I'm confused. PPT vs. Conductivity (mS/cm)

beachsidereefer · Oct 10, 2015

Earlier this week I purchased the RKE SL2v2 module which came with a probe to measure salinity via the conductivity method. Up until this point I've always used a Optical refractometer and calibration fluid for 35 PPT.

I hooked up the SL2V2 and calibrated the salinity probe according to the manufactures directions using the package of fluid that calibrates at 53 mS/ CM. After the calibration process I double checked it by seeing what reading it gave for the calibration fluid. It was dead perfect on 53 mS/CM.

I put the probe into the sump and it almost instantly shot up to 57! which according the the conversion chart would equal 37.94 PPT. So over a period of 24 hours I slow removed salt water and replaced it with RO and the reading slow crept down to 53.3 which is just a little north of 35 PPT which is my target. I'm using this web site for the conversion
https://www.hamzasreef.com/Contents/Calculators/SalinityConversion.php

So I thought I would check to see what my refractometer was showing and I was really surprised to see it was reading 30 PPT! Thinking that it was probably out of calibration I used the fluid I have which is a target of 35 PPT and low and behold... it was almost perfectly calibrated!

so now I'm just confused. It seems like I have two calibrated systems for measuring salinity yet their is a deviation between 4-5 PPT.

Hoping someone can shed some light on what's going and which one I should trust.

Thanks!

Randy Holmes-Farley · Oct 10, 2015

Well, as usual when to measurement disagree, it is not easy to tell which (if either) is accurate.

53 mS/cm is the exact conductivity of 35 ppt seawater when properly corrected for temperature to 25 deg C (or measured exactly at 25 deg C). I do not know anything about the sl2v2 and how it corrects for temperature, but you very likely need the temp probe in the same solution.

That said, what was the calibration fluid used to calibrate the two devices (the brands)?

Was it the same solution?

FWIW, not all fluids intended for conductivity meters are equally suitable for refractometers. It would have to be a seawater type of solution to work for both. In other words, 53 mS/cm conductivity does not imply exactly 35 ppt unless it is seawater, and a conductivity standard might be made in other ways.

beachsidereefer · Oct 10, 2015

Hi Randy,

Thanks for the reply. The sk2v2 comes with calibration fluid and it's not labeled with a brand name but here's a pick from Google on what the package looks like this except of course my package has not expired.
http://www.aquariumspecialty.com/me...e127f42dca9de82fb58b1/i/m/image_photo11_6.jpg

The calibration solution I use for the refractometer is this stuff I buy from amazon: http://www.amazon.com/gp/product/B0...=1444502940&ref_=sr_1_2&s=pet-supplies&sr=1-2

So you raise a good point about temperature. My refractometer compensates for temperature according to the manufacture. The RKE instructions make no mention of temperture calibration and when I calibrated it the solution was at room temperature which is about 78 F.

So is my issue that solution for calibrating the RKE unit wasn't at 25 C?

Randy Holmes-Farley · Oct 10, 2015

So that first one is potassium chloride and cannot be used for the refractometer, but is fine for conductivity.

The second one, from pinpoint, is good for either device, and would be a good check on the conductivity meter.

I'd check both standards in the RKE with the temp probe in the standard solution.

beachsidereefer · Oct 10, 2015

Randy Holmes-Farley said:
So that first one is potassium chloride and cannot be used for the refractometer, but is fine for conductivity.

The second one, from pinpoint, is good for either device, and would be a good check on the conductivity meter.

I'd check both standards in the RKE with the temp probe in the standard solution.

I don't have any more of the potassium chloride testing solution so I stuck the RKE probe into the PinPoint bottle to see what it would read. The reading it shows is 57.7.

I guess the only thing I can do at this point is order a fresh batch of both testing solutions and re-calibrate the refractometer and conductivity probe to see what results I can get?

Randy Holmes-Farley · Oct 10, 2015

Yes, a new batch sounds good, but the calibration of the RKE may have been the problem since it reads the Pinpoint too high.

You can also make a DIY is you have a scale:

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2004-06/rhf/index.htm

beachsidereefer · Oct 13, 2015

Hi Randy,

I obtained a fresh batch of Pinpoint from Amazon. First I tested the refractomoter and it dead on 35 PPT.

Next I went through the re-calibration process of the RKE SL2v2 per their instructions. After calibration the probe read 53 mS/CM in the Pin point solution. I double checked it by first swishing the probe in RO water, dried it and put it back in the calibration solution.

After that I put the probe into the sump and the reading once again shot way up. This time it's registering 73.1 ms/cm! Using my refractometer I checked the salinity and it's reading about 33-34 PPT

At this point is it safe to say the RKE SL2v2 unit/ probe is probably defective?

Randy Holmes-Farley · Oct 13, 2015

You may have some sound of grounding problem or other interference.

Try measuring the conductivity of a cup of the tank water with the RKE, just like you did with the calibration fluid.

beachsidereefer · Oct 13, 2015

will do, I do not have a grounding probe in the tank so I could have stray current and not know it.

Thanks!

beachsidereefer · Oct 13, 2015

ok, I put some tank water in a glass container and the reading and it stayed consistent at 73.1. Just to see what would happen I filled up the container with distilled water and the reading dropped to o.1.

My entire electrical system for the tank is hooked up to a battery back up. I checked unit to see if it was registering a ground issue and it wasn't. I also activated the back up to see if it was working properly or displayed any error codes. After it went through a self test and switched back and forth from house to battery power it did not display any error codes and showed the batteries to be in good health.

Is there any other substance that could be present in the tank water and would throw off a conductivity reading? I guess my next step is to reach out to Reef Keeper and see what they say.

Randy Holmes-Farley · Oct 13, 2015

No, there's nothing in the water that can make 35 ppt seawater have a conductivity of 73 mS/cm.

It reads the Pinpoint fluid OK in a cup, but not the tank water?

beachsidereefer · Oct 27, 2015

HI Randy,

Thought I would circle back with you and let you know the issue has been solved. The calibration instructions were not entirely clear on a couple of steps so it resulted in the abnormal readings. After working with a Digital Aquatics rep. we figured out the problem and now it's spot on what my refractometer is showing for salinity.

Thank you for all your help in troubleshooting the issue. I certainly learned a lot more about salinity, conductivity and a very important lesson of not always trusting testing equipment!

Randy Holmes-Farley · Oct 27, 2015

That's great! I'm glad everything is worked out.

You're welcome and Happy Reefing.

beachsidereefer · Oct 29, 2015

Hi Randy,

Follow up question out of sheer curiosity. I've noticed that when my lights (combination LED and MH) are on I'm seeing a reading of 52.5 ms/cm. In the morning, when the lights are off I'm seeing a reading of 52.2 ms/cm., when the lights come on in the afternoon, sure enough it starts to creep slowly up to 52.5 and stay's there until next morning and once again, back to 52.2. I observed this patern for a week and it's like clock work. What could be contributing to this variation?
My first thought was my ATO but it's a Tuenze optical eye unit and it's very sensitive and is working as expected. When it triggers for a fill it doesn't add more then 3-4 ounces of water at a time. Just for reference my tank is 200 g. and I think my sump holds about 15-20 g of water. My next thought was temperature, when the lights are running the tank temp rises about 2 degrees before the chiller kicks in. Could the rise in temp. alter my conductivity reading?

Always appreciate your thoughts and thank you for your wisdom!

Randy Holmes-Farley · Oct 29, 2015

Conductivity does vary quite a lot with temperature, and conductivity units generally correct for it (to a baseline of 25 deg C, typically), but that correction is not likely to be perfect so imperfect correction may be what you are seeing.

One way to test this for your system is to put the probe and the temperature sensor into a cup of water and allow it to cool to a lower temp than tank temp (or a higher temp) and monitor the change.

The change in conductivity with temperature is different for different solutions. Seawater, for example, will have a different correction than sodium chloride only. Some expensive devices have different choices for the temperature correction, and one of mine has a choice that is supposed to be accurate for "natural waters",b ut that probably means streams and lakes more than seawater.

pickupman66 · Oct 29, 2015

FWIW, the SL2v2 needs to have the temp probe hooked to it and calibrated to function properly. When you calibrate the conductivity probe, first pull the temp probe from the water and let it settle on the air temperature. then with both probes in the air. calibrate the "0" reading. then put both probes in your cal solution and wait for both to settle. then calibrate to that solution. the SL2v2 will then compensate for temp fluctuations in your water.

beachsidereefer · Oct 29, 2015

Thank you for the information! That is definitely not in the instructions I received from RKE.

n2585722 · Oct 31, 2015

Also you can calibrate to 35 for ppt if you input that as the cal point instead of 53.

beachsidereefer · Nov 4, 2015

n2585722 said:
Also you can calibrate to 35 for ppt if you input that as the cal point instead of 53.

great idea! I'll probably try that the next time I re calibrate. Do you think I should do it monthly, quarterly, etc?

n2585722 · Nov 4, 2015

It is up to you but the more often you do the calibration the more likely it is still in calibration. I have a refractometer and use it to double check the readings. I don't use it to control anything but do have it setup as alarms for too low and too high a reading. These shut down the ATO and will shut down the AWC when I get it setup. I also have it setup to email if it gets out of bounds. Just make sure what you normal swing is before you setup alarms.