Ask MAKE: Liquid level sensor for brewing system?

Hi, I am building an automated brewing system for home brew beer. It will be controlled via an Arduino and network interface. I am using 15 gallon stainless steel kegs as brewing vessels. What is the best method or type of liquid level sensor to use to accurately detect liquid levels in the kegs? The liquid won’t be water alone, and all three vessels will be heated. I was thinking of using an external sight glass + a pressure sensor of some kind. Maybe someone in the Make community has a better idea or has done this before?

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Thanks Brett! So, you want to measure the level of a liquid in a tank. These are some of the ways that I can think of to do this:

• Capacitive: Measure difference in dielectric constant between air and water
• Differential pressure measurement: Measure pressure at bottom of liquid
• Optical: Use an array of light sensors to sense where liquid is/isn’t
• Float: Put a float on a stick, then measure the displacement of the stick
• Weight: Weigh the liquid, and calculate volume based on density
• Conductivity: Measure liquid conductivity at a few discrete points

Out of these options, what would be best? I think for your project, the best way to go is probably to use a differential pressure sensor, like you suggested. It seems like it would be the most reliable and least invasive method. Also, it just so happens that the folks who wrote the Practical Arduino book have a Water Tank Depth Sensor project that might be a good starting point.

If you only want an approximation of the amount of liquid in a tank, a simpler way to go would be to test for conductivity at a few points along the side of the tank. You wouldn’t be able to sense the exact amount of liquid present, but it would be a fine way to check if the tank is half-full or not. The folks at electronics labs built a single-point Water level detector using a PIC microcontroller, which could be expanded to measure multiple points

Capacitive sensors might work, but might be hard to set up in a metal tank. Optical sensors work the same way as the conductivity sensors, and would be more expensive, so they probably aren’t worth the trouble. A float could potentially work, but could get gummed up by hops or dead yeast, so it might be unreliable. The weight sensor could also work, but I think it might be tricky to keep it calibrated.

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