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Not My Cup of Tea

By Chuck Newcombe

Recently, in a cafeteria, I selected a tea bag and filled a large paper cup with hot water. No, make that HOT! water, as I soon found out - I sensed the pain in my fingers gripping the cup. I quickly remedied the situation (or so I thought) by slipping one of those little brown insulating sleeves around the cup to make it manageable. That worked for about 10 seconds, but then the sleeve was as hot has the cup had been. I grabbed a second sleeve and placed it around the first. Finally, I found an area where an air gap between the second sleeve and the cup provided the necessary insulation to allow me to hold the cup comfortably.

A. Course Waffle B. Slight Waffle C. Corrugated
Figure 1: Sleeves tested

As I sipped my tea, I inspected the sleeves to see why they were so ineffective. I soon concluded that an oversimplified manufacturing process had intended a slight dimpling, or waffle pattern, of the otherwise solid thin cardboard stock to provide the necessary insulating effect. It clearly didn't work, and the net result was that the cafeteria probably went through twice as many sleeves (or twice as many cups) as would otherwise have been needed if an effective insulating sleeve had been used.

What design makes the sleeve work?

Well, the best I've found so far is one using corrugated cardboard. When slipped onto the cup, it amounts to two thin sleeves (one the cup itself) held apart by the corrugation snaking between them. This decreases the cross-sectional area of the cardboard that can conduct heat directly to the outside surface, thereby lengthening the heat-conduction path, and allowing the outer surface to cool through heat loss to the surrounding air.

That led me to thoughts of thermal conductivity and how to measure it. Could I use non-contact Fluke thermometers? The issue wasn't just about temperature; it was about the flow of heat, and the thermal resistance of the sleeve material.

That's when I decided to experiment with a Fluke Thermal Imager—the model Ti55. I can best describe what I did using pictures. The first (Figure 1) shows the three different sleeves that I tested.

These sleeves were installed on identical cups, which I then filled with hot water. The four following pictures show the results.

Note that I used the temperature cursor to clearly show the temperature of the selected point on an image. The other thing to take note of is that I made all the measurements on a single stored image.

The cups in Figure 2 are arranged with sleeves in the order shown in Figure 1.

Cup Temperature
(no sleeve) 164.1 F
Temp. using sleeve A
144.5 F
Temp. using sleeve B
154.7 F
Temp. using sleeve C
137.6 F

Figure 2: Sleeve Test Results

The pictures in Figure 2 clearly show that the temperature of the unprotected cup was reduced by 20 degrees using the coarse, waffle-patterned sleeve (A). The sleeve with slight waffling (B) showed only about a 10-degree drop, while the corrugated sleeve (C) provided nearly 30 degrees of protection from the hot cup surface.

Even though sleeve C's resulting 137 degrees might seem too hot for comfort, when I picked up the cup with that sleeve, it felt comfortable in my hand.

What was going on?

I figured there must be a way to more clearly show the effects of increased thermal resistance when the cup is held in the hand. I decided to give it a try.

First, I cooled my hand by holding a bag of ice for about 30 seconds and then placed my cold hand around the sleeve for another 30-60 seconds. Then I took my hand away and quickly captured yet another image from the cup. The idea was to show how the surface of the sleeve is cooled significantly by the surrounding heat sink (my hand). My final picture shows the results with the best of the tested sleeves, the corrugated one (C).

Figure 3: Sleeve C with thumbprint

The imager shows the thermal radiation at the measured surface accurately. When I surround the sleeve with my much cooler hand (Figure 3 shows that my thumb's temperature was 94.4 degrees), the thermal resistance of the corrugation slows the heat flow from the cup to my hand, allowing the outside of the sleeve to cool well below the radiated temperature reading, to nearly my hand temperature.

On the other hand, the worst-performing sleeve (slightly waffled sample B) was a fairly efficient conductor of heat from the cup to my hand, quickly bringing my hand temperature to an uncomfortable level.

I suppose a more practical test of insulation using Fluke's family of thermal imagers would be to look for air leaks in outside building walls, or the detection of water leaks into roofing insulation that reduce its effectiveness. Or finding overheated bearings on motors and pumps, loose electrical connections, and so on. But for me on that day, that overly hot cup was the issue at hand.

Whatever warms you up, the usefulness of this handy technology is limited only by your imagination. Can you tell us of a unique application that you have found?

What unusual uses have you found for your Fluke thermal imaging tools? Send your experiences to flukeplus@fluke.com. We'd like hear from you!