Moisture meters: resistance, capacitance, and what each is actually telling you

The moisture meter is the instrument most likely to end up in a complaint. Not because it is unreliable, but because what it measures and what people believe it measures are two different things — and the gap between them usually only becomes visible when somebody disagrees with the report.

Two instruments, two physical properties

Resistance meters (pin meters) pass a small current between two electrodes and measure electrical resistance. Water conducts; dry material does not. The scale is calibrated against the known relationship between resistance and moisture content in timber, and on clean, unsalted softwood it is genuinely quantitative — you can report a percentage and defend it. Timber below about 18% is at low risk of fungal decay; readings sustained above 20% are the zone where decay becomes possible, and that threshold is worth remembering because it is the one number on the display that actually means something.

Point the pins at plaster, brick or a screed and that calibration no longer applies. The instrument is still measuring conductance; it is simply no longer measuring anything you can express as a moisture content. Protimeter labels this scale WME — wood moisture equivalent — which is honest: it means “a piece of timber reading this would be this wet”. It does not mean the wall is 28% water. The display carries on showing a number regardless.

Capacitance meters (pinless, non-invasive, “search mode”) generate a shallow electromagnetic field and measure the dielectric constant of whatever is inside it. Water has a very high dielectric constant compared with most building materials, so wetter material reads higher. The reading is non-destructive and covers an area rather than two points, which makes it excellent for scanning — and it is affected by density, thickness, embedded metal and the air gap behind a finish, which makes it poor for a number you intend to quote. The depth is shallow: Protimeter quotes roughly 19 mm for the Surveymaster’s search mode, Tramex up to about 30 mm for the ME5 on its deep setting, and the MMS3’s sensitivity mode goes deeper still. None of them sees through a dot-and-dab void.

The practical division: capacitance to find, resistance to interrogate. Scan a wall to see where the pattern is, then use pins to test the specific points that pattern suggests.

Salts, and why a dry wall reads wet

Hygroscopic salts — chlorides and nitrates in particular — are the single most common reason a resistance meter shows high readings in masonry that is not, in any meaningful sense, damp. Salts are conductive, and they draw moisture from the air. A wall that has previously been wet, a chimney breast that has had a flue behind it for a century, or plaster contaminated from a long-fixed leak will carry a salt band indefinitely and will keep reading high.

This is the reading most often used to sell a chemical damp-proof course, and the one a surveyor most needs to be careful with. If the pattern suggests salt contamination, that belongs in the report as a possibility to be tested, not as a conclusion either way.

Where a quantitative answer is genuinely needed, the methods are carbide testing (the Protimeter Speedy) on a drilled sample, which gives a moisture content on site in about ten minutes, or gravimetric oven-dry testing in a laboratory, which also separates hygroscopic from free moisture. Both are destructive, both need the owner’s written consent — the buyer cannot give it — and both are outside the scope of a Level 2 or Level 3 inspection. Saying so, and recommending it where the pattern justifies it, is a legitimate professional answer. BRE Digest 245 sets out the method and is still the reference.

Taking a reading you can defend

• Record the conditions. External temperature, internal temperature and relative humidity, weather in the preceding days, and whether the property is occupied and heated. A reading with no conditions attached is very hard to defend two years later.
• Take a profile, not a point. Readings up the wall at fixed heights — 150 mm, 300 mm, 600 mm, 900 mm, 1,200 mm is a workable set — and along the elevation at fixed intervals. The shape of the profile carries the diagnosis; a single figure carries nothing. A band that peaks at skirting height and falls away by a metre is one pattern; a patch behind a downpipe is another; a uniform surface reading that follows the cold corner is a third.
• Include a control. A reading on an internal wall, or on the same wall a room away, gives the rest of the profile meaning.
• Note the instrument and mode. Which meter, pin or search mode, which scale. If the scale is not a timber moisture content, do not present it as a percentage.
• Check the surface temperature against the dew point. If the wall surface is below the dew point of the room air, condensation is forming on it and every surface reading is suspect. This is the single measurement that most often separates condensation from a building fault, and it is the reason the MMS3 or a separate hygrometer earns its place in the bag.
• Photograph the position, not the display. A photograph of a number tells the reader nothing; a photograph showing where the reading was taken, with the meter in shot, tells them a great deal.

Wording it in the report

The phrasing that gets surveyors into difficulty is the confident one: “moisture readings of 28% were recorded to the base of the wall.” On plaster, that number does not mean what the reader will take it to mean, and the first expert to look at it will say so.

The phrasing that holds describes the instrument, the pattern and the inference separately: “Elevated comparative readings (Protimeter Surveymaster, pin mode, WME scale) were recorded to the plaster of the front wall in the living room, in a band from skirting level to about 600 mm, falling to normal above. A control reading on the hall partition was normal. The pattern is consistent with moisture at the base of the wall and with hygroscopic salt contamination from earlier wetting; the two cannot be separated without a drilled sample. Recommend a specialist damp investigation including carbide or gravimetric testing before exchange.” Observation, reasoning and recommendation stay visibly distinct, which is what makes the conclusion reviewable rather than merely assertive.

Elevated comparative readings are evidence. They are not a diagnosis, and the report should not let them look like one.

What to buy

Two makes dominate UK residential work for good reasons, and the choice between them is mostly about which you reach for first.

Model	What it is	Indicative UK price (inc VAT, Oct 2026)	Verdict
Protimeter Surveymaster BLD5375	Pin and non-invasive in one body; WME and relative scales; the BLD5365 it replaced is discontinued	About £540–£585	The default. If you carry one meter, carry this one.
Protimeter MMS3 BLD9800	Pin, a deeper and adjustable non-invasive mode, relative humidity and air temperature via a plug-in Hygrostick, infrared surface temperature with dew-point margin, and logging for 10,000 readings	About £1,100–£1,300	The damp-diagnosis instrument. Buy it if condensation-versus-defect questions are a regular part of your work; the surface-temperature-against-dew-point reading is the feature that pays for it.
Tramex ME5 Moisture Encounter	Non-invasive only, two depth settings, large reading area, very fast	About £510 (£425 ex VAT)	The best scanner. Pair it with any pin meter and you have the “capacitance to find, resistance to interrogate” kit.
Tramex MEX5	The ME5 with a pin socket and an optional relative-humidity probe; replaced the discontinued MRH III	About £715 (check VAT treatment on the listing)	A one-box alternative to the MMS3 at lower cost and without the surface-temperature reading.
Protimeter Aquant BLD5770	Non-invasive only, shallow depth	About £385	A scanner on a budget if you already own a pin meter. The Tramex scans better for a little more.
Protimeter Speedy (standard, 6 g)	Calcium carbide tester: moisture content from a drilled sample, on site	About £900–£1,150 plus reagent (about £30 a tin)	Specialist and destructive. For practices that take on damp investigations as a separate instruction, not for a Level 2 bag.

The sensible order to buy in is a Surveymaster first, then a Tramex ME5 when you find yourself wanting to scan whole walls quickly, then an MMS3 (or a separate hygrometer with surface temperature) once condensation diagnosis becomes a regular part of what you are asked to do. Most surveyors who end up doing damp work carry a Surveymaster and an MMS3 or a Tramex and an MMS3; nobody needs all of them.

Worth paying for: a clear distinction on the display between the timber scale and the relative scale, so you are never tempted to quote the wrong one; deep-wall probes (Protimeter BLD5060) and a hammer electrode (BLD5055) if you investigate timber at depth; a calibration check — the MMS3 has one built in, Protimeter sells a check device for the others — and an actual habit of using it monthly and recording that you did.

Not worth paying for: Bluetooth logging you will never open, a colour screen, and the concrete-specific Tramex CMEX5 unless you regularly test screeds for flooring contractors.

Prices move; check the live price before buying.

References

• BRE Digest 245, Rising damp in walls: diagnosis and treatment, BRE, 2007 revision.
• BS 5250:2021, Management of moisture in buildings — Code of practice, BSI.
• RICS, Historic England and PCA, Investigation of moisture and its effects in traditional buildings — joint position statement, 2022.
• RICS, Home Survey Standard, 1st edition, 2019 (effective March 2021).
• Protimeter, Surveymaster and MMS3 product documentation, Amphenol Advanced Sensors.