Know Your Target
When performing an infrared inspection, obtaining quality data relies on the use of proper equipment, thermographer training and knowledge of the system being inspected. It is imperative that a thermographer be familiar with the construction and operation of the object(s) to be imaged before the inspection begins.
Prior to performing an infrared inspection of an object for the first time, a thermographer should:
1. Become familiar with system construction by reviewing appropriate drawings or blueprints noting insulation materials located on, or within, the subject system and how they might impact findings.
2. Discuss with the end user the reason(s) for conducting the infrared inspection.
3. Review any previous inspection reports and operational data to determine history of the subject system including past problems.
4. Ascertain that the system is under normal operating conditions and how its operation is likely to affect thermal signatures.
5. Ensure that line-of-sight access is available and that environmental conditions and infrared equipment are appropriate for collecting accurate data.
6. Determine if a similar system is available for reference purposes.
Following the above can vastly improve the quality of collected data and help to reduce errors in reporting. As always, remember to work safely.
Monochrome or Multi-color?
With most thermal imagers capable of displaying images in monochrome or multicolor, many new thermographers ask which color palette is the best choice for effective imaging. The answer will depend on a number of factors including application, delta T associated with the exception, and personal preference.
Because it is usually less confusing than multi color palettes, grayscale may be better suited for some applications. Additionally, applications that have a large delta T associated with exceptions or where target recognition is important may be better suited for grayscale imaging. Such applications include electrical distribution systems, building envelopes inspected from the interior of the structure, and low slope roof inspections.
Multicolor palettes offer an advantage when imaging targets having a small delta associated with exceptions or when imaging targets with several discrete temperature zones. Typical applications include mechanical systems, refractory systems, building envelopes inspected from the exterior, and medical/veterinary applications.
For hardcopy reports, printing monochrome images can result in lower cost than multicolor reports. Lastly, the choice to use monochrome or multicolor is largely a matter of personal preference. Thermographers should always use a palette which best represents the observed thermal patterns and provides data that are easily understood.
Thermal Imaging Basics
With tremendous emphasis placed upon the sophistication of today’s modern thermal imagers, it’s easy to forget the basics of thermal imaging. Regardless of imager age or sophistication, there are several basic concepts that can vastly increase the accuracy and success of an infrared inspection.
1. Select the proper spectral response imager for the application.
2. A clear line of sight to the target is required with no obstruction of the imager lens.
3. Imager optics must be clean and calibrated to the imager being used.
4. Target should be dry and at a stable temperature.
5. Imager focus is imperative to accurate diagnosis and temperature measurement. Be sure to focus imager and the viewfinder as well.
6. Knowing the construction, operation and characteristics of the system being inspected is vitally important to anticipating thermal patterns and performance.
7. Adverse atmospheric conditions such as wind, humidity, or solar reflection and solar loading should be avoided.
8. For electrical and mechanical equipment, the systems must be energized and under load; for structural inspections, a delta T of 10 C (18 F) is desired.
9. Discriminating small temperature differentials across targets with low emittance values can prove quite difficult.
10. Whenever safely possible, cross reference observed infrared temperature values with accurate contact temperature readings.
When performing any infrared inspection, be certain to take all necessary safety precautions and always work safely.
Image Storage for Any Thermal Imager
With the advent of digital storage media, nearly every modern thermal imager is capable of storing images electronically. For thermal imagers that do not have image storage capability, there are inexpensive and readily available solutions.
For thermal imagers having a video output jack there are two options for image storage. The first option utilizes a VCR to record desired imagery. Videotape allows for recording of moving objects and/or dynamic processes. Videotape is inexpensive, readily available, and special equipment is not required for playback. Still images can be captured from videotape using a video capture board available for most personal computers.
The second option utilizes a digital recorder that accepts a standard video input. With a digital recorder, single frames of data may be captured to digital media such as PC cards. Camcorders having a digital snapshot camera feature may provide the best solution of all. Several currently available models allow a thermographer to record to videotape and/or to capture still images from the video input. Additionally, the daylight snapshot feature of the camera may be used to capture high quality daylight images.
Once images have been recorded, they can be directly imported into a PC for storage or incorporation into written reports.
Three Legs are Better Than None
When it comes to providing a stable platform for a thermal imager, it’s hard to beat a tripod. As thermal imagers have gotten smaller and lighter, many thermographers have all but forgotten this once requisite accessory that can help provide better quality data while reducing stress.
For many PdM applications that require still images only, using a thermal imager in a hand held configuration is usually sufficient. However, for applications that require imaging from a fixed vantage point over extended periods of time or where videotaping is desired, a tripod can be an invaluable accessory. When selecting a tripod there are several things you should bear in mind:
1. Be sure to select a tripod capable of carrying the weight of your imager.
2. Tripods should connect directly to the imager via a 1/4” x 20 set screw. Tripods with plastic quick releases should be avoided as they are subject to wear and can cause an imager to suddenly fall.
3. Tripods with sturdy hardware and locking systems are more secure and generally last longer than inexpensive models.
4. Fluid head tripods are preferable since they provide smooth motion for videotaping and are less likely to drop an imager if the head is left unlocked.
When carrying a tripod be sure to maintain a safe distance from energized electrical equipment or moving machinery.
IR Imaging for Check Tank Levels
For many facilities, obtaining accurate product level information for tanks and silos is critical for effective inventory management and safety. Under the right conditions, a thermal imager can quickly indicate product levels and serve as a cross reference for calibrating level indicators.
When product is stored in a vessel, the density of the product is usually greater than the head space of air or gas above the stored product. For stored products that generate heat, levels may be observed by imaging the vessel’s exterior and noting the temperature gradient between head space and product.
For stored products that do not generate heat, it is possible to rely on solar loading to create a temperature differential. Under solar loading conditions, vessels will usually exhibit cooler temperatures above the product level during the early to mid-morning hours. As the day progresses, the head space will exceed product temperature and show as a warm area above the product. This thermal pattern may remain for up to several hours after sunset and reverses once the head space cools to below stored product temperature.
In general, this application works best for un-insulated, single-wall vessels having a high emittance. For vessels with a low emittance, it may be possible to modify the surface with a stripe of high emittance paint. This application also works best on storage vessels where product level is not subject to rapid changes.
Thermal imaging of storage tanks and silos is one of the many applications covered in the Level I Certified Infrared Thermographer® training course. For more information on open enrollment classes or our convenient, web-based Distance Learning program, call 609-239-4788 or visit us online at: www.infraspection.com.
IR Inspections of Electrical Bus Ducts
Electrical bus ducts are a common feature found in many commercial and industrial electrical systems. When used to supplement regular PM, infrared inspections can help to detect loose or deteriorated connections that can lead to costly failures.
Electrical bus ducts are used to distribute low voltage power throughout many industrial facilities. Modern bus ducts are unitized structures that contain insulated conductors within a steel casing. Individual sections of bus duct, each typically 10 feet long, are joined with bolted connections at the end of each bus section. Published industry standards recommend that bus duct connections be manually tightened every six months.
Even with regular tightening of bus duct connections, loose/deteriorated connections are difficult to detect. With the bus duct under load, a thermal imager can readily detect the temperature differentials associated with loose connections. Properly functioning bus ducts should exhibit no temperature differential in the vicinity of bolted connections. Because bus duct conductors are hidden from direct line of sight, any inexplicable temperature differentials should be investigated and corrected immediately. Disconnect switches and cable connections should be checked for thermal anomalies as well.
To ensure complete coverage, bus duct should be inspected from both sides of the duct along its entire length. Termination cabinets should also be inspected once the covers have been removed. Annual or semiannual infrared inspections performed by certified, experienced thermographers should be used to supplement regular bus duct maintenance.
September 01, 2003
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IR Inspections of CMU Walls
Concrete Masonry Unit (CMU) walls are a common construction detail frequently used in single –story and low rise commercial block wall construction. An infrared imager can be used to quickly perform quality assurance inspections of reinforcing grout details which are critical to the strength of finished CMU walls.
During the construction of CMU walls, concrete grout is used to fill the cavity spaces of the blocks in order to provide structural integrity. Vertical details extending from the foundation to the top of the wall are usually placed at regularly spaced intervals along the length of the wall. Reinforcing grout is also placed around openings for doors and windows or in areas where extra wall strength is required.
Because grout details change the thermal capacitance and conductance of the wall, temperature differentials will occur wherever grout details are present. Infrared inspections may be performed under solar loading, heat loss, or cooling conditions depending upon local climate and time of day. Properly installed grout details will appear as uniformly cool or warm unbroken lines in the subject wall areas according to time of day and whether the inspection is performed from the interior or exterior of the building.
September 08, 2003
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IR Inspections of Emergency Power Systems
UPS systems and emergency generators are common defenses for facilities where uninterrupted electrical power is critical. Performed properly, IR inspections can help to improve reliability of emergency power systems.
Most facilities perform IR inspections of their electrical distribution systems at least annually as part of a PM program. To help ensure maximum reliability, regularly-scheduled IR inspections should also include the emergency power systems as well. When performing infrared inspections of emergency systems be sure to:
- Inspect all backup generators while running. Begin inspection at generator output leads and proceed to generator bus, breakers, and switchgear.
- Include all Automatic and Manual Transfer Switches. Inspect switches in both normal and emergency positions.
- Inspect UPS system controls, switchgear, battery cells, battery bus and wiring. Battery cell temperatures should be the same between cells with no hotspots on individual cells.
- Have adequate load on the subject emergency circuits This may be accomplished with normal facility load or by utilizing a load bank.
Taking the time to properly include your emergency power equipment in your IR inspection program can pay huge dividends by increasing the likelihood that your backup equipment won’t leave you in the dark should the power fail.
September 15, 2003
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Focus, Focus, Focus
Proper image focus is still one of the most important aspects of performing an infrared inspection. A clear image not only allows for optimal problem diagnosis, but it is also critical to accurate temperature measurement.
Clear focus is not difficult to achieve if you follow a few simple steps:
- Get as close as safely possible to your target
- Take time to carefully focus for optimum clarity. This may take some practice if you have a motorized focus mechanism.
- Ascertain that your target is stationary.
- Only shoot from a stable platform. If imaging from a motor vehicle, it may be desirable to shut off the engine to avoid vibration.
- Be sure your imager is steady as you capture the image. Gently push the store button rather than punching it.
- If using a handheld imager, consider using a tripod or monopod to help stabilize your imager.
Once you’ve stored an image, recall and check for clarity. If the results are less than perfect, start over. In addition to greater accuracy, capturing clear images makes it easier to convey information to the end user and/or the person who will eventually perform corrective actions.
September 22, 2003
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Spare Parts to the Rescue
Seasoned professionals know the value of spare parts when it comes to facility maintenance. The principle of always being prepared can be successfully applied to an infrared inspection program as well.
Like any electrical or mechanical device, thermal imaging systems are subject to wear and tear. Having user-replaceable spare parts on hand can help prevent unscheduled downtime for your infrared inspection program. Building a spare parts inventory is easy if you follow a few simple steps:
1. Examine your equipment for parts that are subject to physical wear such as eyepieces, switch covers or hand/neck straps.
2. Identify which parts are fragile and are most likely to break such as viewfinders or external monitors.
3. Determine which items are critical to operation such as power/video cables, batteries, fuses, screws and external hardware.
4. Inventory items which are easily misplaced such as lens caps and flash cards.
5. Purchase necessary items as soon as possible to ensure availability of specialty or custom parts. For critical items, be sure to purchase extras.
6. Replace spare parts when utilized to maintain inventory.
After building your spare parts inventory, keep mission-critical components in a safe place or with your imager so that you will have them when needed.
September 29, 2003
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Modifying a Surface for Temperature Measurement
Unknown emittance values are often the greatest error source when taking infrared temperature measurements. This error source can be eliminated by modifying a target with a material having a known E value.
Some of the modifying materials that thermographers commonly use include flat-finish spray paint, PVC electrical tape, masking tape, and spray deodorants containing powder.
Prior to modifying any surface:
- Make sure that it is safe to contact the subject equipment.
- Obtain permission to modify the surface from the end user.
- Ascertain that the selected modifying material will not melt, catch fire or emit toxic fumes when heated.
Once you have determined it is safe to modify a surface, proceed as follows:
1. Place radiometer at desired location and distance from target. Aim and focus.
2. Measure and compensate for Reflected Temperature.
3. Apply a surface modifying material having a known E value on target making certain that material is in full contact with target and there are no air pockets. Modifying material should be larger than radiometer’s spot measurement size for the chosen distance from the target.
4. Enter E value of modifying material into radiometer’s E setting.
5. Measure temperature of modifying material once it has reached thermal equilibrium with target.
6. For greater accuracy, repeat measurement three times and average the results.
For more information on the above technique, refer to the Infraspection Institute Guideline for Measuring and Compensating for Reflected Temperature, Emittance and Transmittance available from Infraspection Institute.
October 06, 2003
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Fire Resistant Clothing
Fire Resistant Clothing is required Personal Protective Equipment for many who work in high temperature areas or near energized electrical equipment. If your job requires the use of FRC, there are several important things of which you should be aware.
Not all garments classified as FRC are created equal. When choosing FRC you should be aware that:
- FRC is not fireproof. It is designed to protect the wearer from burns by resisting ignition during brief periods of high temperature exposure such as electrical arc flashes.
- FRC is manufactured with different materials and different weights. Be certain that the chosen material is appropriate for the task at hand.
- FRC effectiveness can be compromised by age, wear, contamination with flammable materials and the attachment of name patches or embroidery. FRC can be permanently damaged by improper cleaning or laundering.
- FRC is only effective when it is worn properly. It should always be worn as the outer-most garment. If worn over other layers clothing, the undergarments should be made of natural fiber and completely covered by the FRC.
Before wearing FRC, be certain to understand its proper application and limitations and how to use it properly. As always, remember to work safely!
October 13, 2003
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The Problem with Glass and Plastic
Glass viewing windows and plastic safety barriers are common features found on medium and high voltage electrical enclosures and devices. Although glass and many plastics are transparent in the visible spectrum, they are opaque in the infrared spectrum.
Because infrared equipment cannot accurately see through glass or plastics, infrared inspections must be conducted with these materials out of the line of sight of the infrared test equipment. When plastic barriers are present, try the following:
- Shift your viewing angle to try to see around or behind the barrier
- Have the qualified assistant temporarily remove the barrier observing proper safety precautions
- If barrier is short and the subject device is connected to insulated conductors, image conductors and report any inexplicable temperature rise.
When safety glass view ports are encountered in switchgear enclosures, it will be necessary to have the qualified assistant open/remove the subject panels. If this cannot be done due to safety interlocks, other types of electrical testing should be performed during regularly-scheduled PM shutdowns. As always, any obstructed equipment or equipment not inspected should be noted as such in the final written report.
October 20, 2003
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Know Your Safety Requirements
When working in a new facility or plant area for the first time, you may encounter safety rules that are new or different. It is always a good idea to review safety requirements with the project manager prior to project start to ensure that you are prepared.
When contacting a project representative concerning safety, be sure to ask the following:
- What general safety training and/or site specific training is required?
- Is special clothing, shoes or other Personal Protective Equipment required?
- Can infrared and related test equipment be used in the subject areas?
- Are respirators or additional safety equipment/monitors required?
- Will the work involve hazardous locations such as confined spaces, scaffolding or elevated platforms?
- What medical conditions might preclude a person from working in the subject area(s)?
- Are there site specific emergency procedures including evacuation, designated rally spots and how to report an incident?
Once the project commences, be sure to maintain good situational awareness and always stay with your qualified assistant. Becoming familiar with area safety rules in advance of a project can help to avoid cancelled projects and embarrassment while helping to maximize safety.
October 27, 2003
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Compensating for Reflected Temperature
When performing infrared temperature measurements, reflected infrared energy can be a significant error source. This potential error source can be overcome by using the proper radiometer and test procedure.
All thermographers have experienced reflected energy when inspecting low emittance targets. For qualitative imaging, single-point reflections may be avoided by changing viewing angle.
With quantitative imaging, failing to compensate for reflected energy can account for significant measurement errors. The infrared energy received by a radiometer is the sum of emitted, reflected and transmitted energy (E+R+T=1.0). For targets with a transmittance of zero, the error sources are emittance and reflectance. Using a quality radiometer, reflected energy can be measured and compensated for by using the Reflector Method described below.
1. Set radiometer Emittance control to 1.00
2. Locate radiometer at desired distance from target to be measured
3. Aim and focus imager
4. Position diffuse reflector in front of, and parallel to, face of target
5. Measure apparent temperature of reflector surface and remove reflector
6. Enter value obtained in Step 5 into radiometer’s computer under reflectance input – commonly labelled Background, TAmbient, or Reflected Temperature.
Be sure to maintain a safe working distance from any energized or potentially dangerous targets. For more information on this subject, refer to Infraspection Institute’s Guideline for Measuring and Compensating for Reflected Temperature, Emittance & Transmittance.
November 03, 2003
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Using a First Surface Mirror
Proper conduct of an infrared inspection requires line of sight access to the object(s) being inspected. A first surface mirror can often be utilized to inspect components that may be obstructed or obscured.
A first surface mirror is a special optical mirror that has a highly reflective coating adhered to the front of the mirror substrate. For infrared inspections, first surface mirrors can be temporarily utilized as reflectors to inspect areas that are inaccessible or unsafe for a thermographer to enter.
When using a first surface mirror with your infrared imager, keep the following in mind:
- Select a mirror of sufficient size for the selected imager and target
- Inspect mirror prior to use for cleanliness and condition
- Place the mirror in the optical path between the imager and object being inspected
- Position the mirror so that the reflective side of the mirror faces the imager
- Inspect object by imaging mirror surface
First surface mirrors are commercially available from a number of scientific suppliers that deal with optics and lasers. When using a first surface mirror, be certain to follow necessary safety precautions, especially when working near energized electrical components.
November 10, 2003
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Outsourcing IR Services
Whether you are considering instituting an IR inspection program or already have one in place, obtaining competent manpower can be a challenge. One potential solution is to outsource services for additional manpower and expertise.
There are many factors that will determine if a person is capable of effectively supporting your infrared program. Your success in qualifying your thermographers can be increased if you keep the following in items in mind when qualifying individual thermographers.
- Proof of formal infrared training and certification level
- Amount of experience with the type(s) of inspections planned
- Experience with the selected test equipment
- Knowledge of the system(s) being inspected
- Documentation of requisite safety training
If you choose to outsource your thermographers through an infrared consulting firm, you may also wish to check the following.
- Number of years in business
- Type of infrared equipment to be utilized and calibration dates
- Insurance coverage
- Safety records and experience modification rating
- Professional references
Depending upon your company requirements, be sure the chosen vendor is capable of complying with security, background screening, and substance abuse policy requirements.
November 17, 2003
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Autumn is the Time for Steam Trap Testing
With the onset of seasonably cooler weather, autumn is the time to prepare your steam system for the upcoming heating season. Testing your steam traps before the season begins can help to pinpoint costly leaks before the heating season begins.
Traditionally, two different non-destructive technologies have been employed to test steam systems – contact ultrasonics and temperature measurement. Used individually, each of these techniques has limitations that can lead to false positive and/or false negative results. Combining temperature measurement with ultrasound can result in a highly accurate test method by following a few simple steps:
- Measure trap inlet to ensure that temperature is above 212º F
- If trap inlet is below 212º F, ascertain why steam is not reaching trap
- Listen to the trap outlet with contact probe of ultrasonic unit
- Continuous hissing or rushing sounds usually indicate a failed trap
- Ascertain that trap is cycling periodically
- Frequent cycling may be caused by an undersized or worn trap
- Tag defective traps and document in written report
- Re-test defective traps after repair to ensure effectiveness of repair.
Always be sure to follow appropriate safety precautions especially when working with high pressure steam or when using ladders or lift equipment.
November 24, 2003
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Do You Have the Correct Time?
Most modern thermal imagers have the ability to record time and date along with thermal images. Taking a moment to ensure that the correct time and date are displayed on your imager before you begin your inspection can help to avoid wasted time and the collection of inaccurate data.
Having the correct time associated with your imagery is important for several reasons. With correctly dated imagery, it is possible to:
- Accurately document when the inspection was performed
- Easily store and uniquely reference image files
- Record the duration of a thermal event
It is always good practice to consciously check your imager’s clock each time you start your imager and make any necessary adjustments. Be certain to check your clock periodically during your inspection and whenever you restart your imager such as after a battery change or power interruption.
If your imager frequently displays incorrect time, it may be indicative of a defective or dead internal battery. To help avoid this problem, arrange for replacement of internal clock batteries whenever you have your imager serviced or repaired.
December 08, 2003
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Splash Protection for Your IR Imager
Taking your infrared imager into dusty or wet environments can have disastrous consequences for your imager. While it is best to wait for such conditions to subside, your can use a polyethylene sheet or trash bag to temporarily protect your imager and accomplish a qualitative inspection.
Since not all imagers and trash bags are created equal, you can follow the following steps to ensure good results.
1. Set up imager looking at thermally stable target with a high emittance. If using an imaging radiometer, note the apparent temperature of the target.
2. Select a clean, unused, polyethylene trash bag with a uniform thickness.
3. Open trash bag and place over imager. Use only a single layer of the bag plastic to cover the lens.
4. Use a rubber band to keep plastic smooth and wrinkle free over the imager lens.
5. Image target in Step 1 again and note image quality and apparent temperature.
6. Repeat above steps using different brand bags and thicknesses until you find a bag that gives minimal attenuation of image and apparent temperature.
7. After selecting the bag that works, trim to fit imager so as to prevent a tripping hazard. If your imager requires air cooling, leave the bottom of the bag open so the imager can ‘breathe’.
8. When finished imaging, remove bag from imager and discard.
While not glamorous, this procedure can allow you to successfully perform a qualitative inspection in an environment that might otherwise harm your imager.
December 15, 2003
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Renting a Thermal Imager
Whether you are facing an equipment shortage or looking to evaluate the characteristics of a new imager prior to purchase, renting a thermal imager may provide a solution. As with purchasing an imager, there are several important things to consider when arranging for a rental unit.
To help ensure that you select an appropriate imager for rental, be certain to:
- Identify appropriate spectral response required for project
- Determine if temperature measurement is required
- Evaluate the system for objective specifications
- Ascertain imager compatibility with reporting software
When arranging for a rental, obtain terms and conditions from the rental agency. These should include, but not be limited to: rental period, extension of rental, shipping costs, and requirements for insurance against loss. One should also consider the rental agency’s ability to provide technical support during the rental period.
For more information on specifying an infrared imager, refer to the article, “Selecting, Specifying, and Purchasing a Thermal Imager” which may be found on this website here.
Lastly, the greatest limiting factor in any infrared inspection is the thermographer. For accurate results, infrared inspections should be performed by properly trained and certified thermographers. For more information on training, please contact Infraspection Institute.
December 22, 2003
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Our Most Important Tip
With the end of the year upon us, we wish to follow in the grand tradition of saving our best for last. In this Tip of the Week, we address some of the most important issues facing predictive maintenance professionals.
With the holiday season in full swing, we invite PdM technologists and thermographers throughout the world to consider the issues of inventory, reliability and communication and offer our best advice as follows:
- Inventory – Take time to reflect on your many blessings such as good health, family and friends.
- Reliability – Set time aside to appreciate having friends and relatives in whom you can confide and trust.
- Communication – Remember to share your feelings with all of the special people in your life by letting them know what they mean to you.
Spreading cheer and holiday spirit is easy; it begins with each of us as we let others know how we feel about them.
As we enjoy this holiday season, we extend a heartfelt Thank You to all of our readers, friends, and associates throughout the world for everything that you do for us all year long.
May your holidays be filled with peace and joy and your New Year with good health and happiness.
~ Jim & Chris Seffrin
December 29, 2003
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Thermography and NFPA 70B 2002 Edition
For the past decade, the document, NFPA 70B Recommended Practice for Electrical Equipment Maintenance, has recommended annual infrared inspections of electrical systems. The 2002 edition of NFPA 70B contains several revisions of which thermographers should be aware.
The 1994 edition of NFPA 70B outlined many important issues surrounding infrared inspections including training, equipment, required conditions, inspection frequency, and reporting. The 1998 edition expanded on these issues and referenced temperature limit benchmarks. The 2002 edition text is nearly identical to the 1998 edition for the chapter, “Infrared Inspections”; however, there are some important changes:
- Specifications for Infrared Inspections have been moved from Chapter 18-17 to
Chapter 20.17 - Chapter 20.17.52 is a new addition, requiring that circuit loading characteristics be included as part of report documentation
The 2002 edition also contains a completely revised Annex I recommending infrared scanning at least once per year for the following equipment:
- Outdoor Substations
- Switchgear Assemblies
- Stationary Batteries and Chargers
- Motor Control Equipment
- Low Voltage Busway
- UPS Systems
As with previous editions, NFPA continues to recommend semi-annual or quarterly infrared inspections for equipment “…where warranted by loss experience, installation of new electrical equipment, or changes in environmental, operational, or load conditions.”
