Andreas Spiess Turns a Blood Pressure Monitor Into a Working Water Level Sensor

Electronics educator Andreas Spiess has built a water level meter with a difference: it's constructed from a cheap cuff-based blood pressure monitor, repurposed for fluid level sensing.

"We insert a small open pipe to the bottom of a tank and add a pressure sensor on the other side," Speiss explains of the idea behind the sensor. "The theory says that the fluid enters the tube from the bottom and, because the top is closed by the sensor, creates a pressure proportional to the weight of the fluid above it. And the weight of the fluid is proportional to the level of the tank. And the best: The sensor can be placed outside the tank in a safe, dry, and non-corrosive area."

The only trouble is: a functional sensor would need to be entirely air- and water-tight to avoid leakage and subsequent accuracy drifts. The fix: The use of a pump to blow any water, which has entered the pipe out before taking the reading. Better still, there's a product already on the market that has both an air pump and a pressure sensor: A cuff-based blood pressure monitor.

"I found this article in the German Make journal where the author used a blood pressure monitor to create such a sensor," Speiss explains. "Such a device obviously contains a pump and a pressure sensor. And it also contains a valve to let the air out in a controlled way to measure the blood pressure."

"If you go without [using] this blood pressure monitor, you have to find another possibility to attach at least three parts: The measuring tube, the pump, and the sensor. [And] do not expect you could 3D-print such a part. As said before: 3D-printed parts are not airtight."

Spiess' full build log, including details on calibrating the resulting sensor, is now available on his YouTube channel.