How hot is the beam of a Fresnel Lens? (page 2)
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"Can the temperatures go higher than your best record of 3800 degrees Fahrenheit (2093 °C)?"
In theory, a Fresnel lens or parabolic mirror can reach an "almost unlimited" temperature build when compared to heating element ovens and gas nozzle based heaters or torches. Because there is no direct contact to the chamber heat, light can be continually "pumped" in without damaging the power source. There is no brass nozzle to melt and no heating element to overheat and destroy. "Unlimited in theory only." Because light needs a path entering the chamber to take advantage of a crucible's thermal insulation or in the case of an insulated oven, a glass window, physical limitations prevent build temperatures above 5000 degrees Fahrenheit. The best method for concentrated solar maximum temperature builds is direct exposure to an object in an open faced vessel with bottom and side insulation. A well insulated crucible or oven like insulation box does experience thermal heat loss from the open top as heat rises but does not experience light filtering and reflections from a glass plate sealing the vessel. For general cooking with a large lens, this is not an issue as the power of the lens is usually much greater than what is needed.
"How does material testing help?"
Build temperatures can also be estimated on very small masses of material like a tungsten bulb element or metals with a specific melting or burning point. Masses of metals with melting points exceeding 1700 degrees Fahrenheit (926 °C) are limited to 5 grams during testing. Metal masses exceeding 5 grams have an increased surface area larger than the optimal focal point resulting in heat leaching or heat sinking to the surrounding atmosphere. Also, molten metals produce a mirror like shiny surface reflecting away up to 50% of the concentrated sunlight. To overcome this effect for maximum material temperature estimates, smaller amounts of material (5 grams or less) are overwhelmed, with the optimal focal point size exceeding the surface area of the metal by at least 50 percent. This offsets reflected sunlight and heat leaching, allowing for good material based estimates. An example is a real copper US penny pre-1982. These pennies are very difficult to melt because of their size and heat transfer. Copper wire however, can be easily melted and fine strand braided copper wire melts almost instantly.
"Pennies are melted all the time on Youtube."
NOT COPPER. All youtube videos (posted by other people) demonstrating penny melting use ZINC pennies dated 1982 to current (1/3 the melting temperature of copper) that are copper coated. These videos are melting zinc, a metal that has a melting point of only 787.2 degrees Fahrenheit (419.5 °C) and not that of copper which is 1,984 degrees Fahrenheit (1,085 °C). Copper and zinc plated copper pennies look identical to most people and a deep scratch or date matching are easy ways to tell them apart. I have seen many video authors claim they are melting copper, but soon the molten mass becomes silver in color. Copper does not do that. We have some of the most powerful Fresnel lenses available and they cannot instantly melt a stack of copper pennies. As for a stack of post 1982 copper plated ZINC pennies - unofficially, (legal reason, melting currency is sort of illegal) our most powerful lens melts a ten penny equivalent stack in 7 seconds. After a 30 second exposure the molten mass begins releasing a dangerous zinc vapor and yellow slag is formed. After 60 seconds, nothing is left, all the metal is gone and only a burn spot remains.
Warning, do not burn zinc. Melting zinc is safe outdoors and works wonderful for lower temperature casting but burning zinc can result in "Metal Fume Fever," a debilitating and potentially fatal health condition.
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Evening time setting sun super solar power giant Fresnel Lens.
Using a Fresnel Lens to heat a large metal surface, the Leidenfrost Effect causes water to dance. Large masses of steel do not melt easily with a Fresnel Lens in open air.