PSU values aren't matching PPT values on a Milwaukee 887

DiZASTiX

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Hi Randy,

I saw somewhere you recommended using the PSU scale on a Milwaukee 887; I'd like to begin doing that. However, I noticed that at 21.7 C, I read 33 or 34 ppt (suggesting perhaps a real value of 33.5 to 34.4, for instance), whereas for PSU, I read 31 to 32 PSU.

Procedure for measuring water:
  1. Cleaned sample area with soap and rinsed thoroughly
  2. For each calibration or measurement, I used a light-proof cap to entirely cover the sample area
  3. Calibrated using 0 TDS water: 0 ppt, 0 PSU @ 21.9 C
  4. Verified using 0 TDS water: 0 ppt, 0 PSU @ 21.9 C
  5. Placed DIY 34ppt standard* in sample area and measured the following (*Standard created using Himalayan salt).
    PPT measurements: 34,34,33,33,34 ppt @ 21.9 C
    (suggesting true value just between 33 and 34?)
    PSU measurements: 32,33,33,33,32 @ 21.9 C
  6. Placed reef aquarium water in sample area and measured the following:
    PPT measurements: 34,34,34,34,34 ppt @ 21.7 C
    PSU measurements: 33,33,33,33,33 @ 21.7 C
Obviously, they're equivalent, and in my research, I found that the values are similar, but I thought the values would've been closer. What do you suppose is the matter? (FWIW, I performed this procedure 3 times and obtained similar, albeit non-identical results; however, each time, the PSU had values a bit lower than those for PPT measurements).

¯\_(ツ)_/¯

Thanks, Randy .. you've always been exceptionally helpful, and inspired an interest in chem in a way that my professor couldn't.
 

SPR1968

No, it wasn’t expensive dear....
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I can’t answer the question but I can bump the thread for you to see if we can get an answer
 

Randy Holmes-Farley

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Thanks, Dizastix. :)

There's no reason this "should be", and no reason I can give to explain why you saw it except to note that all the values are within the claimed accuracy of this device:

ACCURACY: ±2 PSU | ±2 ppt | ±0.002 S.G. (20/20) | ±0.3°C / ±0.5°F
 

DeputyDog95

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Odd, I noticed the same thing with my Milwaukee unit.

And agreed, @Randy Holmes-Farley is an amazing resource to have for the hobby. I've learned a ton of information from his various articles.

After finding out my Hanna conductivity meter was reading 2ppt low resulting in my tank being 2ppt high and jumping up over 1.028-9, I went down the rabbit hole to learn more about salinity, make reference solutions, and use a number of different devices to co witness the results.

I'm just learning about PSU and prior to my research was only using PPT and Specific Gravity.

However, it's my understanding that PPT and PSU numbers should be interchangeable. Unless I'm mistaken, 35 PPT should read 35 PSU.

Is that a correction assumption @Randy Holmes-Farley ?

The Milwaukee unit will read in SG, PPU, and PPT. However, if you test a sample and flip through the measurement types using Randy's DIY reference solution... It will read 1.026 SG, 35 PPT, and 34 PSU.

Not sure what to make of that. Unlike the Hanna, the Milwaukee unit does not read out to one decimal point.

I don't know what type of rounding the Milwaukee does... Is it possible that the unit is seeing 34.9 PSU internally, and only displaying 34 due to the lack of displaying decimal points?
 

Randy Holmes-Farley

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Odd, I noticed the same thing with my Milwaukee unit.

And agreed, @Randy Holmes-Farley is an amazing resource to have for the hobby. I've learned a ton of information from his various articles.

After finding out my Hanna conductivity meter was reading 2ppt low resulting in my tank being 2ppt high and jumping up over 1.028-9, I went down the rabbit hole to learn more about salinity, make reference solutions, and use a number of different devices to co witness the results.

I'm just learning about PSU and prior to my research was only using PPT and Specific Gravity.

However, it's my understanding that PPT and PSU numbers should be interchangeable. Unless I'm mistaken, 35 PPT should read 35 PSU.

Is that a correction assumption @Randy Holmes-Farley ?

The Milwaukee unit will read in SG, PPU, and PPT. However, if you test a sample and flip through the measurement types using Randy's DIY reference solution... It will read 1.026 SG, 35 PPT, and 34 PSU.

Not sure what to make of that. Unlike the Hanna, the Milwaukee unit does not read out to one decimal point.

I don't know what type of rounding the Milwaukee does... Is it possible that the unit is seeing 34.9 PSU internally, and only displaying 34 due to the lack of displaying decimal points?

Thanks! :)

While a chemical oceanographer might quibble, for our purposes, ppt and PSU are interchangeable, and the difference is a matter of how the values are defined, not that they are necessarily any different. They may be different when looked at very closely (especially if ions deviate from NSW values), but I do not believe there is any mathematical conversion that shows a specific difference and cannot see how the device gives different values. .

I think the issues must be rounding effects, but I don't see how they get a different value for ppt and PSU since the calculation should be the same.
 
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