DIY Conductivity Probe/Meter

[xtal]

I miss quoted about the atlas-scientific I not sure what method they use...
It is the OWL2C by EME Systems thats using lmc555 mounted on about 1" x 1"
pca ....The main problem I'm seeing with my setup is the temperature drift of the circuit ...

I was thinking about investing in the Atlas Scientific, but no so sure now,
did you also get their probe @ what K value...

I'm thinking I could compute K by measuring the EC in a calibration solution
Then use a resistor to obtain the same EC then
compute conductance * K = conductivity
K / R = conductivity
K = conductivity * R

If my math and thinking is correct

[bogdanaioane]

Hello All
This is my first time posting on one of these forums so please forgive me if I go wrong.
I have been reading this post and I feel I should contribute with some of my knowledge and understanding on this. I started developing an aquarium control system two years ago and am still in the said process. However I have finished the design and initial tests on the EC Module and am quite happy with the results.
If people are still interested in developing their own EC controller please write in this post and ill be sure to post schematics and more detail instructions on how to build one.
This thing that I'm making will be a commercial system but will most importantly be open source for people to make their own if they want to or feel it would be cheaper that way. So I figure what better place to start to release these plans and instructions freely than on forums such as these with people that are eager to make something out of nothing.

So... EC Meter
In this post I will cover the concepts involved in measuring EC of salt water.
1. Probe
An EC probe doesn't really have a range in the normal sense (like say a PH probe would have a range) it has a cell constant K which is the distance between the two electrodes divided by the contact area of the electrodes (so two plates of 1cm square each placed at 1cm from one another would give you a constant K=1) Its what circuitry you design and build that together with a probe of a certain constant would enable you to measure a specific range. So to answer one user that wasn't sure if a K=10 probe would be ok for 53000us, yes it would provided you take into account what I am mentioning above.
2.Signal
The voltage applied tot the electrodes HAS to be AC. And this doesn't mean that it has to be a sinewave really it could be square wave or any other shape of wave as long as its balanced equally with respect to GND. The frequency of this "pulse" has to be higher than 3Khz (some literature suggests even higher) The reason for the AC condition is that you need to prevent the positive charged ions from migrating and attaching to the negative electrode and the negative charged ions .... to the positive electrode (we don't want electrolysis). A good solution for generating a stable sinewave for this purpose is a oscillator with automatic gain control using an op amp (I will add schematic in my next post)
this signal has to be stable and unaffected by supply voltage fluctuations.
3.Amplification
Once you generate the signal you then pass this through an op amp to get an amplified signal at the other end. In this amplification stage, you replace the resistor that sets the gain with your probe. And that's about it really. In the circuit I designed I used an instrumentation amplifier that improves linearity. And the probe is actually part of something called a Wheatstone Bridge.(schematic to follow)
4. Signal Rectification
Once we amplified the signal, we still need to rectify it to be able to supply it into a measuring MCU or arduino or similar and since we have only used one of the two stages of a typical op amp in signal generation, we will use the other one together with two diodes to rectify the signal. THE END

One important aspect that I haven't seen discussed on this or any other forum for that matter is galvanic insulation (this thing or better yet the lack of, can make for pretty erroneous readings due to stray voltages and ground loops, especially in measurements like EC where your electrodes are in direct contact with the water) Just a thought.

I hope this hasn't been too much words and not enough substance and please feel free to correct me if I'm wrong and post your comments and if interested let me know and I will add some pictures and schematics to this for you.

[geekengineer]

So what is the point of measuring EC for saltwater? is there some assumption it would correlate with specific gravity? I didn't think that totally worked? I think in order to measure EC automagically for a fluid you need something mechanical in nature or a load cell device.

[bogdanaioane]

yes something like that
refractive index, specific gravity, and electrical conductivity are all indirect methods of determining the salinity of water which is what we actually need to control. in alot of cases there is no direct relation between these 3 indirect measurement methods and salinity but in the case of sea water there is. out of the 3 methods EC has the potential of being the most accurate and stable followed by refractive index and then specific gravity. I once tried using a load cell from a precission jewlery scale to make a specific gravity sensor and i can say that it works but is nowhere near the stability and accuracy of a well designed and built EC meter.
Hope this helps

[geekengineer]

ah ok.. I did mean to say "I think in order to measure SG automagically for a fluid" not EC.. looks like you got it.

So you can use it just not direct correlation but more stable measurement ok. I think I understand. If you make a probe yourself I believe you will need some platinum wire for the electrodes so they don't corrode out.

[bogdanaioane]

that's right
platinum is the most appropriate material for such application
also silver and nichrome wire. platinum is the best option i would say and is actually not that expensive considering that you anly need about 5mm of it to make a probe (if you're carefull you might do it with 2mm which costs arround 4USD)

[bogdanaioane]

As promised
this is the power supply side of the module (any questions just ask)

Power.png

and this is the Sine generator with automatic gain control

Oscilator.png

this is all pretty straight forward but if you see anything unclear just post your questions here and ill do my best to answer

this next part is where it gets a bit more complicated but not too much. this is the part that deals with taking the conductivity of the probe and converting that signal into a DC current that has a voltage swing of 0 to 5v representing min to maximum of the measured range. the measuring range is determined by the R17,18,19 group (these have to be high accuracy resistors at least 1%) and the R16 (this resistor establishes the slope)
there is quite a bit to talk about this part of the schematic as it is important to understand its funtion so one can build their own with a usefull range while keeping high resolution and sensitivity (I will go into this if needed in a future post)


measure.jpg



Hope this helps

[geekengineer]

Ok a few comments on your schematics.

1) doesn't GND and GNDA have to connect someplace? if you want them to be isolated you may have to use an isolated power supply maybe..

2) I assuming GNDA is also being used for the other end of the probe? doesn't the probe need two leads?

[bogdanaioane]

hi
1.yes GND and GNDA are electrically connected through a solder joint someware on the pcb.
its to do with the digital side of the schematic
Microcontrollers are in general very noisy devices and it is somewhat common practice to detach the digital ground field on the pcb from the analogue ground field and allow a single connection point between the two. this is said to reduce noise from the MCU. It si however not ment to be some kind of isolation.
to achieve galvanic insulation one must either as you say use an isolated power supply or implement some kind of standalone DC-DC converter galvanic isolator like the ADuM5401
2. Yes that is correct (sorry i assumed that was obvious but i see now that it may not be the case)

[geekengineer]

roger..

Usually I still put a connection on the schematic and put a note that its a small trace or something. But I just being picky. Just wanted to make sure you didn't miss it in layout as well for the probe connection..

Might not hurt to add some .1uF for your OP amp supply decoupling as well.

[bogdanaioane]

good catch on the OP amp decoupling
i hope this info will come handy to someone one day
if anyone decides to go ahead and build this design, please spend a little bit of time to understand the range of measurement with a specific probe constant and the resolution of the data measured.
the resistor values on this schematic work for a probe with a constant of K=10
which puts the range of the device at 266 to 171 ohms which with a cell constant of 10 gives a range of 37.59mS/cm to 58.48mS/cm
if we convert this into salinity and specific gravity at 25DEG Celsius we get a range of 23.82ppt to 39.05ppt Salinity and 1.016 to 1.028 specific gravity
it becomes apparent that this device has been designed to measure a very narrow and specific range of salinity. The reason for this is resolution and accuracy of AD conversion. This setup offers a resolution of 0.02mS/cm or 0.015ppt or 0.000012 SG
the range can be easily adjusted by changing the values of the resistors R16 to R19 but i won't go into that now (unless anyone needs me to)

here are some pictures of the board I made for this. it includes a bit more stuff like an MCU, a communications buffer and some other little bits (like some decoupling capacitors )
EC_Top.jpg
EC_Bottom.jpg

Hope this helps

[geekengineer]

one thing I was curious about is if I made my own probe how would I figure out what it's K constant is? I wasn't quite sure how to do that, but I didn't get a chance to dig into it much either.

[geekengineer]

Ok never mind I didn't read the thread enough you already answered that question.

[bogdanaioane]

Although the probe itself is potentially the simplest and easiest probe to build in this hobby there are actually a fiew things to consider if one needs to build a high quality EC probe with stable output and long life to rival a commercial available one.
If anyone is interested we can go into more detail and in depth conversation about how to go about making one, but for now ill point out the main aspects of it.
1. Cell constant
It is important to build a probe with a K of as close to the target as possible but is by no means critical (in worst case one can put a trimmer instead of the slope resistor and adjust the controller side to bring the probe in range)
2. Almost all major sensing probes in this hobby can be made as a DIY project however, I believe that none of them are the kind of thing that you can put together with a couple of wires and some duct tape. Sorry.
A bit of accuracy and care is involed if one whants to build a brobe that will work just as well as an off the shelf (more expensive) sensor
3. The actual build involves very few materials which are:
Titanium wire (commonly available online is 0.45mm diameter ) 1cm of it
Some plastic tube with an internal diameter of at least 15times the diameter of the wire (in this case at least 7mm)
Some two part epoxy resin with a high dielectric strength (also known as potting compound)
If temperature compensation is needed than also a thermistor should be introduces in the probe
Some wires (multistrand copper)
Some nail polish to help insulate the connection between the titanium and the copper as this is a mechanical contact (titanium can’t be soldered)
THE END

If anyone needs schematics and more in depth instructions on this build let me know

Hope this helps

[geekengineer]

I thought you need something more along platinum wire not titanium wouldn't it rust out over time?

[bogdanaioane]

platinum would work better (i thought to propose a cheaper option)
the problem one ecounters with platinum vs titanium is more to do with its surface finish after high polish sanding than oxidation
in the industry we find cells built out of platinum, titanium and graphite in general. I wouldnt advise the use of graphite in cells with k>1 as graphite does not have enough structural integrity for a DIY application
The procedure for building the electrode is almost identical in platinum and titanium, the only notable difference being that titanium cannot be soldered making it a bit more fiddely to work with. there is also a sanding and polishing difference between the two but it can be considered neglijable.

[geekengineer]

cool thanks for the good insight into all of this. I like to make one but Im still in the process of setting up my tank and this will have to be for a later date..

Hopefully I can get back to this eventually..

[bogdanaioane]

I understand
when you gat to that stage feel free to get in touch if you think there is more I can help with

[cvhjyguj]

Probes just shipped from China yesterday. Once I have them I can test. I am waiting on the edge of my seat.

[bogdanaioane]

what sort of setup are you using for your tests?

[porlock]

http://blea.ch/wiki/index.php/PPM_Meter

This site is not available any more :-(, i found the archive on
https://archive.org/web/

but without a eagle board and schematic , i some one share it with me I will be very very thanks full

[bogdanaioane]

Originally Posted by porlock

http://blea.ch/wiki/index.php/PPM_Meter

This site is not available any more :-(, i found the archive on
https://archive.org/web/

but without a eagle board and schematic , i some one share it with me I will be very very thanks full

what are the specs of the meter you are trying to build?
maybe I can help

[porlock]

bogdanaioane

There is a archive of not existing site.

https://web.archive.org/web/20131028....php/PPM_Meter

I want to build a EC meter based on schemas on this site, schemas are available and that is good information, bad i that the files with the eagle board was on different server and it was not stored on web.archive.org
There is no sense to develop board from a scratch if someone already do this and it works fine.

[porlock]

I think that my last post is not visible

[bogdanaioane]

can you try posting again?