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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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#2
Big Grin 
Quote:I will keep this thread updated with my findings (if Admin doesn't shoot me)

JD, I think this thread could be a very valuable resource for installers or any GC who is wondering about their relative humidity probe readings.

Besides, shooting customers is generally considered poor customer service. Smile

-admin
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#3
I just don't want to be airing dirty laundry or anything... My initial reaction was EEEEKKK!!! My Wagner's don't agree!!!!! All my integrity and trust has been destroyed like a sand castle in a rogue wave!!!

I immediately called Ed and we walked through what I was seeing and it all made such perfect sense. I'm so glad I didn't have a tag along superintendent jumping on initial readings and flying off the handle.

Kudos to Ed Wagner for being available when I needed him (he always is so helpful). It just amazes me to no end when I call Wagner Electronics and can speak to Ed Wagner himself, and then he takes the time and has genuine concern for his customers. The man knows his stuff and really is a leader in a field we truly need some bright minds leading in....

Is there any wonder why I espouse these probes as hard as I do? Thanks to all of you in there lighting up these pixels and helping us out there drilling holes!!! First round is on me tonight! See you at the campfire! Tongue
JD Grafton
Concrete Answers for Flooring Problems
[email protected]
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#4
OK, so if the first ones are going lower and the new ones are going higher, when are they expected to meet or attain equilibrium? At the same time maybe?
Stephen Perrera dba
Top Floor Installation Co.
http://www.tucsonazflooring.com
http://www.floorsavior.com
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#5
The test results are golden after 72 hours.... The dropping probes (the way I understand it) are kind of a wild card....
JD Grafton
Concrete Answers for Flooring Problems
[email protected]
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