1660328958362 Article Flir2

Structured approach ups thermal scan accuracy

July 10, 2009
Your thermal scan accuracy can benefit from a structured approach.

This is Part 2 of a two-part series on the use of infrared thermographic surveys as part of preventive maintenance programs. Part 1 (http://www.plantservices.com/articles/2009/124.html) covered the scheduling and planning of IR surveys.

Infrared (IR) cameras are a commonly used tool in preventive/predictive maintenance (PPM) programs. When used properly, their thermographic images and temperature measurements can warn of potential equipment failures and unsafe working conditions. However, before performing an IR survey, the thermographer needs an action plan that considers site conditions, environment and how the IR camera will be used.

The purpose of IR thermography is to gather accurate data about equipment conditions safely in support of informed decisions. However, an untrained eye can misinterpret thermal images, which can lead to false conclusions. Such errors might result in missed problems, or raise nonexistent anomalies that look like problems. Either way, the result is wasted expense. A little IR survey preplanning and a few precautions in the field can help prevent these problems.
Setting up the job

The primary factor is conducting a safe survey. When working around electrical gear, wear appropriate personal protective equipment (PPE) as required by NFPA 70E. Always look around, above and below when changing position while scanning. If at all possible, work with an assistant and keep your “free” eye open. Don’t wear loose clothing or have things hanging on you (lens cap, ID tag, clipboard). Always carry a means of communication such as a cell phone or radio. If you must scan from atop ladder, have an assistant hold it steady.

Preset as many camera variables at the job site as possible. These settings should be reviewed for each change in the work environment, for example, when going from inside a building to the outdoors. The main camera settings to consider are:

  • Ambient temperature
  • Background temperature
  • Target’s emissivity
  • Distance to target

Depending on circumstances, these factors might be measured, assumed or found in look-up tables stored in camera firmware. Other camera settings typically adjusted during image capture include designation of a target spot or area, temperature range and the color or gray scale for that range.

Data gathering and reporting

Before going to the field, talk to plant management about any special safety hazards and talk to maintenance personnel about any recent failures (leaks, tripped breakers, failed motors, moisture in buildings, recent storms). Make sure the camera batteries you’ll need are fully charged.

[pullquote]

Don’t rely on memory. Take the time to enter data when making the scan list or recording exception temperatures. Use a PDA or laptop for data entry. Select a camera that has easy FireWire or Ethernet connectivity, RAM, AVI storage and CompactFLASH to facilitate data transfer; some cameras also allow voice recording. Get a compatible software package for data analysis and report generation. It’s extra work to transfer data to a PC, but worth it for better detail and image manipulation. Use paper notations sparingly.

Some IR cameras also have a built-in digital photo camera to document problem areas. If the built-in camera doesn’t have adequate resolution, carry a good-quality digital camera and adjust it to a high mega pixel setting to allow you to show thermograph/photo pairs in your report.

Start a file of accumulated images on the PC and save every image (thermal and visual); this is valuable backup data. IR camera firmware is limited by size considerations. PC software designed specifically for IR data manipulation generally provides additional features and functions. Depending on supplier, this software typically provides a post-measurement capability to modify IR image variables such as ambient conditions, spectral responsivity, atmospheric transmission changes, internal and external filters, and other important criteria. It also might allow picture-in-picture capabilities for combining digital photos with thermographic images, automatic report generation, integration with Microsoft Word, predictive trending analysis and many other functions.

Electrical scanning

Safety first - remove any conductive jewelry. If you aren’t a licensed electrician in the state where you’re working, you’ll need one to remove electrical covers. Make sure that other screws are in place before removing any electrical covers, especially on deadfronts, to prevent a slipped cover from causing injury, damaging equipment or tripping a breaker.

Pay attention to approach boundaries mandated by NFPA 70E (these should be posted on electrical covers, safety gates and other places where hazardous voltages exist). Of course, wear safety gear, the minimum being a hard hat, safety glasses, ear protection (where required) and protective boots. Upgrade this to PPE mandated by NFPA 70E when removing electrical covers or entering hazardous voltage areas. The PPE classification should be posted on the cover or safety gate.

Devise an exit strategy when entering an area in case of emergency. Don’t reset a tripped breaker before discussing it with plant management. Look for burned wires visually. There might have been an earlier problem, but without a load, there will be a minimal heat signature for the camera. Use all your senses for any indications of problems. Smell/feel the heat, hear the failing motor, see the arcing connection; all such areas should be scanned.

Electrical scanning data errors

Reflections can be a problem when scanning highly reflective, low-emissivity targets such as bus bars. Adjust the viewing angle to avoid misleading reflections from nearby heat sources and from your own body (Figure 1). Reflections can produce an erroneous thermal image. If not understood, it becomes a data error.

Figure 1. Reflected heat from a transformer (lower left in visual photo) onto the target (upper right in visual photo), appears as an elevated temperature in the thermal image (left view).

Mismatched and wrong-size fuses aren’t considered as acceptable practice, but they’re often found in the field. Maintenance personnel might have used them to get equipment up and running quickly. Thermography can reveal these fuse anomalies. If a fuse rating is too high, its image appears cooler than others in the group. If the rating is too low, it will appear warmer than the rest.

Trip sensors are electrical devices in breakers that measure current. When it exceeds a setpoint, the breaker opens. A common thermographic error is misdiagnosing the thermal signature of these devices as an internal breaker problem.

Be wary of close-up images. When collecting thermal images, ensure there’s enough view of the surrounding area to help find the component that needs attention (Figure 2). Always take the thermal visual images from the same angle and distance. This helps maintenance personnel find the bad component.
Don’t make things difficult

Figure 2. A camera position for the visible light image that doesn’t match the IR camera position used for the thermographic image makes it harder to identify components that need repair.

Scanning outside

Weather is a big factor in any form of thermography because it affects not only ambient temperature, but also that of the target objects. In addition, snow and rain attenuate the IR signal before it reaches the camera. Wind can cool components quickly. For example, if a fuse disconnect enclosure is opened, there might be little time before convective cooling reduces component temperatures, making accurate measurements elusive. Get ready in front of the box, camera running, in focus, and imager settings fixed before opening the door.

Scanning electrical utility gear in snow and rain is a washout. Generally, it’s best to wait for more favorable weather and avoid a wasted trip. Follow these safety considerations when surveying substations and switchgear:

  • Always go into substations and switchyards with an experienced assistant who works for the utility.
  • Wear appropriate PPE.
  • Don’t wander around the yard; stand where the most experienced person is standing.
  • Don’t point at components (doing so can turn you into a good path to ground).
  • Ask for a description of every component that has an exceptional temperature signature (each utility calls switches and other gear by different names).

Roof scanning

The following equipment and techniques help promote safety and useful results in walk-on roof surveys. Different cameras are sensitive to different IR wavelengths; a camera with a midwave (three micron to five micron) detector is highly preferred. Always bring an assistant, whose job will be to mark off roof areas having suspected moisture intrusion and to warn you of dangerous areas, conditions and possible roof damage.

Bring a 10-foot stepladder so you can use a better elevated vantage point when looking for wet spots. Use a flat board under each foot to avoid damaging the roof. Bring fluorescent red, orange or yellow marking paint and plenty of it. Use a dashed line to outline areas; this minimizes paint use and roof impact. Bring flashlights and plenty of batteries; the head-mounted type works well.

Bring a laser pointer if your camera doesn’t have one. Use it to track the assistant’s foot to the first area that shows wetness, and then outline the area with spray paint using a distinctive numbering system immediately before taking the thermograph (fresh paint is cold and will show up in the thermograph).

Data errors

Be aware of solar loading. Scanning a rooftop air conditioner on a sunny day can produce false (high) temperature measurements. Also, scanning a roof for leaks is usually done in the evening hours to better see the temperature differences that moisture ingress causes, but beware of residual heat. As the sun sets, the flashing and stem wall on the west side of a building remains in the sun longer and shows an elevated thermal signature for some time (Figure 3).

Figure 3. Solar loading and debris on a roof can lead to data errors. In this image, yellow and orange colors indicate higher temperatures

Roof debris, such as pine needles, leaves and other material, usually accumulates in corners and at drains as the wind blows them around the roof. After the sun sets, the debris retains heat longer than the rest of the roof and can lead to another data error.

Some roofs are ballasted with gravel. As time passes, gravel gets kicked around or pushed into piles to facilitate repairs. When scanning for leaks, remember that thicker gravel layers retain solar load well into the night. Make sure you know what is under the roof decking being examined. There might be a boiler room or mechanical room generating heat that shows up as an elevated roof temperature reading. Similarly, exhaust fan air usually is warmer that the outside temperature. This too can lead to a data error.

Thermographer safety and temperature measurement accuracy are major considerations during IR surveys. To avoid having to cull a lot of bad data from survey results, and perhaps repeat a survey, it’s best to recognize potential sources of error at the beginning. Infrared thermography is a great tool for finding hidden problems when you know what to look for, and what to ignore.

Harley Denio is president of Oregon Infrared, based near Portland, Ore. Contact him at [email protected] and (503) 680-4550.

Sponsored Recommendations

Arc Flash Prevention: What You Need to Know

March 28, 2024
Download to learn: how an arc flash forms and common causes, safety recommendations to help prevent arc flash exposure (including the use of lockout tagout and energy isolating...

Reduce engineering time by 50%

March 28, 2024
Learn how smart value chain applications are made possible by moving from manually-intensive CAD-based drafting packages to modern CAE software.

Filter Monitoring with Rittal's Blue e Air Conditioner

March 28, 2024
Steve Sullivan, Training Supervisor for Rittal North America, provides an overview of the filter monitoring capabilities of the Blue e line of industrial air conditioners.

Limitations of MERV Ratings for Dust Collector Filters

Feb. 23, 2024
It can be complicated and confusing to select the safest and most efficient dust collector filters for your facility. For the HVAC industry, MERV ratings are king. But MERV ratings...