Most of the asphalt calls we get start the same way. Somebody on a paving crew or in a QC lab needs a temperature reading they can trust, and the thermometer they have isn't cutting it.
We don't sell density gauges or core drills, and we won't pretend to. What we know is temperature. We've been selling and calibrating thermometers out of Elizabeth, Colorado since 1990, so that's what this article sticks to.
Why temperature runs the job
Hot mix asphalt has a working window. If it's too cold at the paver, the crew can't get it compacted before it stiffens. Overheat it at the plant and you can damage the binder. Your agency or project spec sets the actual numbers, so we won't quote them here. But nearly every spec has them, and somebody has to check.
In the lab it's the same idea on a smaller scale. Samples get heated to a set temperature before they're mixed or compacted, and the oven dial by itself isn't good evidence that a sample actually got there.
Probe or infrared?
This is the question we hear most. The honest answer is that a lot of crews need both.
An infrared thermometer reads the surface from a distance. It's fast. You can sweep the mat behind the paver and spot a cold streak in seconds, and nobody has to lean over hot asphalt to do it. Fresh asphalt is dark and dull, which IR thermometers generally read well. But IR only sees the surface, and the surface cools faster than the material under it. Distance matters too. The farther back you stand, the bigger the spot you're averaging, so look at the distance-to-spot ratio before you buy. Our infrared thermometers list it for each model.
A thermocouple probe tells you what's going on inside. Push it into a load in the truck bed or into a lab sample and you get the internal temperature instead of the skin temperature. A rugged handheld like the Cooper-Atkins 35100-K takes any Type K probe with a standard mini connector, so one meter can handle several jobs.
Match the probe to the material. A heavy needle such as the 49135 has a big handle and a stiff 1/8" shaft for pushing into loose mix. For a hard surface, a spring-tip probe like the 33001 puts a 12" shaft between your hand and the heat. Whatever you pick, check the probe's maximum temperature against your hottest reading. Probes and meters don't always share the same limit, and the lower of the two is the one you're working with.
Logging and keeping readings honest
If you need a record instead of a single reading, use a thermocouple data logger. The T&D TR-75nw has two thermocouple channels and records at an interval you choose, which works well for watching a lab oven through a long heating cycle.
And then there's calibration. A thermometer that reads five degrees off is worse than no thermometer at all, because people trust it. Many of our thermometers can be ordered with a NIST traceable certificate as a product option. For instruments you already own, our in-house lab does calibration and repair. Fill out the RA form on our calibration and repair page before you ship anything, or call 800-390-0004, ext 3, with questions.
Not sure what fits your crew or your lab? Browse our asphalt thermometers, or call us and tell us what you're measuring. We'd rather sell you the right probe than the expensive one.
