tape?
any suggestion for best way of doing cable raceway inspection most welcomed.


thanks & regards

Hi guys,

Thanks for all the information..

Actually we tried what Stephen described but we run out of the range of emissivities.. The reflection is too much.

John,

So, i think what we are doing is correct. Measure the temperature of the tapes and forget about the emissivity of bus bar.

Have a nice day!!

Jenish
Another simple way that I use is to use a contact thermoeter to get the temperature and then alter the emissivity on the camera untill it is the same as the contact probe, that will do it for you (but not on live equipment!)

Vrop,

This method can also work but you will, as I suggested in my post above, find it very unreliable for field measurements of low emissivity materials. You just don't have enough emission, especially at lower temperatures, and you have too much reflectivity to be accurate on a repeatable basis.

The only reliable way to get good temperature is by increasing emissivity. Jenish,yes, I would suggest you are better off measuring temperature on the tape or, if near the connection, on a cable insulation.

Vdrop,

This method of measuring is not as easy as using a reference emissivity. You need a thermocouple with a smaller thermal mass than the busbar, you need to hold it in position long enough to reach thermal equilibrium, you need to hold it still, you need good contact, and then the thermocouple will display its own temperature, which may not be as high as the busbarm itself. If this is not done correctly, it may lead to an emissivity determination greater than 1, and this is impossible. Also as John says, you have achived an emissivity figure for something that is almost impossible to measure the temperature of in a field environment.

Bob / John

Thanks for the feedback. I suppose another good thing about this forum is that you soon learn if what you are doing is misguided. I now understand why this is not a good idea having just completed my level 2 with Bob a couple of weeks ago.