Implemented a device to measure permittivity of a liquid.

Thread Starter


Joined Jan 6, 2004
Most probably this post will be labeled under the classification once mentioned by member @jpanhalt: ego massage. I cannot other than to agree.

Yesterday, I completed the journey starting one month ago, after reading this thread in another forum. Thanks @bertus. I got interested in implementing such a device: a VFO fitted with a variable capacitor which inserted into a cuvette full of a certain liquid allows to calculate its permittivity taking into account the changes in capacity in air and immersed. In this particular case, the variable cap measures 4 to 144 pF in air.

For the reasoning supporting the measurement process and calculation, it is worth referring in particular to the OP and the even more specific, #101 by member JimB . Due credit must go to where it belongs; I would not dare to add anything.

First I needed to acquire a reasonable familiarity with VFOs and after several attempts I got a decent BJT Clapp oscillator working in the 1,1 - 1,3 MHz range. Not bad, considering that I had to revisit (bah, relearn) how to properly bias (resistor divider) a BJT amplifier. The sole "conditioning" of the VFO's output was to provide a +2,5V offset prior to applying it straight to the TMR3CKI input pin of the micro. Circuit attached.

Gracias to the so many that helped in my way to get that one working. If I omit a name it is not intentional: @Papabravo @Danko @Bordodynov @MrChips @LvW @OBW0549 @ronsimpson @ericgibbs @DickCappels

Frequency counter implemented with a PIC micro 18F2321 (TMR3 as a counter) with data displayed on a 2 x 24 LCD.

Following the nomenclature employed by Jim in his post, I got the following values:

fa small air: 1,306006 MHz
fb large air: 1,202345 MHz

fa small liq: 1,298703 MHz
fb large liq: 1,088448 MHz

K = 2,38 (for this particular oil stated as 2,5 to 2,6). I am not a chemist, far from that. Now you know.

BTW, the oil to test came from the kitchen: pure genuine sunflower cooking oil (6-bottles boxes get a 25% discount). I manage to cook good tasty french fries with it.







Thread Starter


Joined Jan 6, 2004
Excellent report, and a good example of the "Manhattan" construction technique (tutorial attached, for those not familiar with it).
I am envious of your ability to put together nicely laid out and tidy circuits. Agree with @OBW0549 that it is a very good report.
Considering the heavy weight contributors that all you are, I cannot allow faux pas myself if I can avoid them :oops: :D :D

I started using Manhattan style when reproducing a design by member Dick Cappels in his site. My error has been insisting in making them extremely compact thus really small. For the first oscillators every change / addition was a nightmare. Add to that a new component in this game: essential tremor. I ruined two too small boards. This last came out easily and allowed all changes and adjustments.