I can’t answer that yet. I have used for 2 batches with my house tap water but I don’t know the exact makeup of that water. I plan to go back to RO water for my next batch. Then I will have some insight.
My first batch with the meter was with RO water but the RO water had TDS of 70ppm and I think I messed up the calibration. That result was off from Brunwater but I have no idea what made up the 70 ppm of TDS.
PS. without a meter I did think just using Brunwater with RO water was helping my beer. But, I could never measure my pH. I wanted to know exactly where I was so I can try to correlate to what I am tasting with my lagers.
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For EC/TDS measurements the ATC is very important since the slightest change in temp will affect the reading. There are multiple causes as to why a different value is obtained from an RO system. As far as the meter reading correctly the two things that come to mind are:
Temperature stability
Contamination
Depending on the meter that you have the thermistor used for temperature compensation can be either exposed or built into the body of the meter. An exposed temperature probe will compensate faster than one built inot the body. The one that is built in the body can take time to reach thermo-equilibrium with the sample. Until that happens the readings will drift until the temperature is stable.
As far as contamination, it takes very little of anything to contaminate the sensor. For example if the meter was recently calibrated or used to measure something else then it possible that there is a residue still left on the meter that will increase the EC/TDS reading.
Best practices for EC/TDS measurement include:
Use two beakers/containers for both calibration standard and sample
Use one beaker with calibration buffer for rinsing and the other for the actual calibration
Thoroughly rinse probe with DI/RO water
Use one beaker with RO water to test for a rinse and the other for testing
Place the meter in the beaker. Do not allow to touch the sides or bottom of the beaker. The reason is that there is a fringe field effect with EC sensors. Disrupting the filed can yield different results
As stated previously, 70 ppm is noth very high TDS but I would expect a lower reading from an RO system. I have seen readings from 25-50 uS/cm EC which is equal to 12.5 to 25 ppm TDS.
That’s been my experience too. In fact, the biggest variation I’ve found has come when my LHBS changes malt suppliers. I saw a surprising change in actual pH (lower than Brunwater predicted) when they switched from Best Munich (which yielded a mash pH in-line with Brunwater) to Avanguard Munich (which for a few batches came in lower than Brunwater predicted).
Lately they’ve stuck with the Avanguard but the pH has come back closer to the Brunwater prediction. I know there have been other discussions indicating that some maltsters’ products were sometimes tainted with acidulated or somehow otherwise overly acidic.
I guess my take-away from all that is while I feel pretty confident in the predictions, I still like to test mash pH once per batch even with tried and true recipes. Usually I just test the same sample I’m using for my pre-boil gravity and record it. At that point it’s too late to adjust anything, but at least if any changed I’ll understand how that might relate to any sensory differences I’m experiencing when tasting the beer.