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Old 03/31/2010, 09:05 AM   #1
cyrusthevirus
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DIY Conductivity Probe/Meter

So I have a circuit to connect a probe to, but I can not determine which probe to purchase. From what I can read the EC of saltwater is 54,000uS/cm.

I have read on several threads that people are using the conductivity electrode SDL-1 from http://www.nengshi.com/en/new_lab.asp?id=254.

However from what I can tell, it would not read high enough. Anyone else using one of these probes or have a different source for probes?


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Old 03/31/2010, 11:21 AM   #2
GlassReef
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Marine Depot sells a number of different models. Just do a search on conductivity probe.


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Old 03/31/2010, 11:30 AM   #3
Randy Holmes-Farley
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Unless you are an electrical engineer, I do not see conductivity as a DIY.

If you are, there are many probes available from places like Cole Parmer, and they do show the appropriate range in their catalog. For seawater, you'd want up to at least 60 mS/cm (60,000 uS/cm). Some probes have 2 and many of the best ones have 4 electrodes. They all typically also incorporate a temperature sensor since the value must be temperature compensated.

I discuss the method and complexities a bit here:


Using Conductivity to Measure Salinity
http://web.archive.org/web/200406040....aspx?aid=1804

and this one has more on conductivity:

What is TDS?
http://www.reefkeeping.com/issues/20...ture/index.php


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Old 03/31/2010, 12:01 PM   #4
cyrusthevirus
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From what I understand the difficult part of conductivity is the need for an AC circuit. There are a couple posted online and once I test mine I will share it. As for the temperature, we all run our at between 78-80 degrees so there is not much need for compensation for temp as this can be handled in the software.

The probes listed on Marine Depot and Cole Parmer and much more expensive than on the site I listed.

Anyone use these less expensive probes?


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Old 03/31/2010, 02:31 PM   #5
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OK, I just wanted to be sure you knew what you were getting into and what you'd have when you were done. Without temperature compensation, you'll have to warm up any calibration standards you use as well. I personally wouldn't want to use a conductivity meter that wasn't temperature compensated. Even 2 deg F is enough to change the readings a fair amount.

I didn't see any prices on the site you linked. As you note, the SLD-1 is not suitably high in range.


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Old 03/31/2010, 09:10 PM   #6
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Quote:
Originally Posted by Randy Holmes-Farley View Post
OK, I just wanted to be sure you knew what you were getting into and what you'd have when you were done. Without temperature compensation, you'll have to warm up any calibration standards you use as well. I personally wouldn't want to use a conductivity meter that wasn't temperature compensated. Even 2 deg F is enough to change the readings a fair amount.
Good point, I will need to remember that.

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Originally Posted by Randy Holmes-Farley View Post
I didn't see any prices on the site you linked. As you note, the SLD-1 is not suitably high in range.
$15, but you have to wire the money so unless you are buying several, the costs do not make sense.

Anyone know what probe they are using in these builds:
http://www.nano-reef.com/forums/inde...owtopic=180805

http://sites.google.com/site/richardorme1979/home


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Old 04/01/2010, 08:36 AM   #7
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I do not know what, if anything, was ever implemented in the first one, but the second link talks about using a DC circuit (quoted below), so he is not informed about how to measure conductivity properly and I wouldn't use whatever he used (if, in fact, he ever made it).

"To do this digitally is a little more complex and is accomplished by passing a small DC current through the water sample."


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Old 04/01/2010, 09:09 AM   #8
cyrusthevirus
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Quote:
Originally Posted by Randy Holmes-Farley View Post
"To do this digitally is a little more complex and is accomplished by passing a small DC current through the water sample."
If I understand everything I have read, this would produce three things, a poor reading, hydrogen and oxygen.

I reached out to my contact at nengshi.com and the specs are wrong on the site. The probe is capable of measuring from 1-100mS/cm. I am ordering several other probes so for $15 I added in a couple of these. If they work, great. If not, maybe someday I will have a freshwater aquarium or as you say, "measure the concentration of limewater."


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Old 04/01/2010, 11:20 AM   #9
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Sounds good. Good luck.


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Old 04/06/2010, 10:37 PM   #10
cyrusthevirus
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This is a repeat of my other post, but just incase someone is looking just for salinity here it is:

pH/Salinity and ORP circuit boards have been soldered. They are below. Now I am just waiting on the probes, They should ship next week.

Salinity before soldering:

Salinity after soldering:




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Old 04/16/2010, 10:48 AM   #11
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Any update on your project?


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Old 04/16/2010, 10:50 AM   #12
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Probes just shipped from China yesterday. Once I have them I can test. I am waiting on the edge of my seat.


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Old 04/16/2010, 11:38 AM   #13
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can you share some information about your circuit? I have bread boarded a few but have not been happy with the results so far... too quirky or too much drift.


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Old 04/23/2010, 09:26 PM   #14
cyrusthevirus
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Sorry for the delay, I was out of town. Here is the diagram for my circuit. It should work. I have had it reviewed by a lot of people. PM me if you are interested, I have some extra PCBs of it.



On another note I received all of my probes. I need to setup my standardized solutions so I can begin testing.


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Old 04/23/2010, 09:42 PM   #15
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I may take a PCB or two off your hands. Did you design the circuit or modify one you found? I understand the oscillator section (U1A) and the probe amp (U1B), but am not wraping my head around the bridge amplifier (I assume we are converting AC sinewave into DC but I am going to need to get the op-amp book out to figure out what else is happening.

Do you have a scope?


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Old 04/23/2010, 10:10 PM   #16
cyrusthevirus
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I worked with the one located here, http://www.octiva.net/projects/ppm/ but it was hard to understand. I tried to reach out to the person, but never heard back from them. I also wanted to include the - 12 VAC conversion in the board so I only needed to suppy 12 VAC instead of + and - 12 VAC.

Unfortunately I do not have a scope.

The probes I purchased from China. The range should work for saltwater, but I will not know until I get a working prototype.

PM me on the PCB if you are interested. I think I have 6 or so extra ones.


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Old 05/12/2010, 12:34 PM   #17
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how is the project coming along?


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Old 05/12/2010, 12:36 PM   #18
cyrusthevirus
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I have been focused on my lighting, pH and ORP, so I have not had a chance to play with this. I am happy to report the other probes I ordered from China are working out just fine.


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Old 05/12/2010, 01:58 PM   #19
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I am curious if the circuit is stable. I am placing an order with digikay today (for a pressure transducer for my RO/DI automation project) and was going to toss the parts for the design in the cart.

I want am looking for a stable conductivity circuit so I can run (3) TDS probes on the RO/DI project. The code for the flow sensors, pressure and temp is pretty much done. I just have to find a TDS circuit and interface it so I can write the code.


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Old 05/12/2010, 02:04 PM   #20
cyrusthevirus
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Yeah I am just not sure yet. If you want one of the boards to play with I have extras. The board cost me $11 each. If you want one, let me know. The parts for it are listed above. Although I may have an excel file with all of the parts and Mouser part numbers. I would be glad to have someone test with me.


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Old 05/12/2010, 02:13 PM   #21
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I have most of the parts in my bin but they are SMT. I have the through hole caps, diodes and resistors but not the opamp. I guess I will toss the stuff I need in the cart and order it along with the transducer. Not sure if I need the board yet. I may just do it on the breadboard first. I will order mult-turn cermets for the trimmers and metalized poly MKTs for the two RC caps. Looks like the brdige cap should be aluminum electro to smooth the ripple and the 1n4148 diodes can be just about anything. I wonder of a precision bridge would help?

I have ordered TDS probes from HM digital so don't need the BNC either.

Let me know how things progress.


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Old 05/22/2010, 08:53 PM   #22
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The more I look at that schematic, the more I see room for improvement.

1) I have not taken the time to breadboard the circuit but am not convinced that the Wien Bridge is going to be stable or oscillate at the proper frequency, the component values just don't appear correct. There is no gain control, meaning that the sinewave will be distorted or clipped and the stability will drift with impedence and temperature.

2) I think there should be a buffer stage after the oscillator.

3) An Opamp would likely work much better as an attenuator (instead of the two diodes).

4) I don't see a need for the bridge rectifier. It looks like he is using it in conjunction with an opamp precision rectifier... It does not appear to make sense.

I have a more stable and conventional circuit in the simulator right now. The wien bridge will be equiped with gain control so that it will be self stable. You shouldnt need a scope to set the oscillator then. I have also ordered a sinewave generation chip and will play with that as well.


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Old 05/23/2010, 08:11 PM   #23
cyrusthevirus
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How long until you can do further testing?


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Old 05/23/2010, 10:32 PM   #24
BeanAnimal
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Are you having problems with the circuit you built?


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Old 05/23/2010, 11:28 PM   #25
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Quote:
Originally Posted by cyrusthevirus View Post
How long until you can do further testing?
Killed the last 7905 regulator a while back... just ordered a pile from ebay and will order the prototype parts for my design tomorrow. I also ordered a new bench supply today (Cheap Mastech) because I lost a auction (fell asleep at the wheel) on a really nice triple output Lambda bench PSU.


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