Specific Gravity vs PPT

Discussion in 'Reef Chemistry by Randy Holmes-Farley' started by pdiehm, Oct 12, 2017.

  1. pdiehm

    pdiehm Well-Known Member Build Thread Contributor

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    Let me say I've used both of these, and it wasn't until this week that I noticed some discrepancy.

    Over the past week or so, I've been wondering why my parameters have been elevated. I couldn't figure it out. Alkalinity was at around 9-ish, Calcium just below 500, Magnesium at 1450/1500..The salt mix of choice has been Fritz RPM. This won't become a salt mix debate.

    Since I've rebooted my tank, I've been making water at 35 PPT, which is widely known as NSW (1.0264) or whatever. Cool right?

    Fritz RPM at 35 PPT mixes up at roughly 9.0/475/1500. They say it should be 8.5/430-450/1300-1400.

    Now, I just made 5 gallons of new water, mixed to 1.025, which equates to 33 PPT. The tests were 8.6/440/1400. Basically identical to what the manufacturer has said.

    I have used Red Sea (Blue Bucket) previously and at 35PPT, the Alk is 9.0/500/1500. At 1.026, the results are 8.4/430/1400 (if I recall), but the PPT for this test was 34.

    Regular Instant Ocean: at 35 PPT, Alk was 11.2/465/1480. At 1.026 (34 PPT), 10.0/440/1400, or there abouts.

    Equipment used:

    Milwaukee Digital Refractometer: Rinsed with RODI, Calibrated with 1.025 solution, rinsed with RODI, and tested newly mixed water.

    Hanna Alkalinity dKh checker

    Red Sea Professional Calcium/Magnesium test


    With that being said, I am wondering if I should start mixing to the Specific Gravity instead of the PPT scale? I'm very confused as to why 35PPT is giving higher numbers, even though it says 1.026, but if I test to 1.026, and change the scale to PPT, it shows 34 (PSU 33 I think).
     
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  2. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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  3. nautical_nathaniel

    nautical_nathaniel Goby Whisperer R2R Supporter R2R Excellence Award Build Thread Contributor

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    Now why would they do that?? ;)

    Seriously though, I've always thought that it was blatant false advertising on their part since almost everyone is going to buy a reef-spec salt for reef keeping application at a higher salinity than something like a FOWLR tank at 1.020 sg. If I'm paying extra for the reef-spec salt I want to get all of the advertised gallons worth out of it.

    I always read my refractomer in sg since the increments for it are a bit easier to read IMO. 1.025 is also an easy line to find on the scale.
     
  4. pdiehm

    pdiehm Well-Known Member Build Thread Contributor

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    what's confusing to me @Randy Holmes-Farley is if the salt manufacturers are saying their salt tests at X at 35 PPT (ie: Red Sea), why at 35 PPT are the numbers elevated? If I mix to say 34 PPT, or even 33, the numbers are a lot closer, if not spot on to what the manufacturer is saying.

    if I mix to 1.026, and change my scale on the refractometer to PPT, it's not 35. Almost everytime it's 34.

    And I get the difference is miniscule in the grand scheme of things.
     
  5. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    Salt companies often play pretty loose with such information. :D

    The Fritz web site data might be for sg = 1.025 (not 35 ppt), which is 33.1 ppt. :)
     
  6. pdiehm

    pdiehm Well-Known Member Build Thread Contributor

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    And from what I am seeing 1.025 is good enough to handle the SPS tank.

    So much for the you must maintain 1.026 (34-35PPT)
     
  7. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    That's an error of the refractometer conversion from seawater refractive index (which it measures) to seawater sg and seawater ppt.

    A true seawater refractometer should not have such an error. A normal brine refractometer (often sold to hobbyists) will always have such an error, if perfectly made, since sodium chloride solutions have a different relationship between sg and ppt than does seawater.

    That's why calibrating with a 35 ppt standard is important for those types.
     
  8. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    I have not ever claimed reef aquaria must be 35 ppt or 1.0264. Corals and other marine organisms thrive under a wide range of salinity.

    But in general, I would target 35 ppt as a reasonable goal.
     
  9. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    Optimal Parameters for a Coral Reef Aquarium: By Randy Holmes-Farley
    https://www.reef2reef.com/forums/re...-coral-reef-aquarium-randy-holmes-farley.html



    Salinity

    There are a variety of different ways to measure and report salinity, including conductivity probes, refractometers, and hydrometers. They typically report values for specific gravity (which has no units) or salinity (in units of ppt or parts per thousand, roughly corresponding to the number of grams of dry salt in 1 kg of the water), although conductivity (in units of mS/cm, milliSiemens per centimeter) is sometimes used.

    Somewhat surprisingly, aquarists do not always use units that naturally follow from their measurement technique (specific gravity for hydrometers, refractive index for refractometers, and conductivity for conductivity probes) but rather use the units interchangeably.

    For reference, natural ocean water has an average salinity of about 35 ppt, corresponding to a specific gravity of about 1.0264 and a conductivity of 53 mS/cm. It often ranges from 34-36 ppt over reefs, but can be higher or lower locally for various reasons such as land run off of fresh water, or evaporation from a lagoon.

    As far as I know, there is little real evidence that keeping a coral reef aquarium at anything other than natural levels is preferable. It appears to be common practice to keep marine fish, and in many cases reef aquaria, at somewhat lower than natural salinity levels. This practice stems, at least in part, from the belief that fish are less stressed at reduced salinity. I have no idea if that is true or not, but I've not seen evidence that it is true. Substantial misunderstandings have also arisen in the past among aquarists as to how specific gravity really relates to salinity and density, especially considering temperature effects. For example, the density of seawater is less than the specific gravity, and measurements with glass hydrometers may require temperature correction, but newer devices do not need the aquarist to make corrections. Consequently, older salinity or specific gravity recommendations may not actually be referring to the same measurements that aquarists make today, even if the recommended numbers have been handed down

    My recommendation is to maintain salinity at a natural level. If the organisms in the aquarium are from brackish environments with lower salinity, or from the Red Sea with higher salinity, selecting something other than 35 ppt may make good sense. Otherwise, I suggest targeting a target salinity of 35 ppt (specific gravity = 1.0264; conductivity = 53 mS/cm).
     
  10. pdiehm

    pdiehm Well-Known Member Build Thread Contributor

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    So with my Milwaukee digital Refractometer, am I better off mixing to specific gravity or ppt?

    I know it should be one in the same but it hasn't been in my experience.

    I know with Fritz, 2.5c per 5 gallons is 1.025. Lock stone cold.
     
  11. chipmunkofdoom2

    chipmunkofdoom2 Well-Known Member

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    I didn't observe the same thing with Red Sea blue bucket. At 35ppt (measured as 53 mS/cm using a conductivity meter), I get Ca = 425 ppm, Alk = 8.5 dKh and Mg = 1,275 ppm.

    Very technically speaking, by using your Milwaukee Digital Refractometer, you're not measuring specific gravity or salinity in ppt. The meter is measuring the refractive index of the water, and the Milwaukee is converting it using built-in constants to SG or ppt. The only reason that these two would not agree with one another is if the meter is broken, or if the built-in conversion factor is incorrect.

    Stability is the most important in reefkeeping. Personally, I would use whichever measurement scale is most intuitive to you, calibrate your refractometer regularly, and just measure away.
     
  12. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    I can't tell you which is more accurate since they should match, but I'd use PSU on the Milwaukee MA887. :)
     
  13. pdiehm

    pdiehm Well-Known Member Build Thread Contributor

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    I know I'm going to get an article written by Holmes-Farley...but what's PSU, other than...THE Pennsylvania State University :)
     
  14. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    It's just a theoretical version of ppt. Practical Salinity Units. It's what oceanographers use and is based on conductivity.

    http://reefkeeping.com/issues/2004-06/rhf/index.htm

    "In this article, I will use solutions with the same properties as seawater with a salinity of 35 PSU (often written as S=35). PSU is an acronym for practical salinity units, which is essentially a modern replacement for ppt, since salinity is no longer defined as directly relating to solids in the water, but rather by its conductivity. How each standard is made and used is detailed for each of the different methods in subsequent sections."
     
  15. greg 45

    greg 45 Well-Known Member

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    pdiehm the digital refractometer should be calibrated with distilled water not with a calibration solution.
    I own the ma 887 . I wrote the company a letter asking this question.
    They have there own calibration solution to check the unit s.g. of 1.025
    the distlled water should read 00.00 ppm 00 psu 1.00 s.g.
    This might change your results
     
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  16. pdiehm

    pdiehm Well-Known Member Build Thread Contributor

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    I zeroed it out with rodi. I used their 1.025 calibration verification fluid. Cleaned with rodi and tested.

    Is that proper?
     
  17. greg 45

    greg 45 Well-Known Member

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    I don't know the difference between ro/di and distilled
    This was what they recommended .
    So that how I have been doing it.
    Poss Randy can comment on the difference .
     
  18. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Expert Contributor Article Contributor

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    There's none in this context. RO/DI is fine and so is distilled.

    A true seawater refractometer can be calibrated with pure fresh water. Some digital types, may physically require you to calibrate it with fresh water (and soem don't).

    Then, if you want, and did not calibrate with a standard, you can check it with a 35 ppt standard like my DIY (or other standard, if you have confidence they are made properly and made for the type of device you are calibrating).
     
  19. Stigigemla

    Stigigemla Well-Known Member

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    If you make your own calibration standard it is important how you´store it.
    I did dilute muriatic acid to use for alcalinity testing but in a few months practically all the acid vanished through the usual orange juice PET bottle.
    I made a salinity reference after Randys receipt and stored it in a pp can with a flip on lid. When i wanted to check my condictivity meter after almost a year it had dried a bit in the closed can. Almost 36 ppt instead of my expected 34,5 ppt. Just made a new reference and flushed it out after using. The digital meter was perfect.
     
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