Liquid Level Measurement 5

[Zebedee1]

Can capacitance be used to generate signals (say 4-20mA) proportional to varying level of liquid in a vessel rather than simply detecting an interface and being used as a switching device?

 

[danw2]

When I hear "interface" measurement, I think of a rather specialized application of capacitive level measurement, where the measurement is the interface between liquids, like, oil and water.

Continuous capacitive level measurement has been around for decades, and is routinely used for "ordinary", single media (meaning not an interface between two different liquids), continuous level measurement, typically with as a 2 wire, loop powered, 4-20mA output.

Capacitive point level devices have also been available with on-off switch outputs.

Dan

 

[Ashereng]

The simple answer is yes.

Capacitance probe level measurement is very common. The most common output is 4-20mA.

The limitations is the liquid needs to be conductive (I found out very pure water actually isn't - it needs ions) and temperature is rather limited.


Here is one example from Endress & Hauser (no, I am not a rep)

Liquicap T - FMI21 Capacitance Level Measurement

Capacitance level measurement has conquered a firm position in process instrumentation during recent decades and still offers decisive advantages today. In Liquicap T FMI21, Endress+Hauser introduces a new generation of capacitance instruments for continuous level measurement.

Liquicap T FMI21 is a cost-effective capacitance probe for continuous level measurement in conductive liquids starting at 30 mS/cm. The measuring range covers from 150 mm to max. 2500 mm and the max. permissible process temperature is +100 °C. The probe is pre-calibrated to the ordered length at the factory which makes customer calibration superfluous. Subsequent shortening of the probe rods is easily done at any time using the separate shorting kit. High-quality, corrosion-resistant materials are available for applications in aggressive liquids like acids and lyes. Liquicap T ensures safe operation regardless of tank materials (plastic, steel or concrete) and geometries (baffles, nozzles, etc.).


Many small storage and buffer tanks in all industries have not yet been equipped with instrumentation. The unique price/performance ratio and distribution via E-direct make Liquicap T the optimum solution for such applications. Liquicap T has the following approvals: WHG, General purpose (USA/Canada) and ATEX Zone 2.

 

[w8819096]

Capacitive probe level meter can be used in conductive media and non-conductive media both. just different type of probe. Of course, the temperature is importante, because the property of media will be changed along with tempertature.

 

[Ashereng]

W8819096

Can you give an example of a non-conductive media capacitance probe? I am not familiar with them.

Thanks.

 

[w8819096]

If you use capacitance level meter in non-conductive media, you should use couple probe as a capacitor, the media as dielectric, the capacitance will be changed by the level. It just likes flat capcitor.

If you use capacitance level meter in conductive media, you have to use the insulated probe.

For example, you can review this pdf file as follow:

http://www.americanmagnetics.com/support/brochures/model175.pdf

 

[Ashereng]

w,

Thank you. The link is a good one.

I am not sure why my pure (and de-ionized) water wouldn't work with a capacitance level probe.

 

[rahul4981]

The answer is YES.

Capacitance type transmitter can be used for continuous type level measurement (4-20 mA)

as per the relation C=EA/d
A=Cross sectional area of plate (constant)
d=Distance between two probes (constant)

So Capacitance C is purely a function of dielectric constant E.

So as level rises you will get a continuous change in capacitance. which in turns calibrated in continuous current (4-20 mA) output

 

[kontiki99]

The fuel quantity indication system in every commercial jet I've ever seen uses capacitance probes.