Tuesday, February 03, 2009

This Should Work: Thermistor Senses Liquid Levels

In precision temperature measurement applications using thermistors, RTDs, or other resistive temperature sensors, care must be taken to avoid self-heating errors caused by the excitation current. In some applications, however, the self-heating effect can be put to good use. The design concept presented here should work, but it has not been fully tested.

6 Comments:

At 4:48 PM, Anonymous Anonymous said...

it would have take 5 minutes to test .... lazy!

 
At 4:57 PM, Blogger Analog Dialogue said...

This circuit has been built and tested, but has not been fully characterized to the degree required to support it as a reference design or application note. We're providing this new feature as a way to spark ideas.

 
At 8:24 PM, Blogger Bob Paddock said...

OIL WELL LEVEL SENSOR US Patent: 4747451

"A system for controlling production in an oil well comprises a first level sensor mounted upon the outer surface of the tubing near the lower end of the tubing and a second level sensor mounted upon the outer surface of the tubing above the first level sensor. The first level sensor is tuned to a first electrical signal frequency. The second level sensor is tuned to a second electrical signal frequency. A control device at spaced intervals transmits electrical signals of the first and second frequencies down the oil well and senses the liquid level by detecting impedance."

I wrote the software for this system. The owner of the company put his name on the patent. The Oil (un)covers Thermistors to detect the top/bottom heights of the Oil in the casing.

 
At 9:23 PM, Anonymous Peter Barrett said...

Would this cct work the same if thermistor was placed outside tubing (that contained the liquid) that was conductive or insulating such as glass.

 
At 9:24 AM, Blogger Analog Dialogue said...

The core idea is that the liquid acts as a heat sink for dissipating the ohmic power being pumped into the thermistor. Placing the thermistor outside a tube but in "good" thermal contact with the internal liquid compromises the design. When the liquid is above the level of the thermistor, presumably some cooling takes place through the tube wall and into the liquid, but the thermistor itself will be running hotter than in the immersion case, so care must be taken to ensure this hotter thermistor will not flip the circuit. When the liquid in the tube falls below the level of the themistor there will be a delay in the circuit alerting due to the thermal time constant as the thermistor contact spot now warms gradually. Thus the actual trip point of the circuit in terms of liquid height will become vague to a certain extent and will not be as precise as the original.

This idea will not work with any medium that is a thermal insulator. Glass is probably closer to being a good thermal conductor than to being a bad one so it may work. The poorer thermal transmission medium surrounding the thermistor means the idea becomes less and less effective and more imprecise until it stops working altgether when the thermistor sees no appreciable thermal difference between being "immersed" or "not-immersed", so to speak.

 
At 9:43 AM, Blogger Bob Paddock said...

http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/Sensors/LevelSensors/Page,locale=en.html

See their application notes, for off-the-shelf sensors that do exactly what
you are describing.

 

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