Rotameter Sensor Switch Explained: How It Works, NO vs. NC, and Sensing Range

Bobby Brown

Post 2026-10-12

Rotameter Sensor Switch Explained: How It Works, NO vs. NC, and Sensing Range

A rotameter shows flow rate by float height, which is easy to read by eye but impossible for a PLC or alarm panel to see. An inductive sensor option solves this. It watches the float from outside the tube and turns its position into an ON/OFF electrical signal for flow alarms and computer-connected monitoring.

The inductive sensor option is available on the LORRIC F22, F32B, and F34 rotameters. This article explains how it works, what Normally Open and Normally Closed mean, and how big the sensing range is.

How the Sensor Works

In a rotameter, fluid flows upward through a vertical tapered tube and lifts a float. The higher the flow, the higher the float sits. If you are new to the principle, see What is a Rotameter?

The sensor is mounted on the outside of the tube at a fixed height. When the float enters its sensing zone, the sensor switches ON. When the float moves above or below the zone, it switches OFF. Because the sensor never touches the liquid, it adds no leak point, which pairs well with chemical-resistant bodies such as the F22's PSU.

Normally Open (NO) vs. Normally Closed (NC)

“Normally” describes the switch's resting state, meaning no float in the sensing zone.

State	Normally Open (NO)	Normally Closed (NC)
Float outside sensing zone	Circuit open (no signal)	Circuit closed (signal present)
Float inside sensing zone	Circuit closes	Circuit opens
Typical use	Simple “float has reached this level” detection	Fail-safe monitoring

An NC circuit reports a fault when the signal disappears, so a broken wire or lost power looks the same as an alarm and gets noticed. An NO circuit stays silent in those cases. Choose based on whether your control system should treat “no signal” as normal or as a warning.

Understanding the Sensing Range

The sensing zone is a window, not a single point. It also has hysteresis: the float position that switches the sensor ON is slightly different from the one that switches it OFF. This keeps the signal stable when the float hovers near an edge, instead of flickering ON and OFF with every small ripple in flow. For the general principle, see What is Hysteresis? How it improves flow accuracy?

Sensing Range at a Glance (F22, 1–11 L/min)

All positions are measured from the sensor's centerline to the top face of the float.

Item	Float-top position	Range size
Sensor stays ON while float top is between	5.5 mm below and 21.5 mm above centerline	27 mm
Sensor switches ON when float top enters	0.5 mm below to 18.5 mm above centerline	19 mm
Sensor switches OFF when float top passes	21.5 mm above or 5.5 mm below centerline	—
Hysteresis (upper edge)	21.5 mm vs. 18.5 mm above centerline	3 mm
Hysteresis (lower edge)	5.5 mm vs. 0.5 mm below centerline	5 mm

+21.5 mm Stays ON
27 mm window −5.5 mm

+18.5 mm Switches ON
19 mm window −0.5 mm

OFF

3 mm
hysteresis

Sensor
centerline

5 mm
hysteresis

OFF

Sensing range of the F22 (1–11 L/min) inductive sensor. Positions are measured from the sensor centerline to the top face of the float.

How the Switching Behaves

Current state	Float-top position (relative to centerline)	Result
ON	Stays between 5.5 mm below and 21.5 mm above	Stays ON
ON	Moves beyond that range	Switches OFF
OFF	Stays outside the range from 0.5 mm below to 18.5 mm above	Stays OFF
OFF	Enters that range	Switches ON

In other words, a rising float turns the sensor ON at 0.5 mm below the centerline and OFF once it passes 21.5 mm above. A falling float turns it ON at 18.5 mm above the centerline and OFF once it drops past 5.5 mm below.

Positioning the Sensor

Because the output responds to a window, a float that travels all the way through it produces ON and then OFF again. Mount the sensor so the float sits inside the window at the flow condition you want to detect, and check the signal logic against your alarm requirement before commissioning.

Wiring Note

The sensor is a precision component. It is designed to send a signal to a controller, PLC, or relay input. Do not use it as a power switch to drive a load directly.

Frequently Asked Questions (FAQ)

Which rotameters offer the sensor?

The inductive sensor option is available on the F22, F32B, and F34. On the F22, the sensor versions are 0.5–6.0 L/min (PP float) and 1.0–11 L/min (PVDF float). For model selection or sensor questions, contact our technical team.

Does the sensor affect the flow reading?

No. It sits outside the tube, so it does not interfere with the flow or with reading the float on the scale.

Why does the signal not switch at exactly the same position going up and down?

That is hysteresis, and it is intentional. It prevents the signal from chattering near the switching point. Read more in our hysteresis article.

Can I use it to detect both low and high flow?

A single sensor monitors one float position. For both limits, discuss your setup with our engineers.