Hi, I am finally making the leap from CaCl and was wondering if someone could give me the pros and cons for the different methods of measuring RH. I have access to a GE Protimeter MMS, but was considering using Rapid RH. How do they compare with respect to reliability, cost effectiveness, ease of use, etc. Any other methods that may be preferable?
From everything I can read, there are some basic differences between the Rapid RH and systems, methods, etc. such as GE. GE and others like it use a separate sleeve and external probe that goes into the sleeve when you need to take a reading. The Rapid RH apparently has the sensor integrated into their sleeve and that is why it can apparently get a more valid reading sooner than the others.
The time savings seem like they're pretty substantial with the Rapid RH, and if time is where most of your money is, then that could be important. With the GE, and the methods like it, if you leave the external probe in long enough each time you insert it you will get a valid reading eventually. So if you have enough of the probes and you don’t mind waiting, then you are probably fine.
I think I calculated that if you buy bulk, the Rapid RH method would run about 3 cents/square foot, and you get very quick results. It is also seems to be very easy (I watched the installation video) to insert it to the correct depth. Apparently, you don’t have to be concerned about recalibration with it as opposed to the other methods, but I think the recalibration may not be a big cost with some of the others; don’t know for sure.
Anyway, that's just what I can see.
jf222, Thanks for the response. Your info was helpful. Any idea how long the external probe has to be inserted in the sleeve before you get a valid reading? It appears that when all things are considered, Rapid Rh may be my best alternative.
Which type of probe are you talking about? The Rapid RH?
I know a guy who has done a lot of testing over the years, and has used a number of the different brands. If you have one probe, and are going from hole-to-hole, you cannot just pop it into one and then get out. Seems like he was telling me that the first hole of the day on a job might take 2-3 hours with some of the brands, and then about an hour for each hole after that. I just don't have that kind of time; time problem was one of the main reasons for moving away from CaCl.
I guess the type of sleeve used has a lot to do with it. Which reminds me of another thing I heard, and that is some sleeves are not designed so you are always at the depth you're supposed to be at. I think the ASTM standard calls for readings at 40% of the slab thickness. I think that Vaisala sleeve is OK, but the probes are pretty expensive.
The two test methods are not interchangeable and the values as given mean completely different things..so to eliminate one method for another is probably not a good idea...for example..I believe and have been talking with Ron..that r.h. probes for exact reason they are innacruate for measuring moisture volume in concrete, are invaluable for measuring the curing environment for concrete, which is dependent more on the relative humidity remaining above 80% than what the volume of water is inside the concrete...for example even though a humidity room doesn't have near the voume of water as a curing bath, they appear to be equally efficient in keeping the concrete sufficiently moist.
The way I see it is the resuable probes read the volume of rh from the top of the slab/hole to the bottom of the hole. Unlike the Rapid Rh which only reads the bottom of the hole which I believe is a quater inch. Therefore the other probes readings are invalid.
One of the most important parameters in package design is the control of water vapour transmission into or out of the final package. Robert L Demorest of Mocon - Modern Controls Inc - of the USA reviews the development of water permeability tests and shows why the latest 'generated RH' method is to be preferred.
This is precisely why the humidity probes and meters are necessary for the curing of concrete since it will measure the humidity rather than the volume of water, which is imperative for proper curing of concrete, but VERY misleading when trying to evaluation the amount of moisture available. This graph assumes no change in moisture volume..and is within the 65-85 degree range as stated within the acceptible range for CaCl testing.
My testing with the Wagner Rapid RH test system has proven to me that Wagner is the ONLY test I can depend on.
I have consulted on many jobs with Protometers and CRH probes that gave inaccurate readings.
For true testing of accuracy, you must seal the concrete slab in plastic and set a probe to get its reading, then weigh the slab and dry it in an oven until it loses no more weight, and calculate the relative humidity.
If you decide to trust the CRH meters, I have about 12 of them I could sell you, and 3 meters to go with them...
I won't ever use them again.... Checking calibration, acclimation times, inaccurate readings because you are reading much more of the slab than just the horizon at the 40% or 20% depth where you need the reading.... It's not worth it!
The method you describe cannot be accurate..unless you know the following: volume of pore space, amount of sorptive, semi sorptive materials, and that the temperatures are constant of air AND concrete.
Sorptive materials are capable of lowering the relative humidity within a space (such as some types of aggregate, unhydrate cement, etc.) while the moisture VOLUME increases..giving a false reading using ONLY relative humidity testing.
For example: Humidity STILL changes when the moisture volume stays constant - a 2 degree change in temperature will affect the r.h. reading by an average of 5%!!!! A 4 degree changes humidity by 10%..it is nearly a linear correlation.
Absorbent, semi absorbent materials can hold a disproportionate water to the amount of measurable humidity. A simple experiment using a material like cotton is a great way to illustrate moisture content versus humidity...in a small box containing cotton versus and empty box of the same exact dimensions:
One need only add 8 g. of water to bring the RH from 50% to over 90%, at a constant 20°C. If the box contains 1.7 kg of cotton one must add 150 g of water to the cotton at 50% RH to drive the RH above 90%!
The water vapor flux, which is a quantity, is more relevant to such an investigation than the relative humidity, which is a potential. The difference can be illustrated by letting a kettle boil unattended in the kitchen. The relative humidity of the kitchen air will soon rise to 100% and remain there. But the kettle will continue to release water vapor, which will condense on the walls and influence the room climate for hours after the water vapor source is removed. In other words, r.h. humidity testing ONLY tests how much moisture the AIR can hold NOT THE CONCRETE!!!
A chamber which just applies 100% RH has no way of knowing how much condensation is occurring. In other words..it is impossible to know how much water is in the pores of concrete due to absorbent materials and the effects of condensation (which oddly can actually "steal" humidity from the air under proper circumstances).
This diagram shows the ability of even small quantities of absorbent materials to stabilize the relative humidity of confined spaces against the effects of changing temperature and changing water content.
So to CC Solutions, you may want to rethink your position of over-reliance on any single moisture testing method...many times what WE think is inaccurate, is simply because we do not have enough data to properly interpret the results.