Let’s first of all define “span”. Span is not a temperature. It is the difference between the high and low temperature settings on the span bar. However, the term “level” in thermography refers to a temperature. It is the mid-point of the span. I use the Fluke TiR1 and the Fluke Ti32 infrared cameras, plus the Fluke FlexCams. All of these cameras have auto and manual span bar adjustments. That’s a very useful feature to have on your infrared camera. If I were to ask you to adjust your span bar to 20° at a level of 73° F the top of the span bar would read 83° F and the bottom would read 63° F. (83 minus 63 = 20 and the mid-point is 73.) So, the question is: “What span should I use? Selection of a span is usually a personal preference but here are some tips. If you are looking for a moisture signature due to evaporative cooling the temperature variance will usually be around 3-5° F. You could select the temperature of a dry area of the wall as your level and adjust for a 10 – 15 degree span. If you like to use a 10 degree span and the wall temperature is 73 then your span would read 68° F on the bottom and 78° F on the top. A 15 degree span would be approx 65° F on the bottom and 80° F on the top. What’s the difference? The smaller the span, the more noise or graininess you’ll see in your image. And you will see a more dramatic contrast. Sometimes a very small span is necessary due to a very small delta T (∆T). What if, for example, the moisture signature varied only one degree from the temperature of the dry area? A small span would be needed to get a clear image of the moisture signature. Keep in mind, too, that cameras vary due to the focal plane array. The lower the NETD (noise equivalent temperature differential – see blog #021) the more sensitive the camera and the less noise or graininess you will see.
The following images illustrate the effects of a variety of span bar settings. In all cases the level is set at 73° F. You will see a 5° span, a 10° span, a 15° span, a 20° span and lastly a 40° span. We are looking at a moisture signature due to evaporative cooling. All images are from the one image taken. I changed the span setting in the SmartView software so that you could see the effect of a variety of span settings on the same image.
Although this example was taken with a moisture signature the same holds true with the investigation for insulation issues. Usually a 10-15 degree span setting using the ambient temperature for the level will achieve good results. As you work with your infrared camera and play around with span settings you will become more comfortable and know what works best for you.
How’s your comfort level in thermography?
(The above comments represent my opinion).
Rod Hoff / Restoration Consultants Inc
Thermography Instructor / IR camera sales
3284 Ramos Circle, Sacramento CA 95827
toll free 888-617-3266 ext 301
Provider of Fluke TiS, TiR, TiR1, Ti, Ti25, TiR27, Ti27, TiR29, Ti29, TiR32, Ti32, TiR3, and TiR4 infrared cameras.
See DEMO www.moistureview.com/demo.html