Thermal Imaging 101: How Can It Work For Me

A thermal imager interprets infrared heat by assigning a color scale to temperature values. On the imager’s LCD display, the user sees a heat picture of the object in front of the lens that highlights temperature variations and hotspots.

Thermal Imaging Basics

All real-world materials absorb, reflect and transmit infrared energy depending on their physical properties:

Infrared energy = Absorption + Reflection + Transmission

A material that perfectly absorbs and emits all infrared energy is called a “Black Body” and has an emissivity of (1) – but it doesn’t exist in the field. Most materials that we encounter are called “Gray Bodies” since they are not perfect emitters; close maybe, but not perfect. This simplifies our working formula to:

Emissivity = 1 – Reflected background

The more an object reflects infrared energy, the less it emits. Polished chrome has a very high reflectivity and low emissivity. Brushed stainless steel has less reflectivity and more emissivity. Tarnished brass and copper have even less reflectivity with proportionately more emissivity. Most painted surfaces have very high emissivity and negligible reflectivity.

Adjusting Emissivity and Reflected Background

Thermal imagers have adjustments for both emissivity and reflected background. Both are easy to mea-sure and compensate for when the need for quantitative readings outweighs qualitative readings.

Level and Span

Level and span represent the expected target temperature (level) and the differential from target temperature (span). If level was set to 100°F and span was set to 25°F, the temperature range would be limited to 75°F to 125°F.

Thermal imagers will automatically select the best level and span of the target. Set at automatic, the imager displays the highest and lowest temperature values in the scene. The minimum and maximum values of the scene define the extremes of the color palette. A white spot would not necessarily indicate a very high temperature, only the highest temperature in the scene. The highest temperature in one scene may be 90°F, while another scene may have a white spot indicating 250°F, if that is the highest temperature in that scene.

The palette is proportional to the temperature range of the scene, not a fixed value. A slight relocation of the spot on the tar-get can change the gradient dis-play depending on the range of temperatures now in the scene.

Thermal - Digital Blending

Keep in mind that thermal imagers measure surface temperatures only. The user must understand what is happening beneath those surfaces in order to make accurate judgments. Various materials with differing emissivities within a scene will not report temperature relationships equitably.

Thermal imagers don’t “see” through walls; they detect very fine differences in surface temperature.

Inspecting Ductwork with Thermal Imaging

Thermal imaging is well suited for the dynamic heat analysis of duct heat loss or span and conditioned envelope heat loss or span, but be wary of directly imaging sheet metal ductwork, due to the emissivity and reflection issues described above.

Locating air leakage requires a 4 °F or greater difference between inside and outside air to get accurate readings.

Duct Leaks Behind Insulation or in Walls

When ductwork is located outside the conditioned envelope, air leaks can present conditions for pressure differentials across the conditioned envelope, create opportunities for moisture conditions that support mold growth in or on the ductwork or even hid-den within walls. The effects are similar to unbalanced ventilation and exhaust air.

To find duct leaks beneath duct wrap insulation:
  1. Initiate a heating or cooling demand.
  2. Set emissivity to 0.2 for foil-faced insulation, or 0.95 for vinyl- or PVC-faced insulation.
  3. Scan ductwork with thermal imager.
  4. Dynamic, real-time temperature variations of insulation surface will be displayed and continually updated as they occur.
  5. Indications of an air leak will be displayed as an extreme temperature at the point of the leak, with temperature gradation away from the leak location to areas of ambient temperatures.
  6. The imager can record selected images of interest for download, if desired.
To find duct leaks behind walls:
  1. Make an initial thermal scan of walls concealing ductwork with the system blower off.
  2. Start the blower and repeat the initial scans.
  3. Compare results of the blower-off to the blower-on scans.
  4. Significant temperature deviations may indicate return leaks that are causing pres-sure differentials within the walls that encourage air and moisture migration from out-doors.
  5. Start heating or cooling operations.
  6. Repeat the wall scans.
  7. Even temperature gradients of the wall that follows the duct-work should be expected.
  8. Uneven or spreading temperature gradients may indicate leaking ductwork
  9. Pay attention to temperature gradients along baseboards and around windows
  10. The imager can record selected images of interest for download, if desired.
To see how diffuser discharge affects ceiling temperatures:
  1. Start heating or cooling operation.
  2. Scan diffuser outward along ceiling toward intersecting walls or zones.
  3. Watch temperature change along ceiling to evaluate surface effect.
  4. Watch for temperature change at intersecting wall surfaces to evaluate throw.
  5. This provides a good preliminary analysis prior to breaking out the ladders and air balancing equipment.
  6. Throw should be between 75 % to 110 % of distance from diffuser to intersecting surface.
Insulation effectiveness and air leaks Insulation on all surfaces can be scanned for leakage and losses:
  • Walls that separate conditioned from unconditioned spaces
  • Pipe and duct insulation
  • Higher temperatures are indicated by a shift toward white
  • Lower temperatures are indicated by a shift toward black
Scan conditioned envelope walls or ceilings for even temperatures
  1. Initial scan should be made with HVAC equipment off for insulation effectiveness.
  2. Subsequent scan should be made with blower, economizer and exhaust fans operating to evaluate for air leaks.
  3. Economizers and power exhaust can be temporarily re-adjusted to increase pressure differentials across the conditioned envelope for testing purposes.
  4. Scan on both conditioned and unconditioned side of surface.
  5. Pay special attention to areas of fenestration and along sill plate areas.

Other Remediation Opportunities

Thermal imagers are also ideal for determining the scope of water damage to residential and commercial buildings, including carpeting, walls, roofs, siding, tile, insulation, and other components. Instead of searching for moisture inch-by-inch with a moisture meter, a thermal camera allows inspectors to scan an entire room in a matter of minutes.

By locating the thermal changes from evaporative moisture cooling (EMC), inspectors can moisture map the entire building and isolate problems before providing an estimate for remediation.

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