This week’s EYE ON NPI (video) is PIC GmbH’s LP Series Reed Switch Chains, a range of a new kind of liquid level sensor.
Not surprisingly, this sensor uses a chain of reed switches and a magnetic float to detect the liquid level in a bucket or tank – we’ve never heard of this kind of sensor, so we picked one up to check it out! Arc Extinguishing Chamber
Using a single reed switch for determining liquid level is not new – it’s a common way to have a isolatable sensor for food-safe or caustic fluids.
Compared to some other liquid level sensors like eTape which needs to be ‘squeezed’ by the liquid to give a continuous resistive reading, or sonar which needs to be able to bounce air off, or ToF sensors which can be affected by surface reflectivity/absorption, a reed relay is fully electrically and mechanically isolated. The whole thing can be covered in inert materials and easily cleaned or sanitized.
For more details on selecting fluid level sensors check out this DigiKey article.
The PIC LP Series Reed Switch Chain is made of a thick PCB that is routed down the middle and has tightly packed reed switches soldered one after the other with 1K resistors between each one.
A each reed switch closes, the two contacts will measure a resistance that contains all the ‘lower’ 1K resistors in series. So, say if the first reed switch is triggered the resistance will be 0 ohms, if the next reed is closed the resistance is 1K ohms, all the way up to (n-1)*1K ohms where n is the number of reed switches installed.
In order to have the reed switches activate with the liquid level, you’ll also need a donut/toroid-shaped magnet float which is stocked over at PIC or a generic one can be used if you need particular material.
The PCB comes bare and you’ll need to put it in a tube that matches your material handling requirements. Also, if you need a longer chain, you can abut them end-to-end and solder them – some mechanical stabilization may also be required.
Even though this is designed for liquid float sensing, it could also be adapted to be used as a sliding-platform locator: just watch out that you don’t exceed the switch activation lifetime limit – it’s a few million but these will be hard to replace if they get damaged. Another nifty side-effect of using reed switches is that they can withstand higher voltages, and since they are not powered, can be wired directly into circuitry that is not ground-referenced and don’t care about noise, EMI, static or other common annoyances of industrial automation.
There’s also some other cool items from PIC to check out at DigiKey, such as SMT reed switches which we didn’t know existed, and individual reed floats if you don’t need continuous level detection.
If you want to have a fully mechanically isolated level or position detector that is simple, inexpensive and doesn’t require external power, this could work very nicely! And best of all, PIC GmbH’s LP Series Reed Switch Chains are in stock and available right now for purchase at DigiKey in a range of sizes. Order today and you’ll be floating on air… er… liquid, by tomorrow afternoon when DigiKey packs and ships your order immediately.
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