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Adjusting Level and Span Manually--Part 2 of 2

Recently, I had an otherwise very bright student in a Level I course who had received no training since purchasing his imager three years before. He also had only ever used the system with the automatic image adjustment. An hour into the training, when (as we discussed last week) he realized the limitations of what he’d been doing, his jaw dropped in amazement. My guess is that he had missed about half of what he should have seen had he been using the imager properly!

As I said last week, AUTO adjust can get you in the ballpark, but if you don’t understand how it works, it can also get you into trouble! That’s when you need to be able to adjust the image using the MANUAL function. It is not difficult to do this, but it may require a bit of practice before you become proficient.

The process requires you to think about what an appropriate span of temperatures would be for the best image. Typically we want maximum thermal contrast—minimum span—in the area of interest—in order to see all the detail. When we look at the house, we don’t really care about the sky behind it. We are interested in the wall where there may be only a small temperature variation. As we saw last week, using AUTO adjust didn’t work in this situation. The solution? Simply use the MANUAL function, you can get a great image by narrowing the span.

The second image is inside a large, older commercial building and shows a very hot steam line in the upper left and some cold air infiltration on the right. Think about how it looked last week when we had adjusted it using the AUTO function. The span was defined between 44°F (the cold areas) and 204°F. (the steam line). There was almost no thermal detail on the wall. Again, a simple MANUAL adjustment of the span—narrowed to 16 degrees—and a level set for the wall, produced a remarkable image showing all the framing detail and a wall without insulation. Yes, the steam line and the cold parts of the wall are both “saturated” but we don’t need to see them.

Remember, as long as you are in the Fluke is2 format, both AUTO and MANUAL adjustments can also be made to the image using software. So, if things don’t look perfect when you are in the field, don’t worry! You can make adjustments back in the office. In fact, making adjustments on the computer is another easy way to “get the hang” of these two image adjustment options.

Thanks to my colleague, Matt Schwoegler, for taking and adjusting the images for this and last week’s blogs. In our work Matt and I both see too many instances similar to the ones introduced in this post. If you are not 110% clear about how to adjust level and span both manually and automatically, take some time to practice. Once you’ve mastered these simple techniques, you’ll be seeing more than you ever imagined! Next week we’ll look at the basics of how to locate levels in tanks of all sizes—one of the most lucrative uses for thermography I know!

If you’d like to learn more about adjusting level and span automatically, be sure to check out last week’s post.

Thinking Thermally,

John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner

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