We all know that antifreeze has to be minimum strength (measured by hygrometer) to combat damage by low temperatures, but it hadn't really occurred to me that nowadays it has anti corrosion properties also . Especially as we tend to leave it in for years, i'd best get on and check!!
I would suggest making sure the earth connx of the probe is clean and secure -crocodile clip? and leaving the circuit in place for 20 mins or so to let everything settle down.
if you have any earthing probs with your car this could affect the result.
Part of article and letter from Feb 2020 'Car Mechanics' magazine.
Coolant test for anti corrsion props.pdf Coolant test for anti corrosion props 2 .pdf
