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Relative Humidity Hysteresis

What is Hysteresis and why do we care?

hysteresis |ˌhistəˈrēsis| noun Physics
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.

Layman's terms: RH probes can lag behind the actual reading by a small amount as the concrete humidity changes. You'll have to ask Jason or Ed if hysteresis can cause a lag greater than the percentage of variation inherent in the probes.

If I am close to a target RH before a go / no-go decision is going to be made, I insert a couple fresh probes to get new readings to compare.

I think this is really important if the probes started out reading in the high 90's. The alkalinity is so high at that RH I don't see how the sensors resist damage if they are exposed to it for a long period of time.
JD Grafton
Concrete Answers for Flooring Problems

Jason or Ed can hysteresis can cause a lag greater than the percentage of variation inherent in the probes?

Yes, at very high and very low RH%. This would usually require extended exposure to this environment. The key word to remember here are lag and temporary. In high RH% environments, IF hysteresis is in play, the sensor may lag behind temporarily until the true RH% goes below say 90%. As CC stated, we recommend if you know the slab is going to be high (i.e poured 45 days ago) then do a sampling to begin with and then if there is a question, add more towards the end. Hysteresis is not an absolute.


Have a slab that is sitting at 99% but some probes are dropping. It is new, closed off from elements all winter, no sand under slab. How do I know if the stuff hanging at 99% are subject to Hysteresis?

Mr Adams:

I hope all is well. The easiest way would be to put a couple of additional probes into the slab now to get some comparative data.



project is in Eureka... about as close as I get to you guys in Oregon. So if you get any calls... from SHN Engineers... you have context

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