Talk:PPM Meter Alt

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==Temperature Compensation==
==Temperature Compensation==

Revision as of 10:13, 4 August 2010


PPM Meter Discussion

This page is for:

Temperature Compensation

Has anyone added a temperature compensation function yet??

Shure, why not...

Moved this to its own page Temperature_probe May 31 2009 >:D Wikisysop

Interesting reader comments on probe design

Got this in an email, seems useful so posting here:

"I want to measure conductivity in limewater (for a marine fish tank), so I want an electrode for highly conductive solutions (around 10mS/cm).

After a bit of thought, I used two propelling pencil leads (0.7mm diameter) - I needed only a few mm exposed spaced by say 5mm. Readings after a few months are stable, so graphite leads seem ok for this application. (And to calibrate, just draw a line to make the electrode shorter!)

Clearly they're not suitable for everything, but they are readily available."


seems you're good not only in electronic stuff but also in water analysis...  :)

Thanks for such informative and helpful project.

any update on those meters?

like to see your project.......possible?

how's your old meter?


  • thanks for the encouragement. ^_^ I was considering the other night posting the complete source code for the meter. As far as updates, since the meter is working for the moment I've been using it without much changes. Would really like to add in the temperature compensation circuit provided by ElAMPO, for temperature compensation on the pH/PPM readings. :D Hmmz, lastly been using the meter pictured on the page since 2006. Thx again, ~ eris - 2009/03/13

Is There a Trick to Zeroing


I'm trying to build the conductivity meter but am having trouble getting it to zero. I adjust v2 to midrange (using a meter) and then try to adjust v3 to get pin 14 to read 0 volts. I can't get any closer than -1.87.

V3 is about a 20 turn resister and V2 is probable about 10 so I should have good resolution.

Is there a trick to zeroing it (the first time)


  • hmmz, it should zero... If its otherwise reading okai u may try increasing value of r15 to 100K or a little more. Also check the power supply voltages are symmetrical (should be +/- same voltage) Lemme know if this helps. @_@ Wikisysop

Marine Aquarium Salinity Controller

Hi Isabella,

I was looking for conductivity controllers on the internet and came across your site. AWESOME!

The only problem I'm coming across is getting a cheap 12V dual pole power supply. I have a design for one, but it's much more complicated than the meter circuit itself. Do you think a power supply for a computer would work? The one I'm looking at is only $15. Its outputs include an 18A +12V source and a 1.0A -12V source.

TIA for any thoughts.

If you are interested, here is what I am up to:

I am going to try to build a conductivity meter to automate the tedious process of making the salt solution for a marine aquarium. The idea is to start with a very strong salt solution, and have the meter control a reverse osmosis water supply. At or below a certain salinity, the RO supply will automatically switch off. I'll hold the temperature constant with an aquarium heater, killing two birds with one stone: I have to heat the water to 80° anyway before adding it to the aquarium, plus keeping the temperature constant might make a temperature compensation circuit unnecessary for this application. I will circulate the water with a small submersible pump (a powerhead if you're into aquariums). The RO water supply creates purified water so slowly (10 gallons a day) that the concentration should increase slowly and steadily.

The graphite probe lead* idea posted above is great! That has been one of my biggest hurdles. I bought a surplus commercial probe and experimented with it years ago, but it had a bunch of mystery leads, and I couldn't sort out how to use it in my own circuit. None of the other home-made probes I tried seemed to work. I never could really make a circuit that gave any kind of readable, consistent results. I did manage to burn out a lot of op-amps, though.  : (

If all of that works out, this could be taken farther. The entire process of adding make up water (because of evaporation) and doing weekly 10% water changes could be automated. There would have to be some redundant built-in safeties so nothing could get locked on, which could create a flood or, even worse, kill everything in the tank.

Thanks for all of the excellent information you have posted. Posting all that info looks like quite a bit of work!



  • would that be a lead lead?
  • hmmz, hey, thanks for the interest.. often wonder if anyone reads this stuff hehe. Well making a +/-12V supply should be pretty simple. A center tapped transformer, bridge rectifier, two 3300uf or so caps for filtering, a 7812, 7912, (these are very simple 3 pin +12 and -12 volt regulator ICs) and then two smaller output caps. Something like this: Except without the bottom part.

I used an old modem power supply that happened to have +/-12v and +5v which was perfect for running the controller as well as the ppm circuit. The PSU u described would work, but 18amps seems greatly overkill, the circuit uses less than an amp. If you just want to get things working might try an old PC PSU.

Good luck, if have anymore questions lemme know ^^

Stainless steel probe

Hi there,

My name is Daniel and i'm a electronics/microcontrollers entusiast. I've found you project very interesting. I've also come across a web site mentioning that commercial ppm meters use staineless steel probes, and i'm in the process of experimenting with a homemade one.

I'm building a probe thats very similar at least it looks similar to the ones in those cheap comercial ppm meters. A friend of mine will machine a piece of PVC to shape the probe, and i'm thinking of using staineless steel screws with the screw head removed (a 35mm M4 CHC cutted to make it smaller). They have a threaded part and a non threaded part and it will be the non threaded part that will come in contact with the solution to me mesured.

I was wondering if anyone has tried this because stainless steel screws are cheap and easy to find. It is also easy to make the connections and it is very robust.

I'll post here what results i'll find with this kind of probe. It will take me a couple of weeks because i have other projects going on.

Keep up the good work :)

  • Thanks! will be interesting to see how this works out. ^^ eris

Split Rail

I was researching for a way to measure water conductivity and I stumble upon your project. I've decided to give it a try but I have no experience with op amps.

One of questions I have is whether its possible to power the circuit with a rail splitter such as this: Sijosae-rail-splitter.png

Lastly, is there a way to check the oscillation without using an oscilloscope? Maybe a pair of leds?


  • to check without a scope, a volt-meter set to read AC may work.. AC mode should reject the DC components and just display the AC (or osciallation) voltage. In this case adjust until the voltage jumps up from @ or near 0.
  • The split-rail would work, just be sure that whatever your connecting to read it (such as A/D converter or meter) uses the same ground. I have been considering rebuilding mine using a charge-pump circuit. Another great reference and source of pre-made components is Maxim
  • hope thats helps ^^

Thanks for the info on the charge pumps, I've gone ahead and changed my design to use a charge pump.

I'll be using a single 500mA 5-volt regulator to power the op-amps, the charge pump, and the micro-controller. Any advice on using single supply?

Lastly, I've added a capacitor in series with each leg of the probe to block dc currents.

Thanks for your help and keep up the great work!

Change output voltage range??


Thanks for all the great info and the design.

I'm about to try and solder the PPM meter. Just bought me all the components I would need but before I start I had a small question.

My A/D converter has a range of 0-10v. Is there a way to make this circuit output in that range for EC values of 0-3 mS?

Like by changings the value of some resistors?

Thanks!!!!!!! :)

Eagle source files

Would you consider making the eagle source files available? It'd make it easier to mill/etch the circuit board, get a list of parts, etc.


A few questions...

First off, thank you for posting this interesting project. I am trying to product a prototype of it for myselft, however I have run into a few questions along the way.

1) I noticed that on the Eagle files, C1 and C2 are 0.15uF while on the original document they are labeled as .015uF. Were these values changed for any reason, or is there a preferred value which you would recommend?

2) There are four diodes in the AC to DC conversion section, and they seem to be acting as a rectifier, however I was unsure of their values. It was mentioned that D1 and D2 are 5.1v zenner diodes, but I didn’t know what proper values to use for D3-D6, could you suggest any?

3) C3 is a .22uF capacitor and in the pictures appears to be a electrolytic capacitor (or perhaps a non-polar cap?), but if it was polarized, I was unsure of where to place the anode. Or, I thought maybe I could trace the signal to determine the polarity with respect to pins 13/14 on the IC?

4) Lastly, does this project require 12v and -12v? Or, have you seen any success with running at +/-5v?

Again, many thanks for all of your contributions and hard work.


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