Editor’s Note: The Windshield Repair Forum is a hub for opinion, ideas, comments, venting, questions about windshield repair and headlight restoration, business, and sometimes just life in general. A recent question posted to the forum led to us doing some testing on the topic of drying out a break and the results were surprising.
The discussion centered on the pros and cons of various tools used to heat glass and remove the moisture including blow dryers, a moisture evaporator, and heat guns. The question that got it all going was, “…have you ever measured the heated area by a [moisture evaporator] right away after heating about 20 seconds ? What was the temperature you measured?”
A heat gun and infrared thermometer used to measure glass temperature for windshield repair
The short answer is yes, I have measured the temperature of the heated glass. However I can’t really give you an exact answer due to the number of variables involved. Let me explain.
The temperature of the glass prior to applying the moisture evaporator will vary. We did a test starting with a glass temperature of approximately 76 degrees* and after the moisture evaporator was applied for 20 seconds we got an average glass temperature reading of 140 degrees. We did the same test but this time preheated the glass to 125 degrees, before applying the moisture evaporator for 20 seconds. This time we got an average glass temperature reading of 197 degrees. Never did the glass temperature reach 212 degrees, but the water did begin to bubble in 10-12 seconds and was gone in 16-17 seconds. The interesting thing to note here is that heating the glass to 140 degrees with a hair dryer did not remove the water from the break but heating the glass to 140 degrees with a moisture evaporator did! I’m sure there is a good explanation for that but I have to admit I don’t know what it is.
(Photos Above: A heat gun & infrared thermometer. Below: A break dried with a heat gun.)
There can be significant variances in the measured glass temperature depending on the infrared thermometer that is used. An inexpensive infrared thermometer is fine for finding a general temperature range, but readings are not consistent enough for serious glass temperature testing, especially at hotter temperatures. In our testing we used an inexpensive model (under $50) and a mid-range model (under $150). They were close at lower temperatures but over 200 degrees the variance became increasingly significant. Even the better model failed to produce consistent results in repeated tests.
Each moisture evaporator performs slightly differently. We tried to measure the heating elements with the infrared thermometers but the results were extremely inconsistent. We did however consistently get temperature readings far in excess of 212 degrees with both thermometers. In every test the water began to bubble and then disappeared within the recommended 20 seconds.
Windshield repair break dried with a heat gun
A heat gun most certainly produces enough heat to get the water in a break to boil, as does a torch. We don’t recommend a heat gun because there are even more variables that come into play. There’s just no way for us to tell a technician what heat setting to use, how far from the glass to hold it, or how long to leave it on the glass. The same is true for torches and using an open flame brings up additional safety issues. That’s not to say you cannot use these tools to remove moisture, but the moisture evaporator is a safer, more consistent option.
There was one unexpected result in our testing. We wanted to see how hot we could get the glass with our blow dryer and if we could actually dry out the break with it. In previous tests it took so long that I finally gave up without ever getting all the water out, but today we used a very powerful blow dryer and placed it directly on the glass over the break to see what would happen. The glass temperature reached 170 degrees and although the water never bubbled, after 8 minutes it did evaporate.
So I have to retract my previous comments about it taking all day to remove moisture with a blow dryer. Though given the choice of 20 seconds versus 8 minutes to dry out the break I would opt for the moisture evaporator. But depending on the model used it is obviously possible to dry out a break using a blow dryer (without taking all day). My apologies to those I have told it could not be done.
One other unexpected result from our testing today was that the glass did not crack during any of our moisture evaporator tests. The combination break that we dried out using the blow dryer also did not crack out, but a nearby star break did crack out before we could get the moisture out using the blow dryer. The glass cooled much faster after using the moisture evaporator than it did after using the blow dryer because the blow dryer heated a far larger area of the glass. Also worth noting, was that the inside of the glass got hotter using the blow dryer than it did with the moisture evaporator. One or both of these factors may have contributed to the star break crack out but that is only an assumption.