We had an old WEMA sensor that was enclosed in a rubber accordian shaped membrane to seal it from the tank crud. This worked until the rubber dried out and cracked. Went to WEMA to get another and they said no longer manuf. this type. We got a regular tank sensor (a little donut that slides up and down on center tube). We had to go with this style as we had aluminum holding tank
with access port pre-drilled for EU metric five-hole system. When it stopped working, had to "get the gloves and mask" and open the tank and clean the senser. There is nothing you can put in the tank to do the job of cleaning
a moving sensor. There is too much volume and the concentration of vinegar or muriatic acid you need to dissolve the scale would eat out your plumbing
Tiring of cleaning
the WEMA sensor (2-3 times a year) finally got a Scad Solo Profile Series (TM01), made by Scad Technologies Inc. This was rated tops by "Practical Sailor" last year. No moving parts
. I got the internal tank sensor and hooked it up. Now no need to clean and it is fairly accurate.