Attention Electronics Geeks!

[smcnally]

I have an electro-geek related question. But yet...reef related. What I'm wondering is this...I have I/O ports that can supply voltage and measure voltage changes or resistance. I'm looking for either instructions on how to make (Or an inexpensive) conductivity probe to measure tank salinity. I just need something that will vary resistance that I can then convert in the procesor. So far I have a temp probe, but would like to eventually monitor salitnity and PH. Any ideas out there?

 

[synchiropi]

What about a probe like the one from here?

http://www.omega.com/ppt/pptsc.asp?ref=CDH221_CDH222&Nav=greee02

I think some pH electrodes can also measure conductivity, like this one:

http://www.omega.com/ppt/pptsc.asp?ref=PHB115&Nav=grebb03

I would be a little concerned about long term calibration. I have used some bioreactor probes that remain stable for weeks. There can be a small drift though...

 

[smcnally]

Thanks, I'm on my way out the door, but I'll check those out when I get back.

 

[Jim Tansey]

Probes like that will operate only with the signal conditioning unit it is intended to mate with. My background is in instrumentation design, as far as designing your own conductivity probe to operate directly with your I/O ports, no way.

Jim

 

[smcnally]

Jim, I'm not doubting you, but why wouldn't it work? I'm using a Crestron control system which can do just about anything with analog signals. All I would really need to know is it's minimum and maximum values, and then translate them by testing with known values. Then when it comes time to calibrate just repeat the same procedure. Crestron systems are pretty accurate so I would think it could be done (if done correctly), but I could be wrong. Is it just the conductivity probe your talking about? Or all different types of probes?

 

[synchiropi]

Is the issue that the probe needs to be connected to "something"? From that "something", if it has the correct outputs, then you can get your signal? I also would like to know about this...

 

[smcnally]

I always thought that you basically measured conductivity measuring the resistance that the water has. The more salt, the less resistance right? I thought I could mix some buckets of water all to different known levels of salinity, measure the resistance I got on the probe from each bucket, then translate that in my program. I wouldn't need anything super accurate like from fresh water to natural sea water. I would only want it to read somewhere in the range our tanks should be at.

 

[Reef55]

sounds good in theory... if you have two non-corroding (sp?) conductors that you can fix at a certain distance apart, it might work for a probe. If you could try that for cheap, I would give it a go. If not, buy a salinity / conductivity probe and go from there. I would get one made for a salt water tank though... most conductivity probes have range that is far unsuitable for salt water.

 

[Jim Tansey]

Steve,

Conductivity probes operate by placing a precise stable voltage across two electrodes spaced apart in the meida and the measurement made by measuring the resulting current flow. If you have the ability to provide that small stable exication voltage and measuring the tiny current flow and then calibrate the device then welcome to the business. I know nothing about your I/O port device but the analog requirements of this type of circuit likely are way out of the range of it.

Jim

 

[clamm]

dont tds meters work by measuring conductivity of water?

 

[smcnally]

Jim,
The I/O port I have supplies 4.5V and can read differences up to 1/1000th of a volt, so I thought maybe I could build a circuit off of that voltage, but maybe not. I could get a card for the processor though that can supply up to 24V with a 10bit output resolution. Thanks for all the info so far, I appreciate it. I'm not a "sensor geek", But I am a "control geek". lol

 

[me2003]

The lower the voltage/current the better. 4.5 volts will cause electrolysis.
This will create all sorts of new chemicals.
Aquatic measurement devices minimize this. The voltage across the probe ends
of my conductance meter is about 25 millivolts. The electrodes are also carbon.

 

[smcnally]

Yes, I heard carbon or titanium. I belive I can produce 25 millivolts with that card. I'll have to see.

The lower the voltage/current the better. 4.5 volts will cause electrolysis.
This will create all sorts of new chemicals.
Aquatic measurement devices minimize this. The voltage across the probe ends
of my conductance meter is about 25 millivolts. The electrodes are also carbon.

 

[Jim Tansey]

Titanium is widely used as an electrode material and would work well in this application, with the low excitation voltage required you will likely be measuring current in the nanoamp ranges.

Jim