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Level measurement guide
Float switches and level switches: how to choose
A float switch is the simplest reliable answer to a simple, important question: has the liquid reached this level? This guide explains how magnetic float switches work, when a point-level switch is the right tool (and when it isn't), and exactly what to specify when you ask for a quote — the same questions our engineers ask every customer.
Point level or continuous measurement?
Level instruments answer one of two questions. Point-level devices switch at a fixed height — high alarm, low alarm, pump start/stop — and give you a contact closure, not a reading. Continuous instruments report the actual level as a signal, for inventory, batching or control. Most tanks need both jobs doing, but they are different jobs: if all the process needs is to start the pump at this level and stop it at that one, a continuous transmitter is cost and complexity you don't need. If the process needs to know how much is in the vessel, a switch alone won't tell it. Tell us which question your application is asking and we will match the instrument to it.
How a magnetic float switch works
A sealed float rides on the liquid surface. Inside the body sits a reed switch — a pair of ferromagnetic contacts in a sealed glass envelope. As the float rises or falls with the level, the magnet it carries moves past the reed and the contacts close or open. That is the whole mechanism: no power needed to sense, no calibration, no electronics to configure. The switch simply passes, or breaks, the circuit you wire through it. In a side-mounted design the float pivots through a small arc — around 30 to 40 degrees — so the switching point sits directly at the mounting height, which makes placement predictable at installation.
What float switches do well
Simplicity and price come first: there is no commissioning, no configuration and no signal conditioning — you wire the two leads into your alarm, relay or controller and the job is done. Switching is predictable at a fixed point, with normally-open or normally-closed behaviour selected by float orientation on reversible designs. Clean, water-based liquids are the home ground: water tanks, pump chambers, HVAC circuits, wash systems and OEM equipment. Compact side-entry mounting puts the switch exactly at the level that matters, through a single threaded boss in the tank wall.
Where a float switch is the wrong choice — honestly
Float switches have moving parts, and that sets their limits. Media that coat, crystallise or deposit can slow or stick a float. Liquids much lighter than water may not lift it — check the minimum specific gravity (our HFS 1150 floats on media with SG 0.85 and above). Very viscous products, foams and aggressive solvents belong with other technologies, and a basic reed switch is a resistive-load device: driving inductive loads such as contactors or solenoids without protection shortens contact life. Finally, a mechanical switch cannot tell you it has stuck, so for safety-critical overfill prevention good practice is an independent secondary device rather than one switch doing everything. If any of this describes your application, send it to us anyway — specifying the right alternative is exactly the work we do, and we will say so plainly if a float switch isn't it.
How to specify a float switch — what we will ask you
Six questions decide the specification. What is the liquid, and is it potable water? Materials and approvals follow from that answer. What are the minimum and maximum liquid temperatures? Is this a high alarm, a low alarm or pump control — and should the contact be normally open or normally closed at rest? What is the switch driving, and at what voltage and current? Reed switches are rated for resistive loads; our HFS 1150 switches up to 20 to 25 W, 240 V AC or 120 V DC, at 0.6 A. Is the entry side, top or bottom, and what thread does the tank offer? Finally, what cable length and construction do you need — standard 1 m or 2 m, custom lengths, single or double insulated? Answer any of these, or none: unknowns are our job.
Our current float switch
The QLevel HFS 1150 Series is a compact horizontal float switch for water-based systems: side-entry M16 threaded mounting, magnetic float and reed mechanism, reversible NO/NC switching by float orientation, polypropylene float and body with EPDM or silicone seals, media from -20 to +75 degrees C, and a switching life of over 100,000 cycles. Wetted materials are suitable for potable water, with WRAS / BS 6920 material compliance available upon application. View the HFS 1150 datasheet and specification. For duties beyond a point-level float switch — continuous measurement, aggressive media, solids — send us the application and we will specify and quote the right instrument through the same enquiry route.
What's the difference between a float switch and a level sensor?
A float switch changes a contact at one fixed level; a level sensor (transmitter) continuously reports how full the vessel is. Alarms and pump control usually need a switch; inventory and control loops need a transmitter.
Can a float switch work in any liquid?
No — the liquid must be dense enough to lift the float (SG 0.85 or above for our HFS 1150), reasonably clean, and compatible with the wetted materials. Coating, crystallising or very viscous media suit other technologies better.
Can I use a float switch for drinking water?
Yes — choose one whose wetted materials suit potable water. Our HFS 1150 uses potable-suitable materials, with WRAS / BS 6920 material compliance available upon application.
NO or NC — which do I need?
Fail-safe thinking decides it: for a high-level alarm you generally want the circuit to signal when the level arrives, and to fail obvious rather than silent. On reversible designs like the HFS 1150 you set NO or NC simply by the float's mounting orientation.
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