Looking For Liquid Conductivity Circuits diagram

To measure conductivty in liquids you want an AC signal of say 15v peak to peak or AC coupled pulses to avoid polarising the liquid. The detail depends on the conductivity range you are trying to measure.

Bare in mind conductivity is nonlinear and also very temperature dependant.

Commonly the known stable AC signal would be applied across two stainless steel pins of a probe via a known resitance, the AC across the pins is the n rectified and measured. Maybe a precision rectifier using an op amp would be a good idea to remove the volt drop from the rectifying diodes.

To offer more help I'd need a more specific question, time or incentive (or maybe a tin of tuna )

Del

I need a circuit as many told me to use LM324 I don't know what other component to use or any reference circuit. Some uses gates IC to generate frequency ( oscillator ) but the end result not correct.

Please help

I can ony tell you that LM324 is no a best bet for your measure. you will need more tiny current chip for your use.

say, ICL 7650. TLC2654 etc. then use a multimeter to display the value. DC enough, neednt AC, in order that you have change ac/dc.

Can I request the schematic circuit diagram. I noticed the JCD conductivity monitor module made in china does use LM324 IC but I don't have the schematic diagram for it. Please Help I need to convert any circuit diagram as reference so that I can connect to the PLC input regarding to frequency, current or voltage.

One again many thank in advance.

I dont know this -JCD conductivity monitor - you can tell me about it.

suppose, if the liquid is water, the current will be very small, I think the 324 will nt be good for the measure.

I have designed a commercial chemical dosing unit which uses an AC coupled 5v pulse which is then read straight int an analogue input of a microcontroller with no amplificatin at all
...ok the usual operating range is about 2mS and it's not critical control application...(dish washing)

Actually the lower the conductivity, the bigger the pulse so it is easy to measure!
High conductivities will short circuit the pulse.
Del

it seems I misunderstand the concept of "conductivity".

is it equal to 1/resistance ? (=1/r)

You are correct.

If you apply a 5v pulse via a 200R resistor onto a pair of stainless steel pins immersed in the liquid, and the measure the pulse amplitude across the pins.

A high conductivity will result in a lower pulse amplitude...e.g more current will flow through a higher conductivity, dropping more voltage across the resistor, reducing the pulse amplitude across the pins.

Of course one could measure the pulse amplitude across the resistor and get the opposite result!
However it is convenient to measure across the pins if one is connected to ground/0v

Del

ok.

the liquid you measured must be higher conductivity. like some chemical liquid or reagent.

if it would be water like, the conductivity is very small, you'd better use bridge way and teh 324 will be not able to endure such snall current. because of its higher offset and bias current.

324 can be connect in way of instrument amplifier. I think top thread may hope to get this circuit.

Works with water...
Put a 5v pulse via 200R across 2 pins in water you still have a 5v pulse across the pins, load the water with detergent and the pulse amplitude will drop to say 3v amplitude.

I see,

What I said is if the liquid has a large resistance, say, 200K ohms. its usual. you will get a tiny voltage down. or say very tiny current. then you cannt get a good result from your set.

of cause you can deduce from plate capacitor principle and get back its epsilon (dielectic coefficient)

why dont you all use more common way like bridge to measure?

I had one circuit. I shall search for you, give some time! It is the circuit of Conductivity meter manufactured by Naina Electronics (P) Ltd. They used 6 op-amps out of two LM324 ICs. It uses 1 KHz AC to measure conductivity and cell ratio of 1:10. One good circuit available in Ak Sahaney 's Instrumentation Hand Book.

Thank you

I really appreciated you kind help

Unless you are an experienced electronics design engineer you will find the task very difficult and time consuming. The calibration is nightmareish and aos very time consuming. There are many relatively cheap units on the market which will do this.

Unless you want to build one for your own education/entertainment I would advise you to buy a commercial unit.

Del

Quite. There are any number of proprietary manufacturers of these instruments. Thornton (usual disclaimer) are highly applicable. The OP might want to look at Endress + Hauser (usual disclaimer) as well.

Thank for the info I just need a circuit schematic diagram for my general sturdy knowledge for general lab test i don't think a advice to but a ready made type. Please help not only the theory and condemm the LM324 not accurate. I I mean I need the share knowledge how the schematic diagram constructed. Sorry if I am wrong and hurt you all. Lastly please help I badly need the schematic diagram to help me to start off. Thank You.