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50% fail rate
05-27-2011, 10:55 AM
Post: #1
50% fail rate
@WagnerMeters - Q: Just returned from meeting where cust. says they get 50% false positives with rapidrh other 50% pass, any tips to avoid?

They are having trouble getting reproducible tests. I suspect they are not using rapid rh and so their probes aren't being acclimated and calibrated properly. But if they were using rapid rh what might be the cause of this?

We don't have this problem in California with our subs who use rapid rh. But once and a while we will get one or two test sites that are off. But nowhere close to 50%.
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05-27-2011, 03:55 PM
Post: #2
RE: 50% fail rate
Thanks for your question. Here is Rapid RH Product Specialist, Jason Spangler had to say:

First of all, I would never state “false positive”. The sensor is reading the environment it is installed into. What is there to say, based on the way you state in your question, that there isn’t a 50% “false negative” rate? With that being said, if they are using Rapid RH 4.0 throughout these could be some of the culprits:
1- Depth of hole is critical. Too deep, higher RH% and vice a versa.
2- How well is the hole drilled. Improperly drilled holes will lead to the sensor not sealing properly.
3- Variances in truckloads of concrete. Remember, this is why we are testing every 1000 sqft.
4- Drilling directly into aggregate. If there is a large piece of aggregate that has been drilled into at the bottom of the hole, this is what the Rapid RH 4.0 will read, usually low.
If they are using competitive products(besides the two you mentioned):
1- Sleeve design may be allowing sidewall RH down into the hole, thus giving a lower “average” RH as opposed to the true RH at 40% depth.
2- How high up in the hole does the sensor live? You may be drilling to 40% depth of the slab, but where is that sensor actually living and reading from? The higher it sits up in the hole, the lower the reading.
The question didn’t really give me a lot of specifics to work with, so I made some assumptions. Another think to think about, per ASTM F2170, all sensors are in conformance as long as they are +- 2% from 50-90%RH. This could account for as much as a 4% variance from sensor to sensor with all other conditions being equal.
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05-27-2011, 04:57 PM
Post: #3
RE: 50% fail rate
Yea I am apt to agree. They are in a high humidity climate in the south so I bet they are having trouble keeping the air moisture out of the 'other's' sleeves.
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05-30-2011, 11:18 AM
Post: #4
RE: 50% fail rate
Can you get us more specific values for 'false positive'? Perhaps 50% of the probes were at 74% and 50% were at 76%? Is that a 'false positive'?

JD Grafton
Concrete Answers for Flooring Problems
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05-30-2011, 01:24 PM
Post: #5
RE: 50% fail rate
Tests either came in around 80 or 98-99 so with some bad tests the whole slab fails
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09-12-2011, 05:43 PM
Post: #6
RE: 50% fail rate
Revisiting this thread and I was just thinking that if the probes were cold and put into a warm slab there could be condensation forming on the sensor which will mess with readings.... Typically this is something we need to worry about in the north, not so much in southern climes.

JD Grafton
Concrete Answers for Flooring Problems
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09-13-2011, 03:07 PM
Post: #7
RE: 50% fail rate
The question was posed in Texas. But, my theory is that perhaps the manufacturers of flooring need to have some more nuanced rH specs.

Usually, one test over the rH requirement fails the floor. Perhaps there needs to be an ASTM standard where if you put X probes, you should do X/Y for the number of probes that need to meet rh Spec? Or perhaps some other deviation.

JD - what do you do in a hospital and one probe in one room comes in high? You don't mediate the whole building. Do you retest just there? Do you drill a new hole and new probe? where, how far away from high test?
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09-13-2011, 05:44 PM
Post: #8
RE: 50% fail rate
If one area is high we retest. If it continues to be high, we have a problem in that area and yes we mitigate.
We use reason of course. If the entire slab is at 73% and one area is 76%, well don't tell anyone, but I'll say put the dang floor down. We are pretty dang comfortable with the safety of 80% and under. Now when you start getting to mid 80's I get worried. Do you want a lifetime floor or a one year floor?
Remember I am working with rubber floors and sheet vinyl....

JD Grafton
Concrete Answers for Flooring Problems
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09-13-2011, 05:50 PM
Post: #9
RE: 50% fail rate
So you mitigate the area of the test or the whole slab with one high test?

For example patient room #206 is high, room #207 is fine, #205 is fine, and the hallway is fine. Do you just mitigate room #206?
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09-13-2011, 05:58 PM
Post: #10
RE: 50% fail rate
(09-13-2011 05:50 PM)eaadams Wrote:  So you mitigate the area of the test or the whole slab with one high test?

For example patient room #206 is high, room #207 is fine, #205 is fine, and the hallway is fine. Do you just mitigate room #206?

We retest and figure out why. Then use reasoning to determine the course of action. Like I said, if the rooms are all 78% and one area is 90%, my official recommendation is to follow the book and mitigate it all, but I wouldn't be surprised (and don't believe there would ever be a problem) if we mitigate the high area and let the rest go.

This isn't common. I'll tell you where it happens a lot though.... Trenches for new service. We have mitigated just the trenches. Or areas that are on grade next to basement areas. Areas poured much thicker for footings. It does happen.

JD Grafton
Concrete Answers for Flooring Problems
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