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RH test results varying
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05-07-2012, 01:07 PM #5
CC Solutions Concrete Moisture Evangelist *******
Status: Offline Posts:1,067 Threads:69 Joined:Dec 2009
One point (and I just watched a gentleman do this today) is to not let the reader sit in the probe. You want to insert the reader and remove it within a few seconds and read the results. What could be happening is the thermal mass of the reader is draining into the probe and changing the probe's reading. The best way I can explain this is imagine the probe is acclimated in a concrete slab at 70 degrees and you bring in a cold reader from your vehicle while enjoying your -10 degree Minnesota weather. That freezing reader will cool off the probe as it sits in the hole and throw the RH reading off the charts. I use this extreme scenario to illustrate the point. Within more modest temperature differences a quick plunge of the reader into the probe will not affect the reading.

Do you have a psychometric chart available?
We have tried to correlate the readings we see with a standard psychometric chart but they don't exactly follow the chart. But you would get a pretty good indication of what is happening with the temperature vs humidity by checking your numbers on the chart.

The temperature of the concrete is an important aspect of determining acceptable RH (which is why we would love to be able to follow a psychometric chart). The ASTM states we must be at service temperature, but how much will the RH change if we measure our slab at 75 degrees and we expect service temperature to be 65 degrees? The psychometric chart says the amount of moisture that gives us 72%RH / 75 degrees, is the same as the moisture in 100%RH / 65 degrees!

Unfortunately we haven't been given the green light to use the psychometric chart for calculating RH at service temp yet, but I want you to be aware of how much temperature can affect RH so you can make judgements on what you are seeing when you take readings. If you see a couple of degrees drop in temps, I'd expect to see anywhere from a couple of percent or more of rise in RH.

JD Grafton
Concrete Answers for Flooring Problems

Messages In This Thread
RH test results varying - Erica - 04-30-2012, 02:20 PM
RE: RH test results varying - rapidrhrep - 05-01-2012, 08:13 AM
RE: RH test results varying - Erica - 05-01-2012, 08:27 AM
RE: RH test results varying - eaadams - 05-07-2012, 09:43 AM
RE: RH test results varying - CC Solutions - 05-07-2012, 01:07 PM
RE: RH test results varying - Rubensgt40 - 05-08-2012, 06:49 PM
RE: RH test results varying - CC Solutions - 05-09-2012, 06:05 AM
RE: RH test results varying - Rubensgt40 - 05-09-2012, 04:35 PM

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