Select a float level switch by answering four questions: (1) Mounting: top/bottom (vertical multi-point) or side-wall (horizontal single-point)? (2) Material: 316L stainless steel for most liquids; PVDF for concentrated acids; PP for alkalis. (3) Switch points: 1–4 setpoints on one vertical guide rod. (4) Process rating: standard is 10 bar / 120°C. If continuous measurement is needed instead of switching, upgrade to a magnetostrictive sensor.
How Float Level Switches Work
A float level switch uses a permanent magnet sealed inside a buoyant float. The float slides along a stainless steel or PVDF guide tube. When the liquid level rises, the float rises with it. At the setpoint position, the magnetic field of the float actuates a hermetically sealed reed switch inside the guide tube, opening or closing the electrical circuit without any mechanical contact between the float and the switching element.
This contactless switching principle means the reed switch is isolated from the process fluid — there are no seal failures possible at the switch point. The only wetted component is the float and guide tube material, making material selection the most critical engineering decision in float switch specification.
Vertical vs Horizontal Float Switches
| Parameter | Vertical Float Switch | Horizontal Float Switch |
|---|---|---|
| Mounting | Top or bottom tank nozzle | Side-wall tank nozzle |
| Switch points | 1–4 on one sensor body | 1 per switch |
| Measurement range | 50–6,000 mm | Single point only |
| Best for | Pump control, multi-level alarm, limited side-wall access | Simple high or low alarm, compact installation |
| Typical output | SPST / SPDT reed switch per point; NPN/PNP available | SPST reed switch or microswitch |
| Installation | Single tank penetration for all switch points | One nozzle per switch level required |
Material Selection Guide
The wetted materials (float and guide tube/body) must be chemically compatible with the process liquid at the operating temperature and pressure. Use this guide for initial material selection — always verify with a full chemical compatibility chart for your specific liquid concentration and temperature.
| Material | Best for | Temperature Limit | Avoid |
|---|---|---|---|
| 316L Stainless Steel | Water, oils, cooling fluids, mild acids (pH 4–10), food & beverage | 120°C (standard) / 150°C (HT) | Chlorides >200 ppm, concentrated HCl/HF |
| PVDF | Concentrated acids (HCl, HF, H₂SO₄), chlorine, solvents, aggressive chemicals | 80°C | Strong alkalis, ketones, esters |
| PP (Polypropylene) | Alkalis, weak acids, wastewater, general chemical service | 60°C | Aromatic hydrocarbons, oxidising acids |
| PTFE-coated 316L | Most aggressive chemical applications | 100°C | Physical abrasion that damages coating |
Why 316L is the default float material — its 2–3% molybdenum resists chloride pitting and its low carbon keeps welds corrosion resistant — is explained in what 316L stainless steel is and why sensors use it.
Float Switch vs Magnetostrictive: Which Level Technology?
For the general choice between on/off point detection and a continuous 4–20 mA reading, see float switch vs level transmitter.
├── Yes → Float level switch ✓
└── No → Need continuous 4–20 mA output
├── Short range < 300 mm → Optical or float switch + analog encoder
└── Any range → Magnetostrictive level sensor ✓
Multiple setpoints needed?
├── 1–4 points → Multi-point vertical float switch ✓
└── 5+ points or graduated reading → Magnetostrictive sensor ✓
Liquid is foam-heavy or turbulent?
├── Yes → Float switch with stilling tube + time delay relay ✓
└── Severe foam → Guided wave radar or magnetostrictive
How to Size a Float Level Switch: 5 Key Parameters
Provide these five parameters when ordering to ensure correct sensor specification:
- Tank height and switch point positions: Measure from the tank bottom to each alarm setpoint in mm.
- Process connection: Thread size (G1/2", G3/4", G1", G1¼", flange DN25/DN50) and position (top, bottom, or side).
- Wetted materials: Float material and guide tube material based on liquid compatibility table above.
- Process conditions: Maximum operating temperature (°C) and pressure (bar).
- Output signal: Reed switch (SPST/SPDT), NPN open-collector, PNP open-collector, or analog transmitter.
Common Applications and Configurations
| Application | Configuration | Material | Switch Points |
|---|---|---|---|
| Water tank pump control | Vertical, top-mount | 316L SS | 2 (pump-on / pump-off) |
| CDU coolant expansion tank alarm | Vertical, top-mount | 316L SS | 2 (low alarm / low-low shutdown) |
| Chemical dosing tank | Vertical, top-mount | PVDF | 3 (high / low / low-low) |
| Oil reservoir level | Vertical, top-mount | 316L SS | 2 (high / low) |
| Wastewater sump pump | Vertical, bottom-mount or horizontal | PP | 2 (pump-on / pump-off) |
| Battery coolant tank (EV) | Vertical, top-mount | 316L SS or PVDF | 3 (high / low / low-low) |
Which SEGMENsensor Float Switch Fits Each Case in This Guide?
Every float switch case in this guide maps to the SEGMENsensor SFL float level switch, built with a PP, 316L stainless steel or PVDF float; when you need a continuous level reading instead of switch points, the SGM magnetostrictive level sensors take over. Values below come from the float switch page, the level sensor range page and each series page; the exact build is fixed in the ordering code.
| Case in this guide | SEGMENsensor build | Wetted materials | Switching / output | Published values | Product page |
|---|---|---|---|---|---|
| Water tank pump control, sumps and general liquids | SFL, PP float | PP float and stem | SPDT reed contact (NO + NC); no power needed to switch | Liquid specific gravity ≥0.7; actuation ±5 mm (±2 mm with precision reed switch) | Float switch |
| Oils, hot water, food-grade liquids, coolant expansion tanks | SFL, 316L stainless steel float | 316L float and stem | SPDT reed contact (NO + NC) | Up to +120 °C; +150 °C high-temperature version; liquid specific gravity ≥0.85 for the 316L float | Float switch |
| Concentrated acids, chlorine and other aggressive chemicals (dosing tanks) | SFL, PVDF float | PVDF float and stem, PTFE cable | SPDT reed contact (NO + NC) | Liquid specific gravity ≥0.7 | Float switch |
| Several alarm levels through one tank penetration (high / low / low-low) | SFL multi-point rod | PP, 316L or PVDF | One reed contact per switch point; relay, SSR or 4–20 mA (multi-float) output | Tank depth up to 10 m with a multi-float rod | Level sensor range |
| Single high or low alarm through the side wall | SFL, horizontal mount | PP, 316L or PVDF | Reed switch or microswitch, one switch point per unit | Temperature −40 to +120 °C (high-temperature version +150 °C) | Level sensor range |
| Continuous level reading instead of switch points | SGM-LS10 (magnetostrictive, stainless rod) | 304 stainless tube, float | 4–20 mA / 0–20 mA / 0–10 V / 0–5 V / RS485 | 25–8000 mm; ±0.005 mm repeatability; IP68 | SGM-LS10 |
| Continuous level in strong acids or alkalis | SGM-LC10 (magnetostrictive) | PP head, PFA float and flange | 4–20 mA / 0–20 mA / 0–10 V / 0–5 V / RS485 Modbus RTU | 25–1000 mm; IP68 | SGM-LC10 |
Common SFL options: process connection 3/4" NPT as standard, or G3/4", 1" NPT and 1/2" NPT; cable 1 m as standard and up to 20 m custom, in PVC, silicone or PTFE; IP68 sealing; CE and RoHS. To get a quotation, send the liquid and its specific gravity, maximum temperature and pressure, tank height and each switch point position in mm, process connection, mounting (top, bottom or side wall), output, cable length, application and estimated annual volume. Qualification samples of 1–5 pcs ship in 5–7 working days; production lead time and MOQ are confirmed with the quotation.
Frequently Asked Questions
A float level switch is a liquid level detection device that uses a buoyant float mounted on a guide rod. As the liquid level changes, the float moves with it. A permanent magnet inside the float actuates a reed switch sealed inside the guide tube, providing a discrete on/off signal when the liquid reaches the switch setpoint.
A vertical float switch mounts from the top or bottom of the tank and can have 1–4 switch points on one sensor body. A horizontal float switch mounts through the side wall for single-point alarms. Use vertical for multi-point control with one tank penetration; use horizontal for simple side-wall single-point mounting.
316L stainless steel for water, oils, and most industrial liquids. PVDF for concentrated acids, chlorine, and aggressive solvents. PP for alkalis and wastewater. PTFE-coated for the most aggressive chemical applications. Always verify chemical compatibility at your operating temperature and concentration.
A vertical float switch guide rod can accommodate 1 to 4 reed switches at different positions, providing high-high, high, low, and low-low levels in a single tank penetration. For more than 4 setpoints or continuous level output, a magnetostrictive sensor is the better choice.
Standard 316L stainless steel float switches are rated to 120°C and 10 bar. High-temperature versions reach 150°C and 40 bar. PVDF float switches are limited to 80°C. Rated pressure decreases as temperature increases — verify the combined rating from the datasheet for your specific operating conditions.
A float switch uses a mechanical float buoyed by the liquid — it works on any liquid denser than the float. An optical sensor uses infrared light refraction — faster response, no moving parts, but affected by colored, opaque, or film-forming liquids. Float switches are more universally applicable; optical sensors suit clean, transparent liquids in tight spaces.
Float switches can experience false triggering in foaming or turbulent liquids. Solutions: install a stilling tube around the guide rod to dampen wave action, or add a 5–30 second time delay to the relay output to ignore transient float movements. For severe turbulence, consider guided wave radar or magnetostrictive sensors with built-in wave-filtering algorithms.
Order the SFL float level switch with the float material matched to your liquid: PP for water and general liquids, 316L stainless steel for oils and hot liquids up to +120 °C (+150 °C high-temperature version), and PVDF for aggressive acids. Choose a multi-point rod for several alarm levels, or SGM-LS10 / SGM-LC10 when you need a continuous 4–20 mA reading. For a quote, send the liquid, temperature, switch point positions, connection, output and annual volume.