Using a Liquid Level Switch with Compact DAQ

[Miguel_F]

Hello All,

Hopefully this is the right board to post this under.

I am attempting to add a liquid level switch (http://www.omega.com/pptst/LVN60_70.html) to keep a pump from overflowing a tank. The setup has two pumps feeding liquid to a tank, and one pump removing liquid from the tank to maintain a constant volume. Currently it is manually adjusted to avoid net volume change, however I want to automate the level control. Varying amounts of entrained air prevents me from just matching the rate of the removal pump to the sum of the inlet pumps.

To prevent net accumulation, my plan is to use a float switch (which can be configured to be normally open or closed) to speed up the removal pump when the level rises too high (i.e. switch is activated). To prevent net loss, I just planned to periodically decrease the rate of the removal pump, sort of letting the level drift up until the rate has to be increased again. 

As far as hardware goes, we already have a 0-10 V analog in module, so I thought I could put a low voltage power supply in series with the switch and the module. If I set the switch for normally open, then the module would detect the power supply voltage when the level is too high, and no voltage when the level is correct or low.

I just want to know if this seems like it would work or if someone could offer a more elegant solution using the same float switch. If it makes any difference I also have open ports on a 4-20 mA analog input module, but I'd prefer not to use it becuase I'd have to also buy a current loop isolater (shared COM port with other devices). Thanks for any help!

[Eric_L · Accepted solution]

Hello Miguel,

You can use a switch and power supply as you described. I would suggest adding a pull-up or pull-down resistor to keep the input from floating when the switch is open. Have a look at the following link which describes pull-up and pull-down resistors.

http://en.wikipedia.org/wiki/Pull-up_resistor

Eric

[Miguel_F]

Thanks Eric! I hadn't even thought of the floating voltage. Had to get one of my more EE-oriented coworkers to help me wire the pullup, but it works great.