Apex Salinity probe versus refractometer?

Discussion in 'Equipment' started by rygh, Dec 16, 2015.

  1. rygh

    rygh Supporting Member

    The history is this:
    I updated my old refractomer due to rust.
    I bought a Apex Salinity probe because I wanted real time logging for auto water change monitoring.
    So I have 3 pieces of test equipment.

    I calibrated my Apex salinity monitor with provided 53ms solution.
    (Including floating solution in tank)
    I calibrated my old marine depot refractometer with distilled water.
    I calibrated my new D-D refractometer with "Aqua Craft" 35ppt solution.
    Then I tested them all with my tank water.

    The Apex reports my salinity very high.
    Refractomers both say 1.026. Old one is hard to read, but new one within .0005.
    Apex is reporting 38.7 = 1.029. Ouch!!
    Apex ranges from 38.3 to 38.9 : 1.0289 to 1.0293, which is great, so it is not drifting.

    My questions:

    1) Which should I trust more? Refractometer or Electronic?
    2) Any recommended certified test solutions?
    3) I do not have a temp probe on the salinity Apex module. Should I?
  2. jonmos75

    jonmos75 Supporting Member

    1) If the Apex salinity probe is calibrated correctly it is to be more accurate then a refractometer ...and I see people get into a big discussion how refractometer are not that accurate due to temperature readings and there are different types of refractometers ...you really cant compare the two it is like an orange and a tangerine (they look similar but they are different)

    2) I will be setting up my Salinity probe after Christmas but I have the 53ms solution

    3) Yes you should so you can use the temp compensation on the PM2 to make the results more accurate....(side note) if you move you temp probe to the PM2 or any other module except the base unit you will see that the temp is different then what you saw on the base unit....why I don't know but it is...so just calibrate the temp probe once you have connected it to the PM2 using a NIST certified Thermometer...also if you move the temp probe make sure you change whatever programming you were using in outlets to the new probe location.
  3. Enderturtle

    Enderturtle Volunteer

    Are you using an Apex controller with your probe?

    I recall you using some sort of DIY controller?
  4. rygh

    rygh Supporting Member

    Yes, I have an Apex now.
    Apex does the monitoring, but does little control. Just skimmer right now.

    I like the programming ability of my DIY better, but it has little monitoring ability.
    And having a separate redundant monitor is a big win.
    Plus I really like the Apex Fusion web interface.

    Except: Fusion did not reconnect automatically after a power failure. GRR.
    HiFidelity likes this.
  5. rygh

    rygh Supporting Member

    Just thinking more:

    I calibrated the Apex in a cold garage. While I soaked the fluid in sump, it may
    have cooled quickly.

    I have a grounding point in my sump, pretty close to the probe.

    Time for some new testing and calibration.
  6. aqua-nut

    aqua-nut Supporting Member

    Mark, I'd trust the refractometers. Simpler tech, less to go wrong.

    Salinity probes are, from what I've read, temperamental. I wouldn't trust it to do automated w/c or salinity adjustments.
    Enderturtle likes this.
  7. rygh

    rygh Supporting Member

    I definitely won't do any automation based on that value. The purpose is to send me a text/email saying something is wrong.
  8. Enderturtle

    Enderturtle Volunteer

    At the apex demo, one reefer said he programmed his salinity probe to pump freshwater into his tank for auto top off. Instead of using float switches.

    You could hear the whole room groaning.
  9. wpeterson

    wpeterson Webmaster

    If I had unlimited money, I would consider a salinity probe. However 99% of my salinity measurements target things outside my tank to equalize with the salinity in my tank: water changes, acclimating corals/inverts, etc.

    Unless you do something wrong, your salinity in your tank should be constant. Having an alarm to tell you if that's ever not true might be useful, but not useful enough to pay several hundred dollars for.
    jonmos75 likes this.
  10. jonmos75

    jonmos75 Supporting Member

    I agree with @wpeterson and the only reason I just finally went this route is I will at some point in the future have a salt mixing station in the garage that will add new saltwater daily and remove water from the main tank and I will have it program this to only happen do this if the salinity in the mixing tank is a very close match to the display tank...
    Enderturtle likes this.
  11. Corallus

    Corallus BAR Sponsorship Coordinator

    These may not be the issues but - two things to consider. The Apex probe is measuring electrical conductivity, and the refractometer is measuring the amount of dissolved solids. In clean, pure saltwater, those methods should give basically the same values.

    1 - If there's a decent amount of dissolved organics present (which often don't contribute to conductivity), those values could be a little different. But I would imagine in the opposite direction of what you're seeing.

    2 - I would think any stray voltage source in your tank could affect the conductivity measurement (this is a guess - experienced folks chime in if I'm wrong please!).

    I would try testing newly made salt water in a bucket with both methods and see how close they are, then test the tank. Variation in the bucket - would look to the measurement devices. Consistent in the bucket, variation in the tank - look to what could be causing it in the tank.
  12. rygh

    rygh Supporting Member

    Testing new salt water in a bucket is a good idea, I will try that.

    Stray voltage source/sink should not be an issue since Apex goes on about having great Galvanic isolation.
    Hopefully marketing matches reality.

    New reference fluid is on order as well.
  13. sfsuphysics

    sfsuphysics Supporting Member

    Never calibrate with distilled water, while it COULD work out for you, the unknown factor of it just isn't worth it. You need to calibrate closer to the solution you're trying to measure.

    That said galvanic isolation? So the Apex can isolate ions flowing around in chaotic fashion in a fluid media that the probe is submerged in? Wow that's impressive ;)
  14. jonmos75

    jonmos75 Supporting Member

    On the Neptune Forums there are posts of stray voltage from heaters, pumps and cheep eBay float switches that can cause irregular results in the conductivity probe.... also watch out for micro bubbles that is the salinity probes other kryptonite....lol
  15. rygh

    rygh Supporting Member

    But I tried both as per first post. No significant difference, so in my case, either would have worked.

    Yeah. ;)
    I am sure they really just mean making sure they isolate the conductivity circuit ground from earth ground.
    That will work in most cases. But it depends a lot on that circuit and how much voltage really floats.
    The short term chaotic currents should mostly cancel out with simple averaging.

    Ideally you also measure current both in and out of conductivity probe, and compensate on mismatch.
    I doubt almost anyone does that.
  16. rygh

    rygh Supporting Member

    Interesting. If those reports are accurate, that really implies the isolation is not as good as it should be.

    I wonder why bubbles matter so much. Perhaps surface are of the probe contacts is small?
  17. jonmos75

    jonmos75 Supporting Member

    Not sure the true reason why micro bubbles are such an issue, yet I have heard people installing at a 45 deg so the bubble can escape easier....if you look at the probe you will see two pin holes in the plastic housing towards the bottom of the probe, those holes are there to help vent out air bubble so make sure that those are facing upward if you mount the probe at an angle...the other thing to help with the bubble issue is to have it in a location where water moves fairly fast in the sump.....DO NOT put the probe near the skimmer output....lol
  18. HiFidelity

    HiFidelity Guest

    I wish I had a salinity probe for my RA so I could do the same test & see how that relates to your results.

    Anyone running a Reef Angel w/ salinity probe? I think it'd be great if we could see results from a different brand controller as well.

    Personally I would trust the refractometer over the probe, granted I have no experience with salinity probes the fact is there are more variables on the probe's end than there are with a refractometer.
  19. aqua-nut

    aqua-nut Supporting Member

    In ReefThreads #245, Craig Bingman discusses various methods to test for salinity. Although the whole podcast is interesting, the discussion about testing starts at about 24:15.
  20. Merith

    Merith Guest

    If I remember right BRS had a decent video on calibrating the salinity probe with some helpful tips. One thing I remember reading just can't remember where was that it is best to get their solution and calibrate it a few times as I guess it helps with the the range being off or something can't quite remember but it was confirmed by several places enough that I ordered several packs of the calibration stuff I know not the most helpful but hopefully some of it helps.

    Sent from my iPhone using Tapatalk
    Enderturtle likes this.

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