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Thread: Salinity probe readings

  1. #1
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    Salinity probe readings

    I've been trying to get my Salinity/Conductivity probe to somewhat match or close to my refractometer. But I have no luck. My refractometer is 1.026 and Apex probe is basically 1.023 (30.9). I've been using BRS ATC refractometer and calibrated it with 1.026 solution.

    I've been reading all the thread on here regarding the PM2. I have my temperature probe on PM2. I set up the Temperature comp. to 2.2. Then I calibrated with apex solution 53,000. And yes, I have acclimate the solution by floating it in my sump.

    Any help would be greatly appreciated.

  2. #2
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    Salinity probe readings

    Following.

    I have had exactly the same results. After calibration the probe read 34.9 ppt in calibration solution. Figured accurate enough and I put back in tank and reads 29.4 ppt but BRS refracto reads 35ppt or 1.026 which is where I like my salt. Not sure what to believe anymore. Everything I read is saying conductivity probe is more accurate than refracto.


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  3. #3
    Just a Nerd in the Herd alb's Avatar
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    You've definitely done your homework here.

    However, one thing to note is that salinity is something that is only indirectly measured, and as such the variance is typically somewhat large; for example, I have a reasonably expensive electronic refractometer that only provides +/- 0.002 s.g., so it's not out of the question that both values are correct within the degree of precision they provide.

    The following Randy Holmes-Farley article covers this in his usual exquisite level of detail; it's a good read to really understand the issues involved:

    http://reefkeeping.com/issues/2006-12/rhf/index.php

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    Quote Originally Posted by Mpfaff77 View Post
    Following.

    I have had exactly the same results. After calibration the probe read 34.9 ppt in calibration solution. Figured accurate enough and I put back in tank and reads 29.4 ppt but BRS refracto reads 35ppt or 1.026 which is where I like my salt. Not sure what to believe anymore. Everything I read is saying conductivity probe is more accurate than refracto.


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    Yup, exactly what I did. Put it back, it's giving me 34.8. I even left it over night to see the reading, it went all the way down to 26. Then came back up to 30.9. I am getting very frustrated. Can't get that figure out.

    Actually i've been reading several thread on forums, and they say Refractometer is more accurate. This is 1 of my thread on RC:
    http://reefcentral.com/forums/showth...8#post21271688

    Quote Originally Posted by alb View Post
    You've definitely done your homework here.

    However, one thing to note is that salinity is something that is only indirectly measured, and as such the variance is typically somewhat large; for example, I have a reasonably expensive electronic refractometer that only provides +/- 0.002 s.g., so it's not out of the question that both values are correct within the degree of precision they provide.

    The following Randy Holmes-Farley article covers this in his usual exquisite level of detail; it's a good read to really understand the issues involved:

    http://reefkeeping.com/issues/2006-12/rhf/index.php
    Thanks Alb. I believe I read that article several times. So Are you saying either 1.023 and 1.026 both correct? but don't you think that pretty far off each other?

  5. #5
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    Salinity probe readings

    I mean from 30ppt to 35ppt isn't much of difference as far as SG goes. 1.022 to 1.026 isn't much. But if I raise salinity upto 34ppt or so as it reads now and my refracto is right then ill be too high at 1.029. Not sure what to do. Looks like we have similar issues.

    I also calibrated with 2.2 and 0.0 compensation. I had temp probe connected to PM2 both times. Same result only slightly higher numbers with 2.2 set

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  6. #6
    Just a Nerd in the Herd alb's Avatar
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    Quote Originally Posted by Green_Star View Post
    Thanks Alb. I believe I read that article several times. So Are you saying either 1.023 and 1.026 both correct? but don't you think that pretty far off each other?
    Let's say that I have two of my nifty electronic refractometers. These are spec'ed by the manufacturer to have a precision +/- 0.002 SG. For argument's sake, let's say that I use both to measure a sample with an actual value of 1.024.

    One unit reads 1.023. The manufacturer stated precision is +/- 0.002, so I must remember that the actual value could be as low as 1.021 or as high as 1.025, so it's measuring the 1.024 sample accurately, within the level of precision it can provide.

    Similarly, the other, identical unit reads 1.026. Again, I need to keep in mind that the precision means that this could be as low as 1.024 or as high as 1.028. Thus, again, it's accurate for our hypothetical 1.024 sample, within the stated precision.

    Both units are correct, within the manufacturer-stated precision. Whether or not I think that the values are far off of each other is, sadly, ignored by the devices; they can only give me results within the degree of precision that they're able to provide.

    This will, in fact, cause you to become grumpy if you let it; I think the best approach given the vagueness here is to focus on consistency rather than comparison. That is, I'm personally less concerned as to exactly how accurate my salinity measurement is, as long as it's reasonable; I'm much, much more concerned that it's consistent, i.e., I want to know right away if my ATO has just lost its mind.

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    Quote Originally Posted by alb View Post
    Let's say that I have two of my nifty electronic refractometers. These are spec'ed by the manufacturer to have a precision +/- 0.002 SG. For argument's sake, let's say that I use both to measure a sample with an actual value of 1.024.

    One unit reads 1.023. The manufacturer stated precision is +/- 0.002, so I must remember that the actual value could be as low as 1.021 or as high as 1.025, so it's measuring the 1.024 sample accurately, within the level of precision it can provide.

    Similarly, the other, identical unit reads 1.026. Again, I need to keep in mind that the precision means that this could be as low as 1.024 or as high as 1.028. Thus, again, it's accurate for our hypothetical 1.024 sample, within the stated precision.

    Both units are correct, within the manufacturer-stated precision. Whether or not I think that the values are far off of each other is, sadly, ignored by the devices; they can only give me results within the degree of precision that they're able to provide.

    This will, in fact, cause you to become grumpy if you let it; I think the best approach given the vagueness here is to focus on consistency rather than comparison. That is, I'm personally less concerned as to exactly how accurate my salinity measurement is, as long as it's reasonable; I'm much, much more concerned that it's consistent, i.e., I want to know right away if my ATO has just lost its mind.

    Thank you again Alb. Your explanation is clear. However, that's if you know the "Actual" value. In our case, we are trying to find out the "actual" value. Let's say both Refractometer and Apex has +/- 0.002. IF my "actual" value is 1.024, then yes, both 1.023 and 1.026 are correct within manufacturer-stated precision. How do I know what's my actual value? That could be 1.021, which is correct for 1 device, but it would be way off for the other. That could be also 1.028, which is correct for the other device, but way off for the other. Who know what's the X value is? or the actual value is. That's what we're trying to find out with these devices.

    I totally agreed with consistency.

  8. #8
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    Salinity probe readings

    I totally agree with consistency

    My next thing I will do in new few weeks is send away to aquarium water testing dot com and that will tell me by a lab what my levels are and how accurate my testing is.

    For now I'm just keeping salinity around 30ppt. Another issue I have is I use probe for tank and refractro to measure new salt water for water change. Tank seems happy and have been using refracto for 8 years with no issues in salinity. Just got conductivity probe. Now it bothers me. Go figure.


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  9. #9
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    I didn't have time to read the thread but one thing that effects my probe are my lights. If you haven't already done so check for Electrical interference. My probe goes to 34.9-35.0 as soon as my lights go off.

  10. #10
    Just a Nerd in the Herd alb's Avatar
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    Quote Originally Posted by Mpfaff77 View Post
    I totally agree with consistency

    My next thing I will do in new few weeks is send away to aquarium water testing dot com and that will tell me by a lab what my levels are and how accurate my testing is.
    Please post back when you get the results; I'm sure at least a few of us will find them interesting. I'd presume the lab to use something like an Autosal salinometer, and it'd be interesting to see what the high-end equipment comes back with.

    This discussion reminds me of one I had with a friend many years ago. He'd spent quite some time in the National Reconnaissance Office, i.e., the nice people who bring you the Keyhole series of spy satellites. At the time, the man on the street believed that the satellites could read a copy of Pravda over the shoulder of someone sitting on a park bench in Moscow, so I asked him about that.

    He said, well, first off, that's not a discussion we can have. Secondly, there are easier ways to get a copy of Pravda. Thirdly, and most importantly, the value is not in being able to read the serial number of a tank; rather, it's in knowing how many tanks they have.

    So allow me to paraphrase that. With the equipment we have available to us in the hobby at reasonable cost, there's an associated precision, and so knowing the exact value isn't a discussion we can have unless there's a lab with high-end equipment involved, but what we do have available to us is certainly good enough for our needs, and, in the end, knowing that you've just a had a swing can often be the most important thing.

    But please do post the lab results, since this is certainly an area of interest for many here.

  11. #11
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    Salinity probe readings

    Yea I will absolutely. Hopefully will purchase the test this weekend. I actually have a email out to the website to make sure I can add conductivity to the list of parameters they test. If you curious here is the link. 45 bucks for lab testing of parameters. I think worth it just to confirm how accurate my Red Sea pro test kits are.

    http://www.aquariumwatertesting.com/


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    Salinity probe readings

    I just gave up and changed the compensation value on the probe web page until it matched what my calibrated refractometer was telling me. All I really want the salinity probe to do is tell me if the salinity changes. It does that really well.


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  13. #13
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    Salinity probe readings

    I tried doing this as well and no value brings it high enough to the value I need.


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    Frequent Contributor jrlamountain's Avatar
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    i just got the probe and i am confused what is the ideal number i should be looking for?

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    Salinity probe readings

    Read this it suggests the compensation values relate to mag in the water too with most reef aquariums running 1300-1400 mag levels it suggests the compensation is closer to 0.6 not the 2.0 to 2.3 recommended in the apex manual?

    For some reason I can't copy the page link but go to google and type in

    34 ppt salinity

    And it's the top link


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  16. #16
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    Salinity probe readings

    I have been having trouble with mine after finally calibrating with apex calibration fluid tonight all seemed well probe read 34ppt spot on but as I have sat here over the last 3 hours I have slowly watched it drop to 32 ppt I don't understand what's going on with this either although with I really think its to do with the temp compensation ?


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    Here are my results posted in this thread
    http://forum.neptunesystems.com/show...4741#post14741

    What is the normal range that the salinity probe should be reading after it's calibrated I've calibrated mine and I'm in the high 40s and the parts per thousand read 53.4 with the 53 solution by Neptune.


    Set to Salinity
    Output Reading 48.5 (on the probe calibration raw data its around 699)
    (was zero at the start / 53.4 with the neptune fluid)

    Temp Compensation is set at 2.2

    My refract is reading at 1.023

    ------

    Later I found

    Looks like it was placement. Moved it to 8 different spots in the sump and
    tank.



    Now reading 32.3 ppt next to the return pump



    Looks like it needs flow but not too much.



    It is very accurate as it found pockets where any higher salinity water had
    settled.



    ( made the mistake of adding a high spec gravity mix to the tank seems to
    have stuck in a pocket of the sump also found a spot in the main tank where it
    had settled in a dead spot.)



    Was trying to bring my saline levels up, with a salt mix, that was 2x
    concentrated. Will not make that mistake again!



    also was sent this for reference



    1.023 specific gravity roughly equates to 48 salinity....see here:



    http://www.americanmarineusa.com/salinityconversion.html






    -------------------

    Salinity
    mS

    ‰ Salt
    (ppt)
    True

    Specific
    Gravity






    42.5 27.30 1.0200
    43.0 27.70 1.0203
    43.5 28.10 1.0206
    44.0 28.40 1.0209
    44.5 28.80 1.0211
    45.0 29.20 1.0214
    45.5 29.50 1.0217
    46.0 29.90 1.0220
    46.5 30.20 1.0222
    47.0 30.60 1.0225
    47.5 31.00 1.0228
    48.0 31.30 1.0231
    48.5 31.70 1.0234
    49.0 32.00 1.0236
    49.5 32.40 1.0239
    50.0 32.80 1.0242
    50.5 33.10 1.0245
    51.0 33.50 1.0248
    51.5 33.90 1.0250
    52.0 34.30 1.0253
    52.5 34.60 1.0256
    53.0 35.00 1.0259
    53.5 35.40 1.0262
    54.0 35.70 1.0265
    54.5 36.10 1.0268
    55.0 36.50 1.0270

  18. #18
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    My refract is reading 1.024 and my apex is saying 27.6. If the Apex is correct then my salinity could be reading 1.0204 according to this table above???

    - - - Updated - - -

    scratch that.. refract is reading 1.022

    - - - Updated - - -

    and yes i need to raise my salinity. Just calibrated my refract and so it seems to be reading more true now.. but still if the apex is reading 27.6 then that equals 1.022 salinity?

  19. #19
    Master Control Freak RussM's Avatar
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    Colorado, please do not post about the same issue in multiple places. This is called crossposting which is generally frowned upon. I replied in your other thread.
    Please do not send me PMs with technical questions or requesting assistance - use the forums for Apex help. PM me ONLY if the matter is of a private or personal nature. Thanks.

  20. #20
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    now reading 31.9 and i kept my solution

  21. #21
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    I have had similar issues, but tank readings are high.
    I did some extra testing.

    RF1 = Marine Depot Refractometer
    RF2 = D-D Refractometer
    APX = Apex Salinity Monitor

    Pinpoint 35 reference solution:
    RF1 = RF2 = APX = 35 ppt. All match

    Aqua Craft 35 reference solution:
    RF1 = RF2 = APX = 35 ppt. All match.

    Freshly made salt water, carefully mixed:
    RF1 = RF2 = APX = 35 ppt. All match.

    DISPLAY TANK WATER:
    RF1 = RF2 = 35 ppt
    APX = 39 ppt. WAY OFF!!

    So everything matches great except real tank water.

    I matched temperature pretty well, so unlikely to be that.
    I put tank water in a bucket, with same reading, so not
    stray voltage.

    Something in the real tank water is affecting conductivity.

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    I'm so glad I read this because I thought I could solely rely on the probe. But looks like I'll have to order a refractometer

  23. #23
    Frequent Visitor cadmanof50s's Avatar
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    I'm not sure if this will help, but I would like to share my experience with the Conductivity Probe:
    I purchased a Neptune Conductivity Probe as a spare about two years ago. Its package remained un-opened until a few weeks ago when I noticed that my old probe was giving improper readings..and needed to be replaced.

    I installed the new probe and calibrated it using the exact procedure described in the APEX manuals.

    When I placed the newly calibrated probe in my tank, I would get a reading that was WAY off. I have 3 refractometers...(a digital one and two analog)..all three agreed that my tank salinity was 35, but my probe reading was really low ..like 28!

    I contacted Neptune Support and they suggested stray electrical currents...(possibly)....but really I had no sure-fire way to tell...and all my other probes were working fine.

    After many re-calibrations (with new fluid packets and the same results) I decided to take a different approach. I am fortunate enough to have a couple of PM2 modules and several temperature probes. So I made the temp probe (which was connected to the same PM2 module as my Conductivity probe) a sacrificial temp probe and changed the compensation factor so that the Conductivity probe matched my refractormeters. I needed to change the compensation by almost 10 degrees (F)! Wow...that is really off.
    I have been keeping tracking of the probes output and refractometers measurements at the same time...and they seem to be consistent.....however, the margin of error is still fairly large...and it would be difficult to get values accurate to the first decimal place.

    So I use the salinity measurements from my probe as a guideline and rely on the refractomeetrs as the absolute truth about the salinity readings.

    Why is this happening?....I am at my wits end....I really don't know. However, for what its worth, I too have very high Magnesium and Calcium content in my water...(i.e. 500 ppm Cal and 1570+ Mag).

    Vic

    Vic Brincat - 125G Mixed Reef

  24. #24
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    Quote Originally Posted by cadmanof50s View Post
    I'm not sure if this will help, but I would like to share my experience with the Conductivity Probe:
    I purchased a Neptune Conductivity Probe as a spare about two years ago. Its package remained un-opened until a few weeks ago when I noticed that my old probe was giving improper readings..and needed to be replaced.

    I installed the new probe and calibrated it using the exact procedure described in the APEX manuals.

    When I placed the newly calibrated probe in my tank, I would get a reading that was WAY off. I have 3 refractometers...(a digital one and two analog)..all three agreed that my tank salinity was 35, but my probe reading was really low ..like 28!

    I contacted Neptune Support and they suggested stray electrical currents...(possibly)....but really I had no sure-fire way to tell...and all my other probes were working fine.

    After many re-calibrations (with new fluid packets and the same results) I decided to take a different approach. I am fortunate enough to have a couple of PM2 modules and several temperature probes. So I made the temp probe (which was connected to the same PM2 module as my Conductivity probe) a sacrificial temp probe and changed the compensation factor so that the Conductivity probe matched my refractormeters. I needed to change the compensation by almost 10 degrees (F)! Wow...that is really off.
    I have been keeping tracking of the probes output and refractometers measurements at the same time...and they seem to be consistent.....however, the margin of error is still fairly large...and it would be difficult to get values accurate to the first decimal place.

    So I use the salinity measurements from my probe as a guideline and rely on the refractomeetrs as the absolute truth about the salinity readings.

    Why is this happening?....I am at my wits end....I really don't know. However, for what its worth, I too have very high Magnesium and Calcium content in my water...(i.e. 500 ppm Cal and 1570+ Mag).

    Vic
    Interesting, this whole thread was an interesting read. Guess if I'm seeing highish cond readings I should just watch for changes that are out of the ordinary and not worry too much about a high reading.

  25. #25
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    Maybe I got lucky. After initial calibration my apex is reporting a matching reading to my refractometer. However I am in the early stages of recovering from a low salinity situation (19ppt) I wonder how close it will be at 35?


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