Can anyone interpret damp meter readings?

[RogerL]

I have a meter-type damp meter, an old uncalibrated Protimeter, and
I'm getting some strange fluctuating readings - I've put a new PP3
battery in so it's not a low battery causing the problem.

Most of the readings show as steady 10-12% as you'd expect in a dry caravan - one place shows a steady 28% so I know I have an issue to deal with there.

But
there's one other place where the meter starts at 20% but then drops to
12% in about 5 seconds - it does this consistently in this part of the
caravan.

So, do I have one or two problems? And
what's the nature of the 2nd problem, given that 15-20% is the generally
accepted grey area between dry and damp which the meter reading is
crossing each time?

Has anyone come across anything similar before and what was the outcome?

 

[ProfJohnL]

Hello Roger,

These meters are simply reading the electrical resistance between the probes, so I can think of three possible reasons for the changing reading.

The first is some slightly porous materials will expel moisture when pressure is applied. The pressure of applying the probes could be displacing some of the moisture thus changing the reading.

There could be a capacitive effect, and the meter effectively discharges the capacitor

Similar to the second suggestion, sometimes the moisture content has as slight acid (or alkaline) which can set up a dielectric pile due to the mixture of build materials. This might react with the electrical current the meter imposes on the material. Initially the material has produced a a modest potential difference, but when the meter is applied it provides a load path to discharge the materials PD, and eventually the meters current them swamps the intrinsic battery effect.

 

[RogerL]

Thanks John - I (vaguely) understand why any one of those could be the explanation - do you have any opinion on which is the more correct reading, the initial higher one or the eventually steady lower one?

 

[ProfJohnL]

Sorry Roger,

Not really possible without seeing and playing with the kit and alternatives.

 

[WatsonJohnG]

When my damp reading was done last week the service manager said that you sometimes get a higher reading at first when you open a locker etc
It may be condensation from the air on a cold surface that quickly dries off with fresh air circulation

 

[RogerL]

I don't think condensation is the issue, the areas involved are open, ie not in lockers, and the heating's been on low during this cold weather - natural humidity is very low at the moment and there's no sign of condensation.
I've given the areas a good clean which has reduced the readings in the area where they were dropping so I'll repeat the exercise as this area was always in the firing line of the washroom air freshener spray.

 

[Dustydog]

Roger
Is it worth checking the readings of your meter with another meter which has been calibrated??

 

[RogerL]

I've got to take it to the dealers for a pre-sale inspection so that should give me comparative figures - I guess I'm really trying to judge how much of a hit I'm going to take.

 

[Martin_E]

Calibration of the Protimeter is done as a self-test by
means of using the calibration tool that comes with it.

 

[RogerL]

Calibration of the Protimeter is done as a self-test by
means of using the calibration tool that comes with it.

The calibration tool was missing when I obtained it s/hand.

 

[Sproket]

Calibration of the Protimeter is done as a self-test by
means of using the calibration tool that comes with it.

The calibration tool was missing when I obtained it s/hand.

New ones are available to check the calibration.
Moisture-Meters/Protimeter-Calcheck-device

 

[Gabsgrandad]

Instructions - any use?

http://www.ge-mcs.com/download/sensing-manuals/MMS-Instruction.pdf

 

[Raywood]

You do not say where the readings were obtained. You can get condensation in the toilet area giving a false reading and you may need to wipe the surface first to clear this. We also had false readings round the windows on the last caravan. This turned out to be condensation under the rubber surrounds and again could be wiped away.