Concrete Moisture Meters vs RH Probes
Three kinds of instrument get called a moisture tester, and only two of them produce a number a coating data sheet can be held against. What each one reads, where it reads, and what it is actually good for.
A handheld moisture meter reads about the top 0.75 in of a slab on its own scale, and is good for finding the wet areas fast. An in situ RH probe (ASTM F2170) or a calcium chloride kit (ASTM F1869) is what produces a number in the units a coating data sheet uses. Buy the meter to decide where to test; buy the probes or kits to decide whether to coat.
Three instruments, two kinds of answer
“Moisture tester” covers three quite different things on a flooring job, and they do not answer the same question.
- In situ RH probes. A probe sits in a hole drilled partway into the slab and, once the air in the hole has come into balance with the concrete, reads its relative humidity. The method is ASTM F2170, and its result is a percentage RH
- Calcium chloride kits. A weighed dish of calcium chloride sits under a sealed dome on bare concrete and absorbs the moisture coming out of the surface. The method is ASTM F1869, and its result is a vapor emission rate in pounds per 1,000 ft² per 24 hours
- Surface moisture meters. Handheld meters, pinless (impedance) or with pins, that give a reading in seconds without drilling. ASTM F2659 covers the pinless kind, and its title calls the method a preliminary evaluation of comparative moisture condition
The first two produce the numbers coating limits are written in, which is why the moisture calculator accepts an RH reading or an emission rate and nothing else. The third produces a number that has nowhere to go on a data sheet. That does not make it useless. It makes it a different tool.
What a surface meter actually reads
Two things limit a handheld meter, and both are worth knowing before you put weight on its display.
Depth. Meter makers DeFelsko and Tramex both describe their concrete meters as reading the top 0.75 in of the slab. That is the part of a drying slab that has dried most, because moisture leaves through the surface. The coating has to survive the moisture that is deeper down, which is the whole reason ASTM F2170 puts its probe at a fraction of the slab’s thickness rather than near the top.
Scale. A pin meter’s percent scale is calibrated for wood. The coatings inspection firm KTA puts it plainly: there is no correlation between that percent and the amount of moisture in concrete, and the concrete scale on such a meter is a relative one. Some impedance meters are calibrated against concrete samples of known moisture so that they read a concrete moisture content, but that is still a reading of the top of the slab, and still not an RH or an emission rate.
How much shallower than a probe hole
| Slab thickness | Hole, drying one side (40%) | Hole, drying both sides (20%) | Surface meter |
|---|---|---|---|
| 4 in | 1.6 in | 0.8 in | 0.75 in |
| 5 in | 2 in | 1 in | 0.75 in |
| 6 in | 2.4 in | 1.2 in | 0.75 in |
| 8 in | 3.2 in | 1.6 in | 0.75 in |
Down the side is the slab thickness. Across the top is where each instrument reads: the ASTM F2170 hole depth for a slab drying from one face (on grade, on a vapor retarder or on a steel deck) and from both faces, and the depth meter makers give for a surface meter. Each cell is a depth below the surface, in inches. The hole depths are the standard’s fractions applied to the thickness; the testing guide explains why the depth is set that way.
On every slab in the table the meter reads concrete that sits above where the probe reads, and on a slab drying from one face it reads less than half the depth the standard measures at.
What a meter is genuinely good for
Speed. DeFelsko describes these meters as ideal as a comparator to locate areas of relatively high moisture within a slab, and that is the job to give one. The testing guide makes the point that the required RH or calcium chloride locations should go where the moisture is likely to be rather than on a grid. A meter swept across the floor before you drill is a quick way to find those places: along walls, at joints, below a downpipe, under whatever was stored on the slab.
Read it as a map of where the floor is wetter and drier relative to itself, then put the probes in the wet end of the map. Never read it as a pass.
RH probes: reusable or single-use
Both kinds are used for ASTM F2170. The difference is how the calibration is kept and what each hole costs.
- Reusable probes come out of the hole and go to the next job. The standard requires their calibration to be checked and kept current, so reusable kits are sold with a way of checking it, such as a salt solution chamber held at a known humidity, and the probes go back for recertification from time to time
- Single-use sensors are left in the hole. Wagner, for example, sells its single-use sensor as factory calibrated with a NIST-traceable certificate and says it never needs recalibration. The cost is that every test location consumes one
We do not publish a calibration check interval or tolerance here. Kit makers’ summaries of the standard quote different figures for both, and the requirement has changed between revisions, so take them from the revision of ASTM F2170 that your project specification names.
How many probes a job takes
Every test location runs at the same time, so the number of probes you need is at least the number of locations. ASTM F2170 sets the minimum at three for the first 1,000 ft² and one more for every additional 1,000 ft².
| Floor area | Minimum test locations |
|---|---|
| 1,000 ft² | 3 |
| 2,000 ft² | 4 |
| 5,000 ft² | 7 |
| 10,000 ft² | 12 |
| 20,000 ft² | 22 |
Down the side is the floor area in square feet. The cell is the minimum number of test locations the standard calls for, which is also the minimum number of probes or sensors in use at once. Extra locations placed where the slab is likely to be wet come on top of this.
With single-use sensors, that count is a materials line on every job. With reusable probes, it is the size of kit you need to own: a kit that covers a garage will not cover a warehouse in one visit, and every probe stays in its hole for the whole equilibration period.
Calcium chloride kits
A calcium chloride test is one dish, one dome and one use. The dish is weighed before it goes down and after it comes up, and the emission rate is calculated from the difference, so the other piece of equipment it needs is a scale accurate enough to weigh a difference of a few grams. The kits do not keep: calcium chloride absorbs moisture from the air, so a kit past its date is not a kit. ASTM F1869 scales its test count with floor area the same way, so the table above sizes a box of kits too.
The instruments for the air
Both tests require the space to be at its service temperature and humidity before testing starts, and the evidence that it was is a record of the air, taken with a thermometer and hygrometer in the space. A surface thermometer for the slab itself completes the set. Those same readings are what the dew point calculator needs to tell you whether water will condense on the slab on the day you coat, which is a separate question from anything coming up out of the slab.
What this page deliberately does not contain
- No product reviews or recommendations. Makers are named where their published material is the source for a fact
- No test procedure. Conditioning, hole preparation, timing and record keeping are on how to test concrete moisture
- No moisture limits. What a reading means is the moisture calculator’s job, and the limit that governs is the one on your product’s data sheet
- No conversion from a meter reading to RH or to an emission rate, because none of our sources gives one
Frequently Asked Questions
Can I use a handheld moisture meter instead of an RH test?
Not for the go or no go decision. A surface meter reads roughly the top 3/4 inch of the slab, and ASTM F2659, the standard written for these meters, describes the method as a preliminary evaluation of comparative moisture condition. Coating limits are stated as in situ relative humidity (ASTM F2170) or as a vapor emission rate (ASTM F1869), and a meter reading is neither.
What a meter is good for is finding the wet areas quickly, so that the RH probes or calcium chloride dishes go where the moisture is rather than on a tidy grid.
What does the percent reading on a moisture meter mean on concrete?
Often less than it looks. On a pin meter, KTA notes that the percent scale is for wood, and that there is no correlation between that percent and the amount of moisture in concrete; the concrete scale on such a meter is a relative one. Some impedance meters are calibrated against concrete reference samples to read a concrete moisture content, but that is still a reading of the top of the slab, not the RH or emission rate a data sheet limit is written in.
Reusable or single-use RH sensors?
Both meet the same test method; the difference is calibration and cost per hole. A reusable probe has to have its calibration checked and kept current under ASTM F2170, which means a check kit and periodic recertification. Single-use sensors are sold factory calibrated with a certificate and are left in the slab, so each test location consumes one. The interval and tolerance for checking reusable probes are set by the revision of the standard your spec names, so take them from that revision rather than from a summary.
How many RH probes do I need for a job?
At least as many as the test count, because every location runs at the same time. ASTM F2170 calls for three locations in the first 1,000 ft² and one more for each additional 1,000 ft², so a 10,000 ft² floor needs at least twelve. With reusable probes those twelve are tied up in the slab for the whole equilibration period, so a kit sized for small floors can become the thing that sets the schedule on a big one.
Is a calcium chloride kit reusable?
No. The dish of calcium chloride absorbs moisture and is weighed before and after, so each kit is used once, and the calcium chloride does not keep once the kit is past its date. You also need a scale that can weigh the dish accurately, because the result is calculated from a difference of a few grams.