Precision Pressure In A Piston

[Scott] is building a DIY yeast reactor for his aquarium. What’s a yeast reactor? [Scott] wants to pump carbon dioxide into his aquarium so his aquatic plants grow more. He’s doing this with a gallon of sugary, yeasty water bubbling into a tank of plants and fish. In other words, [Scott] is doing this whole thing completely backward and utilizing the wrong waste product of the yeast metabolism.

However, along the way to pumping carbon dioxide into his aquarium, [Scott] created a very high precision pressure sensor. It’s based on a breakout board featuring the MS5611 air pressure sensor. This has a 24-bit ADC on board, which translates into one ten-thousandths of a pound per square inch of pressure.

To integrate this pressure sensor into the aquarium/unbrewery setup, [Scott] created a pressure meter out of a syringe. With the plunger end of this syringe encased in epoxy and the pointy end still able to accept needles, [Scott] is able to easily plug this sensor into his yeast reactor. The data from the sensor is accessible over I2C, and a simple circuit with an ATmega328 and a character LCD displays the current pressure in the syringe.

We’ve seen these high-resolution pressure sensors used in drones and rockets as altimeters before, but never as a pressure gauge. This, though, is a cheap and novel solution for measuring pressures between a vacuum and a bit over one atmosphere.

26 thoughts on “Precision Pressure In A Piston

  1. Very good project…but in my discussions with people designing MEMS pressure sensors I have learned that the membrane is quite sensitive to the gas pushing on it. So the pressure indicated may or may not be accurate, even though the sensor is highly accurate in air.

  2. I disagree, Scott is doing it the right way –

    Give the plants the CO2 and put the resulting mash in a still for a bit of purification and consume :)

    Plants are happy and Scott is now happy and relaxed watching his fish and sipping on a suitably blended beverage ;)

  3. Holy 56 minute video!
    I get he wanted to be thorough but there’s something to be said for brevity in communicating this stuff.

    I question the need for this sensor in his project though. He’s just got a decorative airlock on the end of his yeast ranch, the pressure will always be equal to the hydrostatic pressure for whatever depth he puts the outlet at, plus a little for drag from the hose and airstone resistance.
    There’s big money in brewing and yeast are a model organism (ie absurdly well studied) so if he’s planning on tracking CO2 output there’s probably whole books written on which formula gives the most accurate metabolism for his strain. Tracking specific gravity or refractive index of the wash are all probably simpler ways to track CO2 input into his aquarium.

    This was ‘just’ an excuse for him to practice I2C and reading data sheets which is an admirable goal in its own right.

  4. I think the error band quoted in the data sheet at 25C and between 700 and 1100mbar is about 1mbar, and increases to 2 mbar between 0C and 50C….

    …so I’m not entirely sure what the other 13 bits achieve in ADC; maybe something marketing related?

    1. Consider precision vs. accuracy. This sensor has 24-bit precision (precise to 0.012 mbar) but less accuracy (accurate to 1mbar). If it is used to detect a “before then after” pressure difference (note the “baseline reset” button), the accuracy becomes less important for the application.

      1. Yep, as the datasheet says, it can give you 10 cm of resolution. This helps to make precise altimeters where you can just reset to zero, you are interested in ho much it move, not so precise where it is. Can be used even in smartphones to GPS assist, best way for sports tracking to see if you gone up / down stairs.

    2. I’m confused because it has a max pressure rating of 1200mbar, which is 17psi. I assumed that was absolute, since the range is 70mbar to 1200mbar, and sea level is ~1,013 mbar.

      He’s shown pumping it up with the syringe to ~ 23-24 gauge PSI. Huh? How is that even possible?

        1. Nearly double its rating. And I’m further confused because he says it’s “rated to 2 atmospheres or something” – nowhere in the data sheet does it say that. They’re pretty clear that the max useable pressure is 1200mbar and the burst/damage pressure is 6 bar.

        1. No, the 6 bar pressure rating is the maximum pressure it can take without the sensor being damaged. That’s not the same as “the range at which the sensor returns values.” It’s sort of like a thermometer that only reads up to 100C, but will not be damaged below 200C. That doesn’t mean you’ll get readings at 150C.

  5. “We’ve seen these high-resolution pressure sensors used in drones and rockets as altimeters before, but never as a pressure gauge. This, though, is a cheap and novel solution for measuring pressures between a vacuum and a bit over one atmosphere.”

    Wonder how well it would do as a blood pressure monitor?

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