Roberto Rinaldi
ITC Italy Technical Training and Support
ITC Level III

This is a typical question from people in the chemical or oil industry using thermography for thermal analysis inside furnaces, but the principles can be extended to any application where an object temperature is desired when viewing through a flame.

The purpose is to view thermal image of an object and, when possible, to measure its temperature through flame and combustion gasses.

When looking through flames, we have to deal with something that has variable transmission and emission characteristics with wavelength based on the fuel being burned. The fuel type determines if you have dusty or clean flames; for example an oil based fuel generates more particles than natural gas or methane. A burner using methane gas is the ideal situation for thermal analysis inside a furnace.

For object analysis in a combustion chamber, such as refractory, pipes or burners, we need to deal with a high concentration of carbon monoxide, carbon dioxide, and water vapor. It is therefore necessary to consider the spectral response of those gasses within the IR band to choose the right IR equipment for the application.

                                                
Visible image through a furnace door               Atmospheric transmission chart for one nautical mile

Observing the spectral transmission chart of air, we can see that for the two IR spectral bands, Mid Wave (2-5 µm) and Long Wave (8-14 µm), there are several wavelength areas where the transmission is very low due to gas absorption. Of course the chart shown below is for a standard air sample of one nautical mile, which is not the case inside a furnace where the above mentioned gas concentrations are much higher.

If we consider the high object temperatures inside a furnace, the related thermal emissions are moved toward the mid wave IR band, according to the Plank curves. The combination of gas transmission and radiation intensity from objects generates a situation where a Mid Wave camera provides better results in term of image quality and measuring reliability.

This is confirmed by the two thermograms below where the same subject is viewed and measured in both IR wavelengths.

      
Mid wave IR image (2-5µm)                  Long wave IR image (8-12µm)
Notice that you can see the flame itself much more clearly in the longwave image.
This influences the apparent temperatures we read from the tubes.

In these two pictures you can see that a mid wave camera using a special flame filter at 3.9 µm provides better clarity and detail. At this wavelength the combustion gasses are much more transparent.

      
Mid wave IR image (3.9µm)                Long wave IR image (8-12µm)
Notice the difference in clarity and the freedom from
interfering combustion gases in the mid wave image.

The image generated by the long wave camera is affected more by water vapor and therefore more "foggy". The apparent temperature level is also much higher due to this effect, therefore the readout is influenced by the combustion gasses and not only related to the analyzed objects.

ITC has a simple procedure that can help you more accurately measure tube temperatures in fired heaters. You can find these documents on the web at the following links:

The inspection procedure

A Heater Reporter template (right click your mouse to save this file with extension ".RET".)

A Heater Template in pdf format (if you don't have a recent version of Reporter)

And a bonus document on heater tube inspection

Straight from the World Wide Web, here is what's going on in the world of infrared thermography.

• Medical Breakthrough -- Arthritis Thermal Scan
  WQAD, IL - February 23, 2005
  New technology gives Tina Hight a chance to see something a normal x-ray can't. Using thermal imaging, researchers can tell if she has early signs of arthritis ...

• Magneto keeps assembly lines running
  Mississauga News, Canada - Feb 21, 2005
  ... industry. They still rewind motors, but they also do vibration analysis and use infrared thermography to detect potential problems. ...

• De plane! De plane!
  Penn Live, PA - Feb 19, 2005
  ... and Bald Eagle State Forest in Union and Centre counties will be among the 250,000 acres of state forest land to be scanned with an infrared camera from a plane ...

• Small UAV provides eyes in the sky for battalions
  United States Army (press release) - Feb 18, 2005
  ... that attach to the nose of the plane, an electrical optical camera that sends data either through a nose camera or a side camera, an infrared camera in the nose ...

• SMOKING: CIGARETTES CAUSE TEMPORARY FINGERTIP AMPUTATION
  Agenzia Giornalistica Italia, Italy - Feb 18, 2005
  ... pressure. Experimental research relied on two analytical methods: remote thermography and Ambulatory Blood Pressure (ABP). Tests ...

• Proof that the Queen should invest in double glazing
  Guardian, UK - Feb 15, 2005
  This image, taken with a £40,000 thermal imaging camera, reveals how much heat is lost from the draughty windows of the royal household on a daily basis. ...
  
   

• Lords turn up the heat in a political hothouse
  Times Online, UK - Feb 15, 2005
  ... The hothouse atmosphere of the Palace of Westminster is shown vividly for the first time in the photograph taken on a thermal imaging camera. It is intended to ...
  
   

• Heat-seeking the invisible killers
  IST Results, Belgium - Feb 11, 2005
  ... guidance – like that provided by a unique European infrared-camera system – on ... Multispectral thermal imaging is not a new idea, admits Sahli: “It detects ...

• Understanding investigation techniques
  Horse & Hound Online, UK - Feb 9, 2005
  ... not need to be specially prepared for thermal imaging, and as the technique is completely non-invasive, there is no recovery period. Thermography costs around ...

• Saturn Has an Unusual Hot Spot
  Universe Today - Feb 4, 2005
  ... Future work observing Saturn will include more high-resolution thermal imaging of Saturn, particularly due to the fact that the larger polar vortex region may ...
  
   

• Waking up to health risk
  Brainerd Daily Dispatch, MN - Feb 3, 2005
  ... eye movement and breathing. An infrared camera lets technicians monitor body positions during sleep. Greatens said sleep disorders ...

Here is a selection of recent new threads by IR Community members. Feel free to click the links, see how people have responded, and post your own response if you like.

• Sip Panels
  I have recently tried many different scans on a home built with sip panels roof and walls, the foundation is insulated blocks. The siding is half log. Is there a right way or wrong way to do such a scan with the temperature differences so close. I have tried to heat the house up cool it down and just leave it the same and it is very difficult to see differences. The owner is having problems with wind and all the air getting sucked out of the house which is causing the heating system to imbalance. The image I have attached is when the temp. outside is 0 degrees and the inside is 75 degrees

• Thermography Inspections in Maryland (Business)
  Does anyone know what the licensing requirements are, if any, to operate as a thermographer in the state of Maryland?
  
  I would like to use thermal technology to detect water intrusion and insulation defects in residential buildings. The state of Maryland does not license home inspectors and there is no funding as of yet. Also there is no licensing classification as far as I know for thermographers in Maryland. I plan to take Level I and Building Science classes from ITC. Does anyone have any recommendations for starting a small thermography inspection consulting service?
   

• Duplicating a page on the report
  To do the IR reports I am using the software that is incorporated with Word. Sometimes one single image has more than one hot spot (a breaker might have two loose connections). If I forget to upload the image twice, is there a way to duplicate that page once at the report?
  
  I tried highlighting the whole page, then copying it and then pasting it, but the reference temperature is fixed on the hottest spot and does not allow to change it (it allows to change the spot marks but the actual temperature readings do not change).
   

• Mobile crane IR inspection
  Is anyone performing inspections on mobile cranes? What are some of your key components to inspect and would you recommend a time based pm on these?

• Possible Transformer Problem
  I have found this over the last week. On this day it was -15 C degrees. The cooling tubes of the transformer are only half working. Li1 ranges from -13 to 11.8, Li2 is -3.3 to 9.6, and Li3 is -12.4 to -4 degrees. I am waiting for some information to get back from the manufacturer but from what I can tell it seems as thought the outer part of the cooling tubes has no oil flow through them. What should happen is hot oil enters the top part of the tubes, then as it cools it flows down the tube and enters back into thw transformer at the bottom. A few days later we had a warm patch of weather so I went and rescanned the transformer. I will add another post for this image.

• Emissivity issue
  I have been facing an issue regarding emissivity.
  I have two Al plates with a small gap in between. The plates are heated to around 300 C. When I point my IR sensor at the plate surface and measure the temperature, it corresponds to the respective thermocouple reading. But at the gap ( air) it shows significantly higher reading compared to the thermo couple at the gap.
  I was wondering if I could do some formula to correct this reading assuming the emissivity of the air is 1.0 (??)

And of course if YOU have a question or want to start a discussion on a topic, we would love to hear from you. Just post a new thread on a message board.

Brainteaser of the Month 

Here is this month's brainteaser. Readers who email us the correct identification and explanation of the thermal pattern in the thermogram are entered into a drawing to win a prize from ITC. Please put "Brainteaser" as the subject of the message.

Click here to email your guess

Do you have an interesting image that you think would stump other thermographers? If so please email me your image (preferably in native .img, .jpg, .tif, .tgw, or .tmw format) with an accompanying visible photo and explanation. If your image is used, you receive a gift as well.

As most of our roof moisture thermographers have probably guessed, this is a thermogram of a flat (no, make that low slope) roof probably showing residual heat from the daytime from trapped moisture under the roofing membrane, a classic pattern. We don't know for sure without a little more investigation, but the pattern is highly suggestive of wet insulation.

Steve Bispo is our winner of last month's contest, and Jim McDonald contributed the image. Both receive an ITC low emissivity traveling coffee mug and a big thanks.

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