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Grant Calibration

I spent some time this Sunday taking volume measurements on my small 1 gallon grant and figuring out how to calculate some really cool information based on that and the float switch timing. I’m sure alot of homebrewers out there wonder why in the heck you would want to go through all the trouble of designing and building a grant into the system when it seems completely unnecessary, but I see significant advantages:

  • Reduced grain bed compaction because the pump does not pull on the bottom of the mash tun
  • Provides a convenient wort sample point for gravity and temperature readings
  • The dead space below the grant vessel outlet may collect / capture grain particles, preventing them from getting into the kettle

In addition to these philosophical points, you can derive some pretty important process data:

  • Sparge rate (qts/min)
  • Approximate accumulated / transferred volume (gal)
  • Pump rate (GPM)
  • Estimated remaining sparge time / end time

These calculations are pretty straight forward, after you think through the basic mass balances:

  • Sparge rate is best captured while filling and when the pump is off. Translating this into my grant design, that is the time between when the bottom float switch is lifted and when the top float switch lifts. The volume between these two events is about 11 1/4 cups, so translating that into a sparge rate = (11.25)/4/Tfill/60 where Tfill is in seconds and the sparge rate is qts/min. This measurement probably has about 1/4 cup measurement error in it.
  • Volume transferred is best captured after a complete pump cycle, being the time between when the low float switch raises (and starts the cycle) and when it then drops down, turning off the pump and ending the cycle. This volume is equal to the volume required to fill the grant (about 11.75 cups) plus any additional volume that flowed in from sparging while the pump was on, plus an additional 1/4 to 1/2 cup that it takes to lift/drop the bottom float switch. Simply put, this means the Vtrans = (sparge rate * (Tfill + Tpump) / 60 + .5) / 4, where Vtrans is in gallons. I suppose you could also just call sparge rate times the pump-off-to-pump-off cycle too… either way, the difference in only a couple hundredths of a quart.
  • The pump transfer rate is then easily back calculated by dividing the volume transferred by the time the pump was on, or = Vtrans / Tpump / 60 to give GPM.

If I totalize the volumes transferred each time the pump goes through a cycle, I should have a rough estimate of how much wort is in the boil kettle. That information could then be used for other things, such as calculating how long the sparge has left = (total volume desired – total volume transferred) / sparge rate. I can also use it to calculate if I am transferring to quickly and run the risk of sucking air into my transfer hoses… there is a delay between when the float switch drops, the pump turns off, and the rotational inertia of the pump impeller dissipates, and flow stops.

The aspect that excites me most about this though is capturing plots of sparge rate vs. time. Why do that? Because it could be used to predict stuck mashes, among other things. If you see the sparge rate drop dramatically, a graphical curve would be useful in deciding if you need to mix things up again or just ride it out. Having a library of these curves would also be useful in predicting mash behavior for things like 100% barley vs. 50% barley, 50% wheat.

Categories: Brew gear, Tech geeks
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