Why Are Crystal IR Lenses Not Suitable For Industrial Applications?
Electrical equipment has evolved over time and their complexity continues to increase as the world’s demands for power increase. Companies strive to improve their processes of monitoring and maintaining their electrical assets to improve safety and protect their bottom line. Inspection and Maintenance Teams need durable tools to use in their inspection procedures. One of these tools is the Infrared Inspection Window; however, not all IR Inspection Windows are created equal. Let’s investigate.
In a previous article, we explored the various types of lens materials used in infrared inspection windows. The choice of lens material is driven by application, environmental factors, wavelength to be measured and cost.
Crystal lenses have been used historically in infrared windows; but, there many disadvantages when used in environments commonly seen in an industrial setting.
1. Humid Environments: Calcium Fluoride and Barium Fluoride lenses are hygroscopic and over time will absorb water vapor into the crystalline structure gradually reducing the infrared transmission through the lens. Some vendors coat the lens to reduce the ingress of water to extend lens life, but this has been observed to be of limited effectiveness.
2. Environmental Stresses: Calcium Fluoride crystal lenses are brittle and unable to withstand impact and mechanical stresses. Mechanical stresses like jarring drops, exposure to high frequency noise or harmonics and equipment vibrations can cause the crystalline structure to degrade or fracture resulting in the elimination of the safety barrier between the electrical component and the inspection team. This is a dangerous situation that could result in bodily harm or even death. Barium Fluoride lenses are even more fragile than Calcium Fluoride. In addition, Barium Fluoride, in powder form as can be found with a broken lens, is a known muscle toxin and suspected carcinogen.
3. Transmission Rates: Lens materials have different transmission rates and some rates will change over time. Barium Fluoride and Calcium Fluoride lenses have excellent transmission properties when they are new; however, as these crystal lenses absorb moisture from the environment, their transmission properties will change, and the thermographer may not realize that a change has occurred until an actual fault occurs.
4. Size Limitations: Crystal lens manufacturers determine the minimum thickness requirement for a given window diameter by calculating its “Modulus of Rupture”, a mathematical formula used to determine lens thickness required to resist a given pressure level. The bigger the crystal lens, the more fragile it becomes unless the thickness of the crystal is increased. But, increasing the thickness of the crystal lens will decrease the transmittance of that lens and will affect temperature readings.
So, why is this a limitation? Switchgear is usually set so the plenum vents open at 25 psi to redirect an arc blast force away from the panels where personnel would be interacting with the equipment. At a pressure of 25 psi, crystal lenses of varying sizes would have these minimum thickness requirements:
* Two Inch – 3.7mm
* Three Inch – 5.5 mm
* Four Inch – 7.3mm
There are no crystal windows that are even close to these minimum thickness requirements. Crystal lenses thinner than the above minimum requirements would shatter at 25 psi. Should an arc flash or blast occur, crystal windows will shatter and expose personnel to a dangerous environment.
What Lens is Suitable for Industrial Applications? Reinforced Polymer infrared window lenses are unaffected by mechanical stress and they maintain the safety barrier and protect the inspection team. Polymer IR lenses have a fixed and stable transmission rate such that one the transmissivity value of the lens is determined, it will not change over time. A polymer lens reinforced with a special grill is malleable and will absorb an impact rather than shatter. Polymer lenses are also non-reactive to exposure of moisture, humidity, seawater and a broad range of acids and alkali so their transmissive properties will not change over time. Another advantage is the polymer lens material can be manufactured in various sizes and shapes and still provide the durability needed in industrial settings. The only disadvantage to using polymer lenses is if the ambient temperature exceeds 392° F (200° C).
IR windows are manufactured with a variety of lens materials. Historically, crystal lenses were used in IR windows however, crystal lenses are fragile and can easily break exposing personnel to safety hazards. Transmission rate drift with crystal lenses is a real problem only slowed with the use of coatings. They are not suitable for industrial applications. Innovative products using polymer lenses are available and provide the durability needed in an industrial environment.