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I am trying to measure the height [level] of the water in a box that goes from -3°C (saltwater) with water and 150°C dry. Objects will be placed inside the box but will not take up the whole area.

I need a sensor that is robust enough to go through those temperatures and measure accurately. I was thinking of using a float switch on a worm gear, but the motor couldn't stand the heat...

What other methods/sensors are there? I also have a clear tube that runs outside of the tank meant for visually inspecting the height of the water.

Can I use a ultrasound sensor to measure water level? is a closely related question but focuses upon ultrasound. That won't work for my particular case because the contents of the box are effectively unknown. And capacitive techniques won't work either as the box will be grounded.

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    $\begingroup$ What's happening to the box/tank? Is it stationary? Is it moving? Can you run wires or tubes to it? Last but not least: what are you ultimately trying to accomplish? Why do you want to measure level? Why such temperature excursions? $\endgroup$ – Nick Alexeev Jun 7 '16 at 22:01
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I didn't understand quite clearly what you want, but it sounds definitely like ultrasound. Although you mention that it probably won't work, I would still try it first. Another possibility would be to use radioactive material (gamma-source). But it might be difficult to get this stuff. The radioactive type is used for measurement of fill height in bottles. Though I don't know how they do it exactly.
I recommend to contact companies, which build bottle filling machines. They probably have ready to use solutions.

Concerning the laser: Any laser should be able to do this. The cheapest should be the best suited. You don't need a high quality beam.

If this doesn't help you, please make a drawing, to make your question more understandable.

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How about using a Capacitive Fluid Level Sensor from Molex. The sensing element can be affixed outside of the tank. Here is an image of the sensing element affixed outside of the a container. .

enter image description here

Below is an expert from the product website.

Capacitive Fluid Level Sensors easily mount to the outside of a container and measure through almost any non-metallic material. They are custom designed and optimized for a range of applications such as medical devices, pump controls, beverage machines etc. A design can use a traditional printed circuit board for a flat surface, or a thin, flexible circuit to accommodate curved surfaces or space-constrained applications. The sensor includes customized embedded software which can be configured for auto-calibration for easy installation, or manual calibration to maximize the accuracy.

With regard to robustness, you will have to work with the vendor. This offering is fairly new and I don't have any details beyond what is published on the company websites.

References:

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I do work for a company that makes sensors that might be applicable. These sensors are used, among other places, to measure levels in ballast tanks of ocean-going ships. The possible gotcha for your case is the size of the tank. These sensors are meant for tanks depths of a few meters at least, up to 30 meters or so. When the tank is too small, the errors in the system usually become too significant. A few mm difference of liquid height in a ballast or fuel tank on a ship is irrelevant, especially considering the ship isn't sitting perfectly still and the liquid always sloshes around a bit anyway.

The best description of the technology I found on their web site is at Jowa-USA Marine Products. Imagine a resistance wire wrapped around a conducting rod, but with insulators in the right places so that at rest the wire doesn't touch the rod. When external pressure is applied, like from being submersed in a liquid, the wire is squished against the rod and makes electrical contact. The resistance between the rod and the wire at the top of the sensor then tells you the distance from the top to the first contact, from which you can derive the level of the liquid.

Of course this is all packaged and encased to be impervious to various fluids. Different materials are used depending on the fluids. The two most common applications are for ballast tanks (seawater) and fuel tanks (diesel fuel). These things last many years on average.

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