Damp meter primarily measure the electrical conductance of salts between two metal pins – so called conductance or resistance meters.
Radio frequency meter measure the echo of radio wave.
Protimeter Datalogger BLE – Amazon et al £436
These are useful for determining whether a wall is drying, and tend to be accepted in tribunal or court cases. The Protimeter Datalogger log changes in resistance across two screws hourly, daily weekly, see dataloggers.
Conductance/ capacitance mode
- Most damp meters measure electrical conductance (or capacitance).
- If all you need is conductance mode, then a simple damp meter works well – Amazon £11.
- These meter are good for measuring improvement in a wall over time, to determine when to paint (typically below 20) – but you may need to apply a second coat later.
- Water is a poor electrical conductor. It is salts in water that conduct electricity. Despite criticism, damp meters are a good test of dryness. Protimeter readings below 20WME are considered dry.
- A high damp meter reading cannot determine the root cause of damp, but is a good starting point for tracing damp back to it’s source and can be used to profile damp.
- 85% of damp issues are caused by condensation, especially at the cold base of a wall.
Tips for using Protimeters
- Damp meters can never determine the root cause of dampness. For instance they can’t be used to determine if there is rising damp as is often erroneously claimed (rising damp is exceptionally rare).
- damp meter are relative accurate for timber, especially with the grain, where above 28%WME means that cells are likely to spilt risking fungal attack. Below 20%WME is considered low risk for fungal attack.
- Damp meters are qualitative tools, not quantitative. That is to say that a reading above the standard accepted threshold of 20WME does not mean a wall is damp.
- False positives damp meter readings can be caused by glue, metal backed wall paper and embedded metal or salts,
- A low damp meter readings typically means a wall is dry,
- False negative can be caused by poor contact or an impermeable surface, such as caused by damp proofers slurry.
- Damp meters should be used in conjunction with
Radio frequency mode
- Radio frequency mode is where radio signals are deflected by dense matter, such as water, metal or concrete, in a similar way to mobile phone signals.
- Radio frequency mode can be used to “see” deep into a wall, behind or under concrete and tiling to some extent, and so it again useful for tracing and damp so that a profile can be mapped.
- RF mode is also useful for quickly scanning a large wall. Above 200 REL needs further investigation.
- Dampness cannot be determined by a damp meter in radio frequency mode alone, a high meter reading does not necessary mean there is damp.
- Much of the damp deep within a wall is caused by interstitial condensation.