To understand the effect of wind or forced convection on a surface, we need to understand the basic movement of heat transfer. Thermal energy always moves from hot to cold—unless there is an outside force that changes the natural movement of heat energy.
Whether you are observing an object outside or inside your plant or your client’s home, a thermographer needs to understand what’s going on with their surroundings. Forced convection can come in the form of wind, a fan or even a pump, and the result of this forced convection will affect the target temperature!
There are a few assumptions that a thermographer can use to help determine the severity of a problem with an elevated temperature being observed when there is forced convection present. The industry suggests that wind can significantly reduce the target temperature and offers the following guidelines:
- It is suggested that a 10MPH wind can reduce the targeted ΔT you are observing and measuring by 1/2 and
- A 15MPH wind reduces the target temperature by 2/3
We conducted a controlled experiment, shown above, using a 3 phase circuit with a measured 40% load. A small fan was used and we measured the wind velocity at approximately 4MPH with our Kestrel 3000. The ambient temperature was 72 degrees; the temperature of the A Phase connector measured 107.1°F. When the forced convection, a mere 4 MPH, was applied, the temperature of the same component decreased to 99.6°F. This is about a 7% decrease in temperature!
The same rules apply to roof scans as well. You need to pay attention to wind conditions while doing roof scans because while it may be easy to find the largest areas affected, finding the source of your moisture problem can be very difficult with a wind.
On the other hand, when doing infrared scans inside of a building, a good thermal practice is to turn off the HVAC system to help eliminate the possibility of the effects of forced convection.
Beaufort offers a wind scale that gives us an opportunity to estimating wind speed when you don’t have an instrument to measurement, such as:
- 0 – 3 Little or no drifting of smoke
- 4 – 7 Wind felt on face, leaves rustle, weather vane moves
- 8 – 12 Leaves in constant motion, small flags are extended
- 13 – 18 Wind raises dust and paper, small branches move
The bottom line? You need to pay attention to your surroundings and understand that there are many factors that can make your job of being a thermographer a little more challenging. For today’s topic, whenever you need to view objects in windy conditions, please note that with a wind present, assume the problem is more severe than it appears and react accordingly!