You might see if you can find a copy of "Instrumental Methods of Analysis" by Willard, Merritt, Dean, and Settle. Good chapter (Ch. 24 in my copy - 7th Ed.) on conductance measurement methods.
I have to build a conductivity sensor with very low accuracy requirement (50% is good enough in the 10 to 10,000 uS/cm range). I am not all that familiar with the inductive method using 2 torroids, one for the excitation, and the other for detection.
What is a good starting point for coil inductance, coil dimensions, separation, excitation frequency, etc?
If anyone has a good reference book or informative web site on this topic, please let me know.
Thanks.
This method is often used to measure the PH of a liquid. I have been looking into this for one of my clients and have conducted some experiments. Try an inductance of about 10mH. Drive that coil through a capacitor with a single pulse, whose pulse width is picked to equal one half of the period of the frequency of this series resonant LC circuit. This pulse will cause the network to ring for many cycles. You can use a CMOS transistor gate driver IC or you can gang several CMOS inverters to form the driver. The pulse rate can be pretty slow, about 50Hz or so. This keeps the power drain low but does launch a hefty signal. Use a second coil of similar inductance, to form a parallel resonance circuit with a capacitor. Place a resistor in parallel with the network to limit the Q to about 5 or so. The ring signal from the exciter coil will produce a nice ring signal in the receiver coil, which lasts several cycles, provided there is a conductive fluid through the center of the two toroid coils. Use a peak detector to capture the ring amplitude. Do a little filtering and you will then have a nice DC voltage, which is proportional to the conductivity of the fluid.
David A. Johnson, P.E. --- Consulting Engineer
Sounds like a good idea. I was actually hoping to use a continuous excitation at about 10KHz and use a synchronous detection (lock-in amp) on the receiving side.
Did you have about 10mH on both coils?
The intent is to induce a sizeable current to flow in the fluid, so you want keep the amp-turns of the primary coil high. Thus, an inductance of about 5mH or 10mH seemed about right. I used a frequency of 25KHz to keep it well outside the audio range. Coils of equal inductance might lower the cost of the product.
Also, you might consider a pair of open air core coils. This would allow the two coils to be sealed inside some PVC pipes. I have considered using this method to monitor the level of the fluid, in addition to its conductivity.
Dave Johnson
I wonder if it would be viable to resistively load (or even short circuit) the secondary and then look at the Q, or watts-loss, of the primary. That would give a 2- -wire sensor.
-- Tony Williams.
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