Regardless if you’re inspecting buildings, roofs, or electrical or mechanical apparatus, when you have an abundance of air movement across a surface, there is an increase in heat loss or gain on that surface from convective cooling or heating.
Convective cooling is often discussed in many applications of thermography. Often the result is cooling of anomalous conditions to the point that they’re not easily detectable. Sometimes, however, when the temperature difference is large enough, the convective cooling can take place and thermal abnormalities are still detectable. The issue at hand, and the question that is posed to the thermographer, is the quantifiable impact that convection has on surface temperature. We want to know this because once the air movement slows or stops, the surface temperature will increase even if the object in question doesn’t change otherwise.
This phenomenon is well known to most thermographers who have had some type of formal training. So, why are we bringing it up in this blog? We’re trying to battle misinformation. There are formulas that supposedly help thermographers calculate varying temperature measurement corrections when given certain velocities of air movement. Readers beware; don’t use these formulas or attempt to use them to correct temperature measurements as they are not accurate except in tightly controlled circumstances when examining one specific piece of equipment.
If you’ve taken a Level I Thermographic Applications course, the formula below should be familiar to you:
This is Newton’s Law of Cooling. It states that the amount of heat transferred convectively is dependent upon the Coefficient of Convective Cooling (h), the difference in temperature between the cooling medium and a point on the surface, and the area over which the transfer takes place. The coefficient of convective cooling is dependent upon many variables, one of which is velocity. Velocity however isn’t the only factor that impacts the value of h. Orientation to flow, surface condition, geometry, and the viscosity of the fluid are all additional factors impacting h, so if the only one you can quantify is velocity you can’t know with any precision what h actually is. The result is that any attempt to correct temperatures based on velocity alone are flawed, hence our aforementioned disclaimer urging thermographers not to use these formulas. There are a couple of “rules of thumb” that we will discuss in an upcoming blog post, so remember to check back often to learn more.
Think Thermally, www.thesnellgroup.com The Snell Group, a Fluke Thermal Imaging Blog content partner