Emissivity and Temperature Measurement

In the operator's manual for the IR temp sensor I use, it says that for certain types of surfaces a better reading will be obtained by putting a piece of ordinary masking on the surface, then measuring the temp of the masking tape.

Yes quite simple once you understand the nature of emissivity, and what you are actually measuring...My ir thermo gun has emissivity settings on it....

http://energy-models.com/forum/why-should-roofs-have-high-emissivity

Nice find!

The first one shows that a polished metal roof has less emissivity, which impacts on its ability to radiate heat away. It does not prevent it from getting hot or have a high temperature. As a matter of fact the low emissivity means it will have to become hotter to radiate the same amount of energy away.

Does this mean that temperature measurements of the ground from satellites are incorrect?

Thanks, SE. There's lots of good info there. I like the part in the link where they talk about how the data has been fiddled with, and how the surface temperature measurements with all its problems is still more accurate than the satellite data. I think we need a lot of deep ocean sensors. - GA

Of course the reading will not be perfect. With IR measurements, you would need to identify all objects in the view and characterize the emissivity of each surface. In practice, an assumption is made on the average emissivity. The final interpreted result will depend on the initial assumption and the implied calibration.

A large part of the current debate is the questioning of the initial assumptions and the bias factors loaded into the calibrations.

It's unfortunate that most people seem to think IR temperature readings are accurate, not realising how important emissivity is and how most materials have variable emmissivity throughout the IR spectrum.

Most are IR thermomoeters are single sensor units so have to have a manual input of the emissivity, which is a guess at best. Two sensor units are better as they calculate the emissivity, but they are pricey.

https://www.sciencedirect.com/science/article/abs/pii/092427169390061Q

Yes I think most people just use them as a comparative tool after using them for some time, this was working good at an 80° reading it's now reading 70° and the conditions are the same, something has happened....all these breakers read 80°, and this one is 130°....after checking hundreds of motors I know the operating temp is good below about 140° give or take and if it hits 170°, there's a problem....In other words checking equipment on a regular basis you get to know the normal reading, and if it reads much higher, there is a problem....so it's not so much the accuracy as it is the accumulated knowledge of readings at normal operating conditions...I always set my ir thermo gun to save the highest reading while shooting the area...I have found it to be one of my most useful and time saving tools....

Fastest troubleshooter in the west.....er South...

It's enlightening to think that those who know not of what they speak, would presume to use phrases such as, " biased presentation," and "fiddled with, when they know nothing of the subject.

Temperature and Radiation

..."Total solar irradiance (TSI) and spectral solar irradiance (SSI) upon Earth

Total solar irradiance (TSI) – the amount of solar radiation received at the top of Earth's atmosphere – has been measured since 1978 by a series of overlapping NASA and ESA satellite experiments to be 1.361 kilo⁠watts per square meter (kW/m²).^{[18][20][21][22]} TSI observations are continuing today with the ACRIMSAT/ACRIM3, SOHO/VIRGO and SORCE/TIM satellite experiments.^[23] Variation of TSI has been discovered on many timescales including the solar magnetic cycle ^[24] and many shorter periodic cycles.^[25] TSI provides the energy that drives Earth's climate, so continuation of the TSI time series database is critical to understanding the role of solar variability in climate change.

Spectral solar irradiance (SSI) – the spectral distribution of the TSI – has been monitored since 2003 by the SORCE Spectral Irradiance Monitor(SIM). It has been found that SSI at UV (ultraviolet) wavelength corresponds in a less clear, and probably more complicated fashion, with Earth's climate responses than earlier assumed, fueling broad avenues of new research in “the connection of the Sun and stratosphere, troposphere, biosphere, ocean, and Earth’s climate”.^[26] "...

..."Solar intensity variation

Further information: Solar variation

Space-based observations of solar irradiance started in 1978. These measurements show that the solar constant is not constant. It varies on many time scales, including the 11-year sunspot solar cycle.^[24] When going further back in time, one has to rely on irradiance reconstructions, using sunspots for the past 400 years or cosmogenic radionuclides for going back 10,000 years. Such reconstructions have been done.^{[34][35][36][37]} These studies show that in addition to the solar irradiance variation with the solar cycle (the (Schwabe) cycle), the solar activitiy varies with longer cycles, such as the proposed 88 year (Gleisberg cycle), 208 year (DeVries cycle) and 1,000 year (Eddy cycle)."...

There are forces at work that have not been as of yet fully studied and understood...

https://en.wikipedia.org/wiki/Sunlight

http://ktar.com/story/1995130/new-solar-minimum-could-change-the-world-economy/

https://www.sciencedirect.com/science/article/pii/S0031018207005214

The link you provided needs correction:

".... Both the sun and Earth's surface behave as blackbodies. An object that absorbs and emits all possible radiation at 100 percent efficiency is called a blackbody. For this reason, the following two laws (Stefan-Boltzmann and Wein's laws) can be used to explain the correlation between temperature and radiation for the sun and Earth. ...."

Saying the Earth behaves as a blackbody is disingenuous or ill informed as to the meaning of a blackbody. If the Earth's surface behaved as a blackbody, you would not be able to see it at its current temperature as insufficient radiation in the visual range would be emitted and no reflection would be available.

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Now, what was that you were saying about people "who know not of what they speak''?

Another gotcha is reflection. A stainless steel refrigerator door with a matte finish is a polished mirror at 10 microns. You can be easily fooled by the reflection of your body which glows brightly in the thermal infrared.

I was once looking at the A/C unit with the thermal camera. I spotted a hot spot which turned out to be my reflection in a metal hose clamp.

Accounting for or correcting for emissivity is a major error in IR scan reports by people who don't understand the topic. There are good lists of emissivity of materials, which can be used to help. The suggestion of a piece of masking tape--or in my case, electrical tape--to give a surface a known/reliable emissivity is a very good suggestion. Many times, what one wants is a comparison of "temperatures" of similar surfaces or devices.

Often, when doing scans, I would do a check of a suspect surface or material with a multimeter (Fluke 87V) and thermocouple. The highest temperature I ever recorded in an electrical panel was approx. 650-degF on a busbar to which a breaker was bolted. Traced it to a very loose nearby bolted connection, and confirmed it with a thermocouple! Needless to say, we did an immediate replacement of the panel and the damaged breakers.

--JMM

The camera will normalize its picture with black for the "coldest" object in the view and white for the "hottest". The false color scale only shows color equivalents for measured thermal energy received by the camera, and can be with a range as little as 2ºC or as much as 400ºC. What was the temperature range for the false colors the camera image showed?

The image is impressive, but without the color scale it is meaningless.

--JMM

In a case like your picture above, calibration is mute. It's the difference that matters. This picture is worth a thousand mathematical formulas.