It’s all relative… humidity
Relative humidity (RH) is the amount of water vapour in the air, expressed as a percentage of the maximum amount that the air could hold at the given temperature; The ratio of the actual water vapour pressure to the saturation vapour pressure.
Standards of Relative Humidity
When it comes to relative humidity, the different standards that are used start to become a minefield! We’ve had a look into a few different standards and have found information on what we think are the most used and popular standards.
- ASTM F2170-09 = the US standards institute (American Standards for Test & Measurement) and the F2170 relates only to in-situ method of testing. They had the hood method in another standard called F2420 but it was dropped about 2 years ago as the Americans thought the Hood test was not reliable. (Tramex have mentioned that they would tend to agree with them except it is very useful when you cannot drill) This standard has now been updated in 2011 so it is now called the ASTM F2170-11 instead of the 09 version.
- BS 8203 is the British Standard for resilient floor covering and mentions only the Hood Method instead of the in-situ method.
- BS 8201 is the Wood Flooring Standard and mentions both the Hood Method and the In-situ method (the 1st British Standard to mention the in-situ method but 8203 is following soon) The problem with the British Standard mentioning the in-situ method is that it specifies a reading of 75% for both tests. It is impossible for both tests to read the same unless the floor is completely bone dry (which is never basically) and when the Hood method reads 75% then the in-situ is normally 80% or even 85%. British Standards should not specify a reading at all as this should by rights be left to the Manufacturers of floor coverings to decide what moisture level their products can tolerate.
Advantages and disadvantages of using RH meters
Using RH meters has quite a lot of advantages which include quantitative & accurate measurements, long-term average of relative humidity, the ability to measure moisture at depths of up to 40% thickness and they are the BEST available predictor of slab behaviour. Nevertheless there are some downfalls to using the meters, one being that a non-calibrated sensor can provide wildly inaccurate results. (However, this is true of any sensor, not just RH Meters.) As well as this, contamination of the reading site can lead to an inaccurate reading.
Overall, a well-calibrated RH meter can be a priceless tool for making sure your contracting project is completed without a hitch.
Hygrohood and Tramex
There are different machines and meters which can assess RH, the first one being the Hygrohood. “The Hygrohood is a Strong, Robust Surface Humidity Box tester for Concrete and Screeds. It can be used in conjunction with the Tramex CME4 or the Tramex CMEX2 to corroborate the Non-Destructive moisture content test and to conform to British Standard.” (Tramex)
“Tramex are international leaders in the design and production of Moisture Meters, with a reputation for the highest standards in quality and innovation.”
There are different meters when measuring RH or moisture content but we deal with Tramex meters as they are a more convenient method and is also accurate and speedy. The Tramex meter is an easy to use electronic device which can provide various measurement results. After choosing the CM measurement option from the menu, the meter simply needs to be put against the screeded surface to get the moisture content displayed in %.
Below identifies the most important points about the Hygrohood:
- Ideal method of RH (or more correctly ERH – Equilibrium Relative Humidity) testing when drilling the floor is not possible. I.e. Underfloor heating pipes installed.
- Very sensitive to temperature changes in the ambient conditions and so readings should be taken at 4 hour intervals to ensure the temperature is stable and readings don’t change in that period.
- Time must be given to allow the hood to come to equilibrium:
- Un-bonded screed on top of plastic sheet – 4hrs and 4hrs later. (No change in reading then reading is final)
- Bonded screed or normal concrete floor (up to 6 inches) – 72 hrs
- Extra deep concrete floors (over 6 inches) can take up to 7 days.
- It is very important to use the non-invasive, capacitance instrument to scan for the best location to place the RH Hood (Highest reading locations) and to verify that the readings with the hood are correct.
- In-situ probes (drilled into the floor) when possible, provide a faster and less temperature sensitive method of testing ERH.
(Pictured above is the Hygrohood)
The Hygrohood can sometimes be paired up with the Tramex CME4 or CMEX2 but neither of these give indications of RH because they test something completely different. They show moisture content which is basically a measurement of how much water by weight is present.
RH is dependent on a number of factors including temperature and ambient humidity conditions which will affect the Hygrohood more than the in-situ test. Therefore it could be said that the in-situ method is a much more stable (and therefore accurate) reading.
Tramex CME4 & CMEX2
The CME4 & CMEX2 are the easiest/fastest method of testing and are much less affected by environmental conditions and so they are probably the most accurate test. They also cover the whole area instead of single points in a floor which means the information is much more comprehensive.
The important point to highlight is that using the CME meters is most important for choosing where to place the Hygrohood or the in-situ test, otherwise it’s like playing pin the tail on the donkey as the RH test, if placed in the wrong position, can completely miss the moisture.
(CME4 on the left and the CMEX2 on the right)
If the moisture content of the concrete or screed exceeds the critical moisture content of the materials that are in contact with it then problems tend to occur in floors such as:
- Adhesion failure between floor covering and concrete slab.
- Vinyl floor covering movement. Dampness in concrete may aggravate alkali related dimensional changes in sheet vinyl.
- Emissions of harmful substances from floor covering materials. E.g. the emission of ammonia from some screeds.
- Excessive expansion of solid wood floor coverings and the decay and degradation of wood based floorboards.
We hope this article has been helpful in understanding what relative humidity is, what meters can be used and how important it is to make sure everything is correct before testing concrete slabs or floors.
We give thanks to Tramex for helping us with the information in this article.