A simple floor hygrometer is about £100... The hygrometer is a European standard recognised test but it can be tricky as its affected by temperature and humidity changes. I've used them dozens if not hundreds if times. You cant always take the readings for granted. You do not need a wireless sensor. It simply stick to the floor using water free tape and you take readings after about 4 hours (if it's over about 80% you might as well take it up as it's still wet) if after 4 hours its below 80% read it after a full 24 hours and then again after 48 hours. If both readings are below 75% its dry. If both are above 80 % it's not dry, if both readings are between 75 and 80 you can do some interpretation with experience but that takes a while to work out as you have to take account of lots of variables finally If one reading is below 75 and one above it usually means its almost dry and responding to environmental conditions. The underfloor heating must be off for 48 hours prior to testing. My preference is tge carbide bomb but that's expensive and not really a DIY job. Bear in mind that ALL the same issues and uncertainties exist with any type of screed or concrete.
If you use a gypsum based tile adhesive on anhydrite screed you reduce risk of failure very significantly as you remove any chemical incompatibility problems.
