exploring the use of conductivity monitoring to measure salinity

jlinzmaier

Premium Member
I'm exploring the option of monitoring my reef tanks salinity level based on conductivity readings. I currently have a pinpiont conductivity monitor and before I delve into monitoring my salinity with it, I have a few quesions.

I assume any stray electical current (even minute) will affect a conductivity monitors reading correct?? I'm sure the answer is yes, therefore the best practice is to measure conductivity by taking a sample of water from the tank and measuring it away from any electrical equipment - correct?

I'm able to find several conductivity to salinity conversion calculators, however, it appears that temperature plays a significant role in that conversion. Are there any other significant factors that will affect my conductivity monitoring that I need to take into account when converting to a salinity reading? Will higher or lower ca, alk, or mg levels affect conductivity?

Jeremy
 

Randy Holmes-Farley

Reef Chemist
Premium Member
Stray electrical current won't impact it. It is a high frequency current, which is unlikely to be in the tank. Measuring in the tank is generally OK. I do it, but confirming it away from the tank is a fine idea.

The Pinpoint and all such meters for hobbyists are temperature corrected to 25 deg C, so temp is mostly not a concern unless you are far from 25 deg C using a poor correction capability probe/meter.

No, typical ionic imbalances do not impact it appreciably.

I discuss these issues in these articles:

Using Conductivity to Measure Salinity
http://www.aquariumfish.com/aquariumfish/detail.aspx?aid=1804

Reef Aquarium Salinity: Homemade Calibration Standards
http://reefkeeping.com/issues/2004-06/rhf/index.htm

The second one has a table converting conductivity to salinity and several other parameters (such as specific gravity and refractive index). ;)
 

jlinzmaier

Premium Member
Thanks Randy! I had read the first article but hadn't read the second. Appreciate your help.

Can you explain the "refractive index" a bit more? Not sure what that refers to and how it's pertinent in the first table of the second article you posted.

Jeremy
 

jlinzmaier

Premium Member
I was wondering how it's pertinant in the first table in that second link you provided.

Not sure what the refractive index numbers mean in relation to the SG and conductivity measurements.

Jeremy
 

Randy Holmes-Farley

Reef Chemist
Premium Member
it is only pertinent to understanding how refractometers work and what is a suitable standard for use in refractometers to match 35 ppt seawater. :)
 

jlinzmaier

Premium Member
<a href=showthread.php?s=&postid=14772237#post14772237 target=_blank>Originally posted</a> by Randy Holmes-Farley
Stray electrical current won't impact it. It is a high frequency current, which is unlikely to be in the tank. Measuring in the tank is generally OK. I do it, but confirming it away from the tank is a fine idea.


Never thought I'd say this but I think your wrong Randy.

I purchased a salinity monitor and have been monitoring my tanks salinity after very careful calibration. Just this morning I came down to monkey around with my tank while the lights were still off. The salinity monitor read 49.1mS (I know - very low, damned refractometer!). When my actinics turned on I happened to glance at the monitor and noticed the salinity was suddenly 46.5mS!!! After some initial panic, I did some thorough checking to make sure my float valve didn't get stuck and suddenly dump a lot of FW into the system. Much to my surprise everything was fine and working properly. I swirled the salinity probe a bit and it still read 46.5mS. I then took a plastic cup and scooped out a cup of tank water and set it on the wooden stand next to the tank. It read 49.1mS!!!! I put the probe back in the water and it again read 46.5. I moved back and fourth between the two at least a half a dozen times to confirm my findings and got the same results every time. I then put the probe back in the tank and turned the actinics off and it jumped back up to 49.1mS. Turned the actinics on and it dropped down to 46.5mS. I have four actinics and after a bit of experimentation I was able to determine that one individual set of lights was causing the problem.

An interesting concept surrounding the situation is the fact that I have a grounding probe properly installed into a grounded GFC outlet. That grounding probe happened to be plugged into the the same receptacle as the timer for the actinics. When I pulled the grounding probed out of the water the salinity went back up to 49.1mS. Hmmmm??? Also, when I moved the grouding probe to another outlet the monitor read 49.1mS. Again, I moved the grounding probe back and fourth among receptacles and it continually dropped the salinity on my monitor when it was plugged into the receptacle that the actinics were plugged into.

As a third and final confirmation that there was no stray electricity (at least none that is affecting the salinity monitor) I tripped all the GFC's and turned off all the breakers that are powering my tank and the salinity still read 49.1mS.

Pretty interesting discovery IMO. Is it possible that the grounding probe could have been transferring electicity into the tank since when I removed the grouding probe from the tank it no longer affected the salinity monitor? I'd like to blame the issue on some sort of fault with the receptacle but I can't becuase it was only showing the anomaly when there was one specific light plugged in and only when that light was plugged in.

Any thoughts???

Jeremy
 

jim.l

New member
Electronic ballasts emit a lot of RFI, and work on high frequency switched current to the lamps. Mine used to knock out FM reception in my house. I would have to turn off my lights to listen to the radio. That might be related to your problem?
 

tmz

ReefKeeping Mag staff
Premium Member
Jeremy. Do you run it with a battery or an adapter. Mine will not work properly with an AC to DC adapter. A good friend ,who is an electrical engineer, explained that adapted ac current is not a perfect in/out but a dc battery is. Otherwise some draw on teh outlet where your unit was plugged in could disrupt it.
 

Boomer

Bomb Technician (EOD)
Premium Member
I agree with Tom. However, there are other issue with this so called stray voltage which really does not exist as you have been told. It is really the magnetic fields generated in the tank by heaters, pumps etc. that are normal and which give false voltage readings on a meter. Then there are the properties of the seawater itself. When you then ground the water with a probe it loses the false voltage reading and the meter reads zero. This may affect the meter when the water is grounded. This so called "induced voltage" or decoupled voltage" is real but normal.

If one has a real voltage leak it will not be 10, 35 volts or so it will be on the order of 80 or more volts. Even equipment not in the tank water can induce voltage into it. For example, just lights suspended above the tank can induce voltage. Take a power head and put it by itself in a bare bottom 10 gal tank and test it and you will see this so called voltage. Hook up the GP and poof it is gone. Any heater, power head etc. will do this. If one has a GP they had better have a GFC as a grounding probe, for if you don’t' and have just a GP it is even more dangerous for you and your fish. A few years ago two electrical engineers spent countless hours testing this and then wrote articles on the subject, which I have posted here in the past IIRC.
 

tmz

ReefKeeping Mag staff
Premium Member
PS I cross check mine with my old rusty refractometer from time to time. Just a security blanket I guess.
 

kaskiles

New member
I've also confirmed that my Neptune Aquacontroller 3pro Salinity probe measurement get interference from something in my tank. When I measure with the probe in the sump, I get a high reading, when I pull the probe and a glass of tank water, it goes back to normal.

I called Neptune support and they said it was a known issue, and that I'd have to setup a drip type isolation cup for the probe...
 

MrPike

New member
There are quite a few instances of the AC adapter causing the pinpoint meter to give screwy readings.
 

tmz

ReefKeeping Mag staff
Premium Member
Jeremey. I had a similar problem a few years ago. It showed on a ph monitor which is more sensitive to interference ,I think. I also got a buzz when I touched the water. In my case the fixutre housing was touching a bare wire inside and reaching the water via a bit of salt creep. Very dangerous. When a ground was added the loose voltage tuned to amperage and flowed. When the ground was removed no sign of the problem lurking for me to be the ground. You may wan't to check if your fixture is plugged into a grounded timer/outlet. Could also be a wiring problem in the outlet. Checking with a volt meter won't show it when it's amperage but you would feel it. Be careful.
 

Boomer

Bomb Technician (EOD)
Premium Member
If one has a GP they had better have a GFC as a grounding probe, for if you don’t' and have just a GP it is even more dangerous for you and your fish.


If one has a GP they had better have a GFC with it, for if you don’t' and have just a GP it is even more dangerous for you and your fish.
 

jlinzmaier

Premium Member
Thanks for all the replies everyone!!

Pending the calibration solution is correct, I believe I'm getting accurate readings from my salinity monitor since even when I trip all the GFI's and turn off the breakers at the breaker box it doesn't change the reading on the salinity monitor.

I am currently running the meter with an AC adapter. Out of curiosity I'll put in a battery tonight and see what happens. Either way, I don't get any fluctuation in the readings since I've moved the GP to another outlet.

The actinic lighting is plugged into a timer with a ground (3 way plug) and the timer is plugged into a receptacle on a GFCI circuit.

Since I just got the salinity monitor and don't fully trust it yet, I have also been spot checking with my refractometer. This leads me to another minor quandry. The refractometer is freshly calibrated as well as the salinity monitor (used the same solution for both). My Salinity monitor is currently reading about 50.0 mS (32.8PPT salinity) and the refractometer is reading about 34PPT salinity. That's a bit of a difference and I don't know why. When I calibrated the salinity monitor I did let it sit for about 15 min in the 53.0 mS solution to allow it to compensate for the temp and then made the proper calibration. It may be that I simply am not seeing the lines clearly enough with the refractometer. Regardless, the corals have responded with better tissue expansion and polyp extension since I've been bringing the salinity up. I guess a visualization of the animals is sometimes as important and helpful as all the instruments and test kits we use.

Thanks again everyone. I aplogize if I missed responding to anyone's comments or questions. Post back if there is any other info which can provide some answers to the situation.

Jeremy
 

tmz

ReefKeeping Mag staff
Premium Member
Good luck with it. I like it but the battery exhausts quickly if you leave it on constantly so I have to use mine in more of a dip turn on/off test .I don't use the adapter because it runs erratically when I do.
 

jlinzmaier

Premium Member
<a href=showthread.php?s=&postid=14997717#post14997717 target=_blank>Originally posted</a> by tmz
I don't use the adapter because it runs erratically when I do.

When you say that, do you mean it doesn't always turn on and remain on properly or do you mean you get varied results when using the adapter?

Thanks Tom.

Jeremy
 
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