How to measure a level sender

How to measure a level sender?

Having the correct length sender is important, not only so it shows the correct amount of liquid in the tank but also to ensure maximum longevity.

The ideal length of a level sender is the distance from the top of the outside of the tank to the inside bottom of the tank, minus 10mm.

For plastic tanks that flex during use it is sensible to have a 25mm gap.

The important thing is that the sender does not touch the bottom of the tank as this can cause any vibrations to be transmitted through the sender, decreasing its useful life.

For black-water tanks or tanks where there will be a build-up of sludge, consider leaving a 50mm gap so any solids that sink to the bottom don't fowl the sender.

For static tanks the gap at the bottom is not important from an operational point of view but you may wish to factor a reserve, if this is important to your application.


To measure an S3 or S3H sender (with screw in head) measure from the top of the thread to the bottom of the stem.

How to measure Wema S3 sender


To measure an S5 sender (with bolt down head) measure from the underside of the flange to the bottom of the stem

 How to measure Wema S5 sender