We've recently had an anhydrite liquid screed floor laid in our house. We are now waiting for it to dry before porcelain floor tiles are laid (over a decoupling layer).
There is some disagreement between the tradesmen involved about how to confirm the screed is dry, and what level of dryness is required.
The tiler (and screed supplier) says "maximum 70% relative humidity" (which seems to align with various online sources), but offers no suggestions on how to measure that, and all my searches around RH only seem to really reference calculating relative humidity in air.
The builder has this Brennenstuhl Moisture Detector (Manual), and says "this meter goes to up to a max of 33%, so that's the equivalent of 100%, so 70% of 33% means we need to target a reading of 23%" - As a software engineer I can't emphasise enough how completely wrong this calculation feels to me, but willing to accept I'm not an expert in this topic.
These are the ranges from the back of the meter (showing the max reading of 33% when in building materials mode. The builders suggested target of 23% also falls well into the 'high' category on the meter, another indication that seems to suggest he's very wrong):
So I suppose the core questions are:
If 70% RH is the target, how do I convert that to the % reading this meter shows?
What is this moisture content meter actually reading, as it's clearly not the RH?