When people say a slab “tested at” something, the first question is: tested how? An in-situ relative humidity result and a calcium chloride emission rate come from different methods, describe different parts of the slab and use different units. A limit written for one never applies to the other.
| Property | In-situ RH (ASTM F2170) | MVER (ASTM F1869) |
|---|---|---|
| Question it answers | How humid is the concrete at a set depth? | How much vapor leaves the bare surface over time? |
| Unit | % relative humidity (plus temperature) | lb per 1,000 sq ft per 24 hours |
| Where it looks | Inside the slab, at a depth set by how the slab dries | The prepared surface and the concrete just below it |
| Time on site | Conditioning, then a 24-hour equilibration after drilling | Conditioning, then 60 to 72 hours under the dome |
| Surface prep | Drilled, cleaned and sleeved holes | Bare, clean concrete patches |
| Pass/fail? | Neither. Each result is compared with the limit the selected products publish for that specific method. | |
Timing and units: see F2170 summary and F1869 kit instructions.
Why the manufacturer’s written limit rules
Flooring, adhesives, underlayments, patching compounds and coatings all react to moisture differently. That is why each manufacturer tests its own products and writes its own limits, often naming which method it accepts. A result only means something next to those words. Two practical consequences:
- Same slab, different answers. The same recorded result can sit inside one adhesive’s limit and outside another’s. Changing products can change the conclusion.
- The weakest link decides. When a floor uses several products, such as a primer, a patch, an adhesive and the flooring itself, the most restrictive written limit governs the whole assembly.
Why a slab can “pass” one test and “fail” the other
Because the two tests look at different depths and react to different conditions, they can point in different directions on the same day. A slab with a dry skin and a wetter core can show a modest surface emission rate while the probes at depth read higher. Conditions in the room during an emission test can push the result around as well. That is not a contradiction; it is two instruments answering two questions.
What not to do
- Do not convert between RH and MVER with a rule of thumb or a chart. There is no reliable conversion.
- Do not compare a result with a limit from a different product, or with a “typical” number from a forum.
- Do not rely on a handheld surface meter reading when the instructions name F2170 or F1869. Surface meters can be useful for surveying, but they are not those test methods.
Read more about each method: in-situ RH probes and calcium chloride.