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Hysteresis and you:

#1
Hysteresis: the phenomenon in which the value of a physical property lags behind changes in the effect causing it, as for instance when magnetic induction lags behind the magnetizing force.

Forgive me Ed if I am out of place, but I wanted to get this in writing as I understand it before the weekend brain cell slaughter Big Grin .

Relative humidity probes (all probes) are subject to the effects of hysteresis, which is the slow change from one state of being to another, or in the case of probes measuring concrete moisture, going from dry to wet, or wet to dry.

So when a probe is inserted in a slab that is very wet and the probe is dry, the probe will slowly climb up from reading low humidity until it reads very close to accurate (the accuracy percentages are listed by each probe manufacturer).

Conversely, when a probe is reading more moisture and the concrete dries, the probe can tend to lag a bit behind the actual slab moisture.

For this reason (and this is in the Wagner instruction manual under 'Best Practices') if you are testing a slab that is very wet it is advisable to only insert enough probes to get an idea of the RH you have. If the slab is drying rapidly the probes subjected to very high RH readings (above 90%) may lag a bit behind the slab humidity as it falls. New probes inserted will be coming up from a drier state to read RH and you may see some discrepancy between the readings of an old probe and a more recently installed probe.

Now as I understand it, this discrepancy will be rather small considering the percentages these probes are allowed to be off. Here's my real life experience for you to consider:

Previous probes are reading 95% at 69 degrees F. These probes have been in the concrete for many weeks and are coming down from the 98% range. New probes are reading 89% after 22 hours, and are coming up. When we look at the allowable variation (2% between 50% - 90% and 3% between 90% and 95%) these readings make sense.

The existing probes are coming down and reading 95%.... the slab is probably much closer to 93%. There is aggressive drying taking place with HVAC and dehumidifiers running 24/7. The probes are lagging a little bit. The new probes are reading 89% and they were at 50% before I put them in the freshly drilled holes, so we know they will come up as they acclimate for 72 hrs. I suspect they will stabilize somewhere in the low 90's. If they only hit 90% there is an allowed 3% variation which means as the new ones come up and the existing ones come down, the lag plus the variable actually overlap already in the 92% to 93% range. Pretty dang accurate when we are measuring just grains of water in a variable like concrete!!!!

I did not publish or discuss my data for this job yet. I do not want to show a variation in readings before I can fully explain the readings, and I believe the new probes will come up a bit which will make my explanation even easier.

I will keep this thread updated with my findings (if Admin doesn't shoot me Sad ) so we all can learn from my adventure.
JD Grafton
Concrete Answers for Flooring Problems
[email protected]
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Messages In This Thread
Hysteresis and you: - CC Solutions - 08-12-2011, 12:13 PM
Concrete Hysteresis - EWagner - 08-12-2011, 12:38 PM
RE: Hysteresis and you: - CC Solutions - 08-12-2011, 01:45 PM
RE: Hysteresis and you: - Ernesto - 08-14-2011, 08:08 PM
RE: Hysteresis and you: - CC Solutions - 08-14-2011, 09:36 PM

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