Liquid Level Sensors

Short, fairly complete but a little vague.

You want to design a sensor? Are you sure? or do you mean, you want to design a level and flow measurement system using optical devices? Those are two different things.

Optical level sensors, though relatively new, have been around for several years now. They use laser technology. They're based on reflecting a laser beam off the top of the liquid and measuring the distance of the reflection from the emitter.

This is an exaggerated drawing, just to show the principle.

As for flow, the only application I can think of using an optical device like a laser would be an open weir. You measure flow by measuring the level in the weir so it's still level measurement except that you convert your units to flow. For flow inside a pipe or tube, I can't think of a way to measure it using optical methods.

"For flow inside a pipe or tube, I can't think of a way to measure it using optical methods."

A rotometer with an optical pick-up?

A rotometer with an optical pick-up?

Actually, it's "rotameter".

Well, yes, you could use a sight flow indicator that has a glass window. It's got an impeller that spins at a rate proportional to the flow. You shine a light through it and a photo-sensor at the other side can be used to determine the speed of the impeller and, hence, the flow rate.

But why use a photo-sensor when you can use a magnetic pickup? It's less complicated and there are models available in the market.

Personally, however, I avoid such types of flow sensors (the impeller, vane, propeller, or turbine type). For clean, low pressure fluids, they're fine but the accuracy isn't very good.

Simple liquid level switches, based upon the total internal reflection or otherwise of an infra-red light beam, are readily available for a few tens of £ each. Their only drawback is that some types need to be protected against sunlight ingress if used outdoors. One could use a number of switches in an array to do level. Then, there are other techniques which may prove more attractive: ultrasonics, radar, nucleonics, magnetic follower, etc.

The question that follows on is: why use optical for flow (rhetorical question)? Magnetic follower coupled to a variable area flowmeter may be more suitable.