05-26-2013, 10:38 AM
What is hysteresis?
frequency to read RH
05-26-2013, 10:38 AM
What is hysteresis?
05-27-2013, 09:48 AM
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.
05-31-2013, 09:49 AM
<<So if you absolutely have to guarantee the RH is below 98%, I'd wait until the probes read 95% and use a new probe for each reading.>>
This is what gets so frustrating as a subcontractor stuck between the MFG, the specifications, and the 'right thing to do' on one side and the contractor and the owner and the project deadlines on the other!
When I take initial readings and tell the GC we have a RH problem, they want to know A) what the readings are, and B) where we need to be to start, and they want the documentation to go with it. When I forward them the paperwork from the mfg that states 90% (for example) and then I tell them it is reading 92%; it doesn't matter to them if there is a margin of error of 2% at this level, all they hear/see is that magical 90% number. I can talk about margin of error until I am blue in the face, and tell them I can't install until it actually reads 88%, and it won't matter to them. They see the magical 90% number, and as soon as I get that reading, they will be on me to install or be in breach of my contract.
I get extremely frustrated that we are always the ones stuck with our backs against the wall over something that we, really, have no control over. I am seeing a shift in our area where specifiers are finally requiring the testing be done by independent 3rd parties (YAY!!) and although it won't cure the margin of error problem I am ranting about above, I still believe it is a step in the right direction.
05-31-2013, 10:00 AM
No, the ASTM doesn't say that you have to take the margin of error into account. If the manufacturer says 90%, by following the ASTM and getting <90% is sufficient. Manufacturers should have built in that safety factor / margin of error into their specs. An architect might specify 88% as their own artificial and arbitrary safety factor but that is different.
05-31-2013, 10:03 AM
Good to know someone else interprets it that way. I have never taken the M.O.E into account, but wondered if it would come up were there ever a problem. I know if I *tried* to tell a GC that I had to account for that, then I would have the problem I detailed above.
05-31-2013, 12:40 PM
The margin of error is a technicality, a necessary evil, a manufacturer's fudge factor.
Nothing is perfect. Everything has some variation. Probes are the same way.
Manufacturers are using the same equipment you are. Of course they have fudge factors built into their warranties also. They may require 75% Rh readings knowing full well they have never seen a failure happen at levels below 80%.
In fact you see the industry changing now, as competition drives manufacturers to loosen the restrictions a bit. Concrete stops hydrating below ~85% RH, and is relatively stable. Most manufacturers are bumping their threshold from 75% to 80%, and they probably know 85% is safe, if it is properly measured at 85%.
Proper measurement means knowing the exact thickness where you are testing and placing the probe at 40% of that thickness. It means having the ambient conditions the same as they will be when in service. And it means testing with calibrated and NIST certified equipment.
So yes there's some fudge built into every side of the equation.
But if there is a failure and an investigation, and your readings were on the border and the new readings come in at 2% or 3% higher, all these fudge factors will be considered.
If you are the person making the go / no-go decision you should figure this in. If you are testing for the decision maker, you should explain your equipment's tolerances.
With Wagner equipment I don't have any qualms. With the RH testing equipment we had before Wagner.... I lost a lot of sleep at night.
05-31-2013, 02:00 PM
As an independent tester, I live in a black and white world. My job is to conduct ASTM tests to precise standards and document the results concisely. I can only "guarantee" that I do the job to testing specifications and industry standards - period. If the conditions are outside of ASTM F2170-11 (i.e. space not acclimated), it is noted in my report. Of course, I always lay out the basics of test requirements beforehand and hope the customer takes heed, and 99% do. When results are high, I'm almost always asked to make mitigation recommendations. But to protect myself, I tell them that as a third party testing agency, I can only make general recommendations, and I cannot recommend specific manufacturers. Last thing I need is for some contractor to install product XYZ improperly and have someone come back on me saying I recommended the product and it failed.
Bottom line it that the slab is ready or it's not, and when I'm done, the companies involved have thorough documentation of testing done exactly per ASTM standards, with a disclaimer that "future results may vary with changes in ambient conditions". That's the best I can do, and not once have my testing documents or protocol been questioned.
Concrete Restoration Services, LLC
05-31-2013, 02:23 PM
I understand your position CCR, but you must consider the parameters of the ASTM.
Using Wagner probes it's pretty simple. Each probe is NIST certified. There is no required calibration. No leap-frogging probes. No tubes to open and close.
However the ASTM governs a process, not a manufacturer or specific probe. I am speaking a bit more broadly, not concentrating solely on Wagner probes in case that is not what the tester is using.
Some probes may be off a considerable amount when calibration is checked, then also be prone to additional error inherent in the probe. These variables must be noted on the final report.
It would be possible to use another RH testing manufacturer's probe and see a displayed reading of 78%RH, but actually have the true RH of the slab at 82%RH accounting for the allowable probe calibration deviation and the acceptable probe reading variation.
If the go / no-go decision is at 75% and the GC is thinking a 78% reading with a 2% error means the slab is really at 76%... Well you may know better.
Especially if the slab is not at service conditions.
This is all to cover your butt. I see no harm in stating the readings can vary by a small percentage and letting the decision makers take the risk.
It is for that very reason that I explain the depth of my probes is based on the design thickness of the slab, and for more precise readings I must drill at each probe location to measure the slab thickness at that location.
05-31-2013, 02:26 PM
My thread has been threadjacked
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