Results 1 to 7 of 7

Thread: Salinity Probe Question

  1. #1
    New User
    Join Date
    Jul 2018
    Location
    UK
    Posts
    12

    Question Salinity Probe Question

    Ok, this is a new build. Only weeks old, no livestock yet. Brand new Apex system. I'm brand new to Apex too although I've been in the hobby many years.

    So I went through several days of communicating with Neptune Systems through a support ticket to get the salinity probe calibrated (there was a technical reason why I couldn't calibrate the probe and they really helped me out). Once calibrated I asked the question "Why does the Neptune Systems 53,000 uS/cm calibration fluid not work on refractometers (as they had indicated to me during the support ticket exchange)? Out of curiosity, I tested the 53,000 uS/cm fluid on my refractometer and it read 25 ppt not the 35 ppt I was expecting (which does indeed confirm their statement that you can't use that fluid on refractometers)." This was Neptune Systems Response:

    Refractometers use a brine calibration solution typically calibrate them. Some use RO water to calibrate the refractometer. I would check with the manufacture of your refractometer. Also below is information about our probe and other measuring devices and how they will not read exactly.

    The Apex probe measures the conductivity of the water, the refractometer measures the change in the speed of light when it goes from air to water, a hydrometer measures how dense the water is. These are 3 different things you are measuring, none of them is a count of the particles of stuff in a million particles of water. All 3 will give different readings if you dissolve different things, 35.0 ppt of sodium chloride will read different than sea water with 35.0 ppt of stuff in a refractometer. Our cond probe will not read number to number identically with milwakee's calibration or any other third party tools for checking this measurement. It can read +/- 2ppt different then the other measurement devices. If you calibrated the Cond probe properly when placed In the 53mS solution it should read around 34.8~35.2. If it did read that after you calibrated it than you successfully calibrated the probe. That's what's important. That it read 34.8~35.2 in the calibration solution from Neptune systems.Can you put the probe back in the 53mS solution and what does it read? If it does not read within that range then calibrate the Cond probe again. If it did, than you're done.

    After that, if you drop it in to a tank or any environment it will read what the conductivity is of that tank. It's not going to be the same as your refractometer, hydrometer, or anything. Our probe reads the conductivity. It's not the same.

    To clarify, the conductivity probe functions by measuring the displacement of NaCl ions in the water. The Conductivity probe is not a refractometer. It's not going to read like your refractometer. This is common. It's a conductive sensing probe. Your refractometer is designed to detect specific gravity/density of all the salts in your water like KCl, MgCl, not just NaCl, but the conductivity probe will give you a closer look at the the true salinity (ppt) reading in your aquarium.

    Typically most refractometers are not calibrated for Seawater, but a brine solution, thus they are reading the SG/salinity of many other salts in your water other than NaCl. So they will read slightly differently than a conductivity probe which only measures your actual NaCl concentration.
    Here is ant article on Reef Builders illustrating this discrepancy (http://reefbuilders.com/2014/05/14/r...rium-salinity/
    ). So it is not uncommon to see this difference between most refratometers and salinity probes.

    If you follow the link above you will see that not all Refractometers are considered equal. So I bought the Red Sea Refractometer (referenced in the link) and after calibrating it to RODI water (as their instructions guided) I measured my water. This is what I got:

    - Aquarium = 31-32 ppt
    - Salt Water Reservoir (for auto water changes) = 30 ppt

    Now compare this to the graphs from my Apex:

    2018-09-05 - Temp-pH-Salinity Graphs.jpg

    Note: The salinity probe changes with changes in temperature. So, I know why the salinity probe changes.

    So, here we are at the question of this post.....which device do I believe when deciding if I need to add salt or not?

    1) The Apex probe says don't add more salt, but
    2) The Red Sea Refractometer says I need to add more salt.

    I understand why they could be different but which one is right or should be trusted first?

    By the way, my old Refractometer that is not calibrated to Seawater or aquarium temperature as my new Red Sea Refractometer is, measures the aquarium and salt water reservoir at 35 ppt.







  2. #2
    Regular Vistor JJreef's Avatar
    Join Date
    Aug 2018
    Location
    London
    Posts
    21
    Following as I’ve been having similar issues.


    Sent from my iPhone using Tapatalk

  3. #3
    Frequent Contributor zombie's Avatar
    Join Date
    Dec 2013
    Location
    Denver, CO
    Posts
    13,176
    I would not trust the refractometer to be correct until you calibrate with refractometer calibration fluid or make some yourself with a 3.65 weight percent solution of table salt in RODI water.

    Sent from my SM-G965U using Tapatalk

  4. #4
    Regular Vistor JJreef's Avatar
    Join Date
    Aug 2018
    Location
    London
    Posts
    21
    So you can’t use refractometer 35pt solution to calibrate the Apex either? Only the Apex own solution?


    Sent from my iPhone using Tapatalk

  5. #5
    Frequent Contributor zombie's Avatar
    Join Date
    Dec 2013
    Location
    Denver, CO
    Posts
    13,176
    Quote Originally Posted by JJreef View Post
    So you can’t use refractometer 35pt solution to calibrate the Apex either? Only the Apex own solution?


    Sent from my iPhone using Tapatalk
    Sometimes you can, but very rarely. Pinpoint makes a solution similar to purified sea water that has both a 35ppt refractive index and also a 53mS/cm conductivity, but most solutions use brine solution which do not match. A 53mS/cm solution made from table salt (brine) will read 35ppt on a cond probe but will read 31.6ppt on a refractometer. A 53mS/cm seawater solution will read 35ppt on both.

    Sent from my SM-G965U using Tapatalk

  6. #6
    Just a Nerd in the Herd alb's Avatar
    Join Date
    Jan 2013
    Location
    San Jose, California
    Posts
    426
    This is the perennial story of someone with one watch always knowing the correct time, and someone with two watches never being quite certain.

    Given a choice, I'll go with the probe, because I am by nature quite lazy, but obviously, I'm biased.

    However, to me, it's actually irrelevant which method you choose to perform the measurement, so long as you do so consistently, and you use the measurement obtained to provide consistency. It's much less important that the level is a specific value; it's much more important that the level is consistent.

  7. #7
    New User
    Join Date
    Jul 2018
    Location
    UK
    Posts
    12
    Ok, so here's an update. I sent the graphs in my original post to Neptune Systems via the support ticket I have open. This is what they said:

    Thank you for your inquiry. No need to add salt.

    Looking at the graph overlay the temp is causing swings with the salinity. This is normal as tempature can have a direct effect to salinity.You should turn on Temp Compensation to (2.2) for the salinity probe and then place the temp probe in the package of 53,000 calibration solution with the salinity probe and re-calibrate the probe. The affects of temperature on a salinity reading is that at higher temperatures the migration of the ions between the negative and positive pole in the probe is going to happen more quickly. This doesn't mean you have a higher conductivity reading at a higher temperature reading but without temperature compensation the probe will respond in that way. The compensation basically adjusts for the rate of ion migration at a given temperature. In my experience, the best % compensation to have is about 1.8-2.2%, but really it depends on your salt and environment.

    So if you are using temperature compensation it is extremely important to make sure that the temperature probe and the conductivity calibration solution are at exactly the same temperature. To do this make sure that the calibration solution remains in the tank during the settling period. AND more important than anything, make sure that you have given the probe a couple shakes in the calibration solution and a couple swirls to remove any air-bubbles.

    Both measurements are correct (Probe and refractometer) as we gave you an explanation that our probe can read +/- 2-3ppt difference between a refractometer.
    Note: I returned my Red Sea Refractometer as the one I had would not hold its calibration after each use, I found the calibration to be off. (hopefully you do not have the same experience that I did.

    I would calibrate the refractometer, test my tank and then test the calibration with RO right afterwards and the calibration would be off.

    Now, I retested the calibration of my Red Sea Refractometer and it was uncalibrated!!!!!!!! So, I did as he suggested:

    1) I set the Temp Compensation for the Salinity probe (Advanced Tab) to 2.2.
    2) I put the temp and salinity probes in the same (new) packet of Neptune Systems 53,000 uS/cm calibration solution and recalibrated the salinity probe (using manual calibration inside the advanced tab of the salinity probe). Note: I first dried the probes using a can of compressed air as the salinity probe can have some fluid left inside the jacket. I noted the reading I got after calibration with the probes still in the calibration solution.
    3) I put the probes back in the aquarium and noted the reading.

    4) I put some RODI water in a clean jug and tested the temp (22C). So I heated it in the microwave for 7 secs to get 25.1C
    5) I recalibrated the Red Sea Refractometer to the 25C RODI water.
    6) I tested both my aquarium and salt water reservoirs with the Red Sea Refractometer.
    7) I re-tested the Red Sea Refractometer with 25C RODI water and confirmed it was still in calibration.

    And the results:
    - Salinity and Temp Probes in 53,000 uS/cm solution = 34.9 ppt (GOOD)
    -
    Salinity and Temp Probes in aquarium = 33.9 ppt (GOOD)
    - Red Sea Refractometer of Aquarium = 33 ppt (GOOD)
    - Red Sea Refractometer of Salt Water Reservoirs = 33 ppt (GOOD)
    - Calibration of the Red Sea Refractometer confirmed after tests (GOOD)

    So, I'm now getting what I consider good consistency between the probes and the Red Sea Refractometer. However, there have been several lessons learned:

    1) Don't think of calibration as a job you do once and then never do again!!!
    2) Probes and other devices (even one refractometer to another) may measure salinity in the same units (ppt) but they are probably actually doing different things.
    3) Make sure the Temp Compensation for your salinity probe is set
    4) Make sure when you calibrate the salinity probe you put the temperature probe in the solution as well.
    5) I think it is advisable to use Neptune Systems Calibration solution when calibrating a Neptune Systems probe. Other methods may work but may add that extra level of uncertainty to a system with several moving parts anyway.

    Enough for today....I've eared a beer I think!!!

Similar Threads

  1. Salinity Probe Question
    By nyc reefer in forum AquaBus Modules, Probes, and Breakout Boxes
    Replies: 1
    Last Post: 04-11-2019, 12:10
  2. Salinity probe question
    By curryb15 in forum A2 Apex/ApexEL and A3 Apex Pro/Apex/Apex Jr
    Replies: 3
    Last Post: 08-13-2017, 19:15
  3. Salinity probe question
    By Edolan in forum AquaBus Modules, Probes, and Breakout Boxes
    Replies: 5
    Last Post: 06-10-2017, 08:51
  4. Salinity Probe question
    By bcpitts in forum Apex Classic/Apex Lite/Apex Jr
    Replies: 4
    Last Post: 05-08-2017, 10:34
  5. Salinity Probe Question
    By yetman in forum AquaBus Modules, Probes, and Breakout Boxes
    Replies: 2
    Last Post: 07-08-2015, 16:42

Tags for this Thread

Bookmarks

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •