I have a capacitance based liquid level sensor that works great when the fluid fills the clean container. The trouble is, when draining, the semi-conductive liquid being measured (milk) leaves a film on the inside of the container. This fools the capacitance sensor into thinking it is still full. The film of milk coats and connects both side of the container (inside), which acts as a capacitive low impedance for the sensing plates (outside).

Please: How do I get around this effect?

I am working on a solution to ignore the imaginary part of the as voltage by comparing the phase of the signal source against the signal measured elsewhere. Any ideas?

 

Most probes of this type, need to be calibrated in the process and should work quite well. It can be most noticeable in heavy liquids like chocolate and paraffin.

Here is an excerpt from the Almeg manual located at: http://www.almegcontrols.com/operating_manual.htm

1. Operating Principles

The switch senses level using a RF measurement technique together with micro-controller technology for high-resolution point level detection. The probe can either form a capacitor with the vessel wall or it can be supplied with a concentric shield around the probe for non-metallic vessels (ground reference). The capacitance of the configuration is measured by the NALRT and is used to provide point level switch control. The NALRT RF capacitance measurement technique combined with filtering algorithms minimizes the effects of coating and electrical noise on the probe and thus focuses only on the vessel capacitance.

The NRALT comes complete with an enclosure, transmitter and probe......etc

Hi and Low alarms are set with the push buttons for differential level applications such as pump control. For single point applications, the high and low points can be set in the same manner.

Bob Hogg

 

You don't say whether your application is on/off or a continuous measurement or whose system you are using. Ametek Drexelbrook has systems that should not be affected by a film such as your.

L.Kolbert

 

If you're talking about a single point or multiple single point controls, use a float switch.

 

Hi Joe,

Your right of course. The trick is in the definition of MTBF. In may cases it relates to the failure rate within the nominated operating lifetime so its the bottom of the bathtub curve.

This is a major issue with hard disks for example. MTBF figures only relate to failure rates within the first 3 to 5 years depending on model. Beyond that failure rates increase dramatically as either the bearings fail or the electronics fails because of accelerated ageing cause by heat.

Regards

Peter Whalley

 

Use EID's Conductivity boards and cells see:

http://www.eidusa.com/Interface_conductivity_cell.htm

http://www.eidusa.com/Conductivity_Cells.htm

Enjoy,

Ron

 

You can find capacitance probes that will work. However tuning fork probes are usually beter suited. I have used then with many filmy materials including peanut butter. The best feature is that they do not need to be calibrated to the material. Endress and Houser makes them in sanitary versions.