I want to measure continuously varying water levels in a small rectangular steel tank (40mm x 200mm x 300mm deep, internal dimensions). I require resolution to +/-3mm. The tank acts as a stilling chamber, so the surface is smooth. 4-20mA analog output signal is required of the sensor. What continuous level sensing technology would you use?

 

I would use a guided wave radar, form rosemount.

 

This is an easy one, so you can use a wide variety of them. Anything from a float, or a magnetostrictive or a capacitance/RF Admittance probe to a dP transmitter or an ultrasonic sensor...the application is so plain vanilla that you are going to have to make your cut, not on what will work, but on what you are willing to pay.

Walt Boyes
Editor in Chief
Control and ControlGlobal.com
555 W. Pierce Rd Suite 301
Itasca, IL 60143

wboyes [at] putman.net
www.controlglobal.com

 

Hi Pete

It looks like the previous replies did not look at the dimensions you specified. Use a normal DP transmitter with a small range. Example would be the Rosemount 3051CD1A or 2A. If it is a open tank, just connect the HP side to the tank bottom and leave the LP open to atmosphere and also install a 3 way manifold. If it is a close tank connect the LP to the top of the tank.

Remember to measure the distance from the transmitter diaphragm to the bottom tap off point and multiply this value by the density to get to a true zero. Then measure from the transmitter up to the top 100% mark and multiply this value by your density to get to span value.

To determine the transmitter range you need to measure from the position where you want to install the transmitter to the top of the tank. The range of the transmitter must be JUST bigger than this. If it is to big, you will not get the accuracy that you require. So if this measured value is say 500mm the positive measurable range of the transmitter must be just bigger than 500mmH2O.(Exp:-600 to +600mmH2O) Take this H2O measured value and convert it into the units of measure that is specified on the transmitter to see if it will be big enough.
Afterward you might want to increase the damping on the transmitter since it will be very sensitive to even the smallest of level changes due to the small transmitter range.

 

Thanks Sam.

I wasn't sure if I could apply a DP xmitter to this small a range. I've been experimenting with a very narrow beam, short range Flowline EchoPod ultrasonic xmitter with limited success. I'll give the Rosemount DP unit a try. Tx again.

 

Pete,

Please check the list of level sensors manufacturers
at http://www.sensorsportal.com/HTML/SENSORS/Level_Sens_Manuf.htm

There are some models that should be suitable for you.