Salinity - PSU vs PPT

[Alexander1312]

I am using the milwaukee refractometer below (marketing picture, not my parameter ) for over a year and had recently a Fauna Marin ICP done. The ICP showed the values as per the picture below:

My Milwaukee has been consistently measuring 35 ppt. I have had a couple of Triton ICP who confirmed this. This ICP however states that my PSU is 33.8. When I measure PSU, it is indeed only 34, but I always select the ppt option.

Should I now aim to go up to 35 PSU (= 36 ppt) when my goal is to have a 35 salinity?

I am aware of the accuracy limitations with these devices (+/- 2), but it looks like I am getting the correct PSU value, and PPT, but if targeting PPT, the salinity is considered too low.

 

[Randy Holmes-Farley]

For any reefing purpose ppt and PSU are functionally identical.

 

[Alexander1312]

For any reefing purpose ppt and PSU are functionally identical.

Thank you. So the fact the device reads consistently PSU one unit lower than PPT is pretty much a deficiency with the device as it should show the same value?

 

[Randy Holmes-Farley]

Thank you. So the fact the device reads consistently PSU one unit lower than PPT is pretty much a deficiency with the device as it should show the same value?

It should yes, if both refer to seawater salinity. Does the manual say anything about the ppt unit?

 

[Alexander1312]

The below is from their manual, page 10. Seems like they are using an algorithm to transfer the results into PSU/PPT/S.G.

PSU is defined as the conductivity ratio of seawater to a standard KCl solution. It is based upon the work of the UNESCO, ICES, SCOR and IAPSO. This information is published in The Joint Panel of Oceanographic Tables and Standards.

An older salinity scale is ppt (10-3), where salinity is defined by “the salt content is the weight of the inorganic salts contained in 1 kg of seawater if all bromide and iodide are replaced
by an equivalent amount of oxides” (Knudsen, 1901).

In summary, I should aim for a PSU of 35 (vs 35 PPT). This would give me a PPT of 36, but knowing that the ICP confirmed my ICP to be 33.8 PSU (too low) while consistently measuring 35 PPT, this seems to be the practical course of action.

 

[Randy Holmes-Farley]

It’s a machine error if those ppt and psu values are not identical, but since it is within the stated error of the device, it’s no big deal. You could make a 35 ppt (= psu) standard and see what it reads, but just picking psu or ppt is fine.