What to check on a Float Level Switch.
1 models from Panasia in Float Level Switch. Below: the full model list.
What Makes This Type Different
A float level switch is a point-level device, not a continuous gauge: a float rides on the liquid surface, and when it reaches a fixed position it trips a magnetic reed switch or a mechanical microswitch through a sealed tube or a hinged arm. It gives one bit of information — liquid present or absent at that height — rather than a full column reading. That simplicity is the reason it survives as an independent overfill or low-level alarm even on tanks already fitted with a servo or radar gauge: the switch has no shared electronics with the primary instrument, so a fault in one system does not blind the other.
Read the full guide
Main Components
Float
Sealed stainless steel or plastic float sized for the specific gravity of the cargo it will see — a float calibrated for seawater will sit differently, or not trip at all, in a lighter cargo such as gasoline.
Guide Tube or Stem
Keeps the float travelling in a straight line so it cannot jam against the tank wall or an internal structure as the level rises and falls.
Switch Mechanism
Reed switch actuated by a magnet in the float, with no mechanical linkage through the tank wall, or a mechanical microswitch on a pivot arm; the reed type is preferred in cargo tanks because it needs no moving seal at the pressure boundary.
Housing and Cable Entry
Pressure-rated housing at the tank penetration, with the cable gland certified for the tank's hazardous area classification.
Selection and Sizing
- Specific gravity range: the float must be matched to the lightest and heaviest cargo the tank will carry, or it will under- or over-read the trip point.
- Trip point: fixed at the design stage — high-high alarm, high alarm, low alarm — and not something adjusted casually once installed.
- Pressure and temperature rating: must match the tank's design conditions, especially on gas carriers or heated cargo tanks.
- Material compatibility: stainless grade or elastomer seals suited to the cargo chemistry, checked against the cargo's compatibility list.
Regulations and Class
On oil and chemical tankers, an independent high-level alarm separate from the main gauging system is required under overfill protection provisions linked to MARPOL Annex I/II and reinforced by class rules and the ship's cargo operations manual; float switches are one accepted means of providing that independent signal. Class surveys check that the alarm actually trips at the set point during a functional test, not just that the wiring reads continuity.
Typical Faults
- Float stuck in guide tube due to wax or sludge build-up — alarm never trips even though the tank is overfilling.
- Reed switch degraded by repeated cycling — intermittent or false alarms that crews learn to ignore, which defeats the purpose of the alarm.
- Float punctured or waterlogged — loss of buoyancy, switch trips late or not at all.
- Wrong float fitted after a spares mix-up — trip point shifts because the float's specific gravity no longer matches the cargo.
- Cable gland leaking at the tank penetration — moisture ingress into the switch housing, corrosion of contacts.
What to Look for in a Supplier
- Float material and specific gravity rating stated for the actual cargo range, not a generic water-service default.
- Certification for the tank's hazardous area zone and gas group.
- Documented trip-point repeatability, since this is a safety alarm, not a rough indication.
Test the float switch by physically raising the float during tank cleaning, not only by checking continuity at the terminal box — a switch that reads correctly on the bench can still be jammed by sludge in service.