Recommendation for Salinity Tester

[gbroadbridge]

This was my concern. If I'm aiming for 34 it could be 36 and it could be 32. But all the anecdotal reports are that it reads the same reliably all the time, although some claim it's .002 off in SG (I'm not sure what the corresponding ppt is).
On the hydrometer it sounds like I need to get a bigger cylinder than it comes with to fill with water and test it in. I haven't looked up the temp conversion part yet but I'm curious how far off it will be if the water temp is 78 or even 80 in the summer.

This has been an awesome discussion though and I'm learning, so it's fun.

Mine is consistently 0.002 sg high compared to my calibrated Hanna Salinity checker. It only shows integer values for PSU.

It wins on simplicity, but if I want to know where I'm at in absolute terms, I aim for 34.8 on the Hanna.

I only keep the MA887 as a backup, not for primary daily testing.

 

[KStatefan]

This was my concern. If I'm aiming for 34 it could be 36 and it could be 32. But all the anecdotal reports are that it reads the same reliably all the time, although some claim it's .002 off in SG (I'm not sure what the corresponding ppt is).
On the hydrometer it sounds like I need to get a bigger cylinder than it comes with to fill with water and test it in. I haven't looked up the temp conversion part yet but I'm curious how far off it will be if the water temp is 78 or even 80 in the summer.

This has been an awesome discussion though and I'm learning, so it's fun.

I use a 500 ml graduated cylinder with 450 ml of water in it.

You can calculate the actual SG if you are not at 77° F.

 

[BradVol]

I use a 500 ml graduated cylinder with 450 ml of water in it.

You can calculate the actual SG if you are not at 77° F.

That's a huge cylinder. Is this thing that big? Haha. If you use that much water do you return it to the tank?

 

[FiddlersReef]

That's a huge cylinder. Is this thing that big? Haha. If you use that much water do you return it to the tank?

I use the same thing. Looks cool on my Dr. Science desk. I put the water right back into the tank.

 

[KStatefan]

That's a huge cylinder. Is this thing that big? Haha. If you use that much water do you return it to the tank?

Yes. I return the water. That is how much you need to float the hydrometer.

 

[BradVol]

I found a Milwaukee MA887 in like new condition on ebay for $48 so I've got that coming too. So I'll be more confused than anything this week, I'm sure. Looking forward to it.

 

[BradVol]

My MA887 showed up and the fun begins. I think the hydrometer will get here tomorrow. I need to do a little research on whether people usually use ppt or PSU for the salinity. They seem to be one number different in the range of measurements we use.

I cleaned and calibrated the MA887 and tested my tank water and got 34 ppt, which has been my target, so that's good. Then I tested the two calibration solutions I have. Both are 35 ppt solutions. The Brightwell is older and there is a chance I left the top unscrewed one night early on. It measured 37 ppt. The Fritz is newer and it measured 35 ppt. This lines up with what I got on my refractometer so all seems good so far. Just waiting for the wrench to get thrown in the works.

 

[Randy Holmes-Farley Verified Community Expert]

My MA887 showed up and the fun begins. I think the hydrometer will get here tomorrow. I need to do a little research on whether people usually use ppt or PSU for the salinity. They seem to be one number different in the range of measurements we use.

I do not think there's a difference as ordinary reefers use the values, just in the definitions of what they mean.

PSU comes from conductivity by definition. It literally is a ratio of conductivity of the solution to the conductivity of a specific concentration of potassium chloride solution. 35 ppt = 35 PSU

PPT can come from many different ways, but it's historic root is the actual weight of solids in the water. 35 ppt = 35 grams per liter.

 

[BradVol]

I do not think there's a difference as ordinary reefers use the values, just in the definitions of what they mean.

PSU comes from conductivity by definition. It literally is a ratio of conductivity of the solution to the conductivity of a specific concentration of potassium chloride solution. 35 ppt = 35 PSU

PPT can come from many different ways, but it's historic root is the actual weight of solids in the water. 35 ppt = 35 grams per liter.

I thought they were practically the same but when I measure I get 33 PSU and 34 ppt on the same measurement. It's also 1.026 SG.

 

[Randy Holmes-Farley Verified Community Expert]

I thought they were practically the same but when I measure I get 33 PSU and 34 ppt on the same measurement. It's also 1.026 SG.

You mean the device reports those all on the same water sample measurement?

Assuming so, that's not a an actual difference based on the units. it's likely rounding and calculating errors.

33.5 ppt = 33.5 PSU = sg 1.0254

 

[Matty Ice]

Not sure if I missed it or if it wasn't mentioned but using a refractometer...are you cleaning it with fresh RO/DI water and drying it after you calibrate and after testing?

 

[BradVol]

I use a 500 ml graduated cylinder with 450 ml of water in it.

You can calculate the actual SG if you are not at 77° F.

Is there any reason I can't just use the hydrometer in my sump with flow turned off? The water gets perfectly still and it has a glass front and is easy to access. I got a graduated cylinder but realized life would be a lot easier if I can just let the water settle and float it in the sump.

 

[Randy Holmes-Farley Verified Community Expert]

Is there any reason I can't just use the hydrometer in my sump with flow turned off? The water gets perfectly still and it has a glass front and is easy to access. I got a graduated cylinder but realized life would be a lot easier if I can just let the water settle and float it in the sump.

If it is deep and still enough, yes. You may need a temperature correction if temp doesn't match the hydrometer calibration temp.

 

[BradVol]

Not sure if I missed it or if it wasn't mentioned but using a refractometer...are you cleaning it with fresh RO/DI water and drying it after you calibrate and after testing?

Yes, I have an RODI supply in my sink and clean and dry it between measurements.

You mean the device reports those all on the same water sample measurement?

Assuming so, that's not a an actual difference based on the units. it's likely rounding and calculating errors.

33.5 ppt = 33.5 PSU = sg 1.0254

That's what I thought. But the same reading of the same sample gives the different numbers.

I just calibrated again and ran a few tests:
Tank water: 1.026 SG, 33 PSU, 34 ppt
Brightwell 35ppt: 1.028 SG, 36 PSU, 37 ppt
Fritz 35ppt: 1.026 SG, 34 PSU, 35 ppt

 

[Randy Holmes-Farley Verified Community Expert]

Yes, I have an RODI supply in my sink and clean and dry it between measurements.



That's what I thought. But the same reading of the same sample gives the different numbers.

I just calibrated again and ran a few tests:
Tank water: 1.026 SG, 33 PSU, 34 ppt
Brightwell 35ppt: 1.028 SG, 36 PSU, 37 ppt
Fritz 35ppt: 1.026 SG, 34 PSU, 35 ppt

Then it's a device calculation issue. I'd just pick a unit and use it for everything.

There's no calculable difference between PSU and ppt.

Here's a discuss of it:

https://static.fishersci.com/cmsassets/downloads/segment/Scientific/pdf/WaterAnalysis/Log92TipConductivitySalinity.pdf

 

[BradVol]

Then it's a device calculation issue. I'd just pick a unit and use it for everything.

There's no calculable difference between PSU and ppt.

Here's a discuss of it:

https://static.fishersci.com/cmsassets/downloads/segment/Scientific/pdf/WaterAnalysis/Log92TipConductivitySalinity.pdf

Thanks. I'll read up on that. I'm more curious for the sake of learning now than anything. Here's what the Milwaukee manual says:

In the MA887 refractometer, light from an LED passes through a prism in contact with the sample. An image sensor determines the critical angle at which the light is no longer refracted through the sample. Specialized algorithms then apply temperature compensation to the measurement and convert the refractive index to: PSU (Practical Salinity Units), ppt (part per thousand) or S.G. (Specific Gravity) (20/20). 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). Specific Gravity (20/20) ia based upon the published relationship between density at 20 °C and the mass of dissolved salts in the seawater sample (CRC Handbook of Chemistry and Physics, 87th) Edition).

I tried to look up those references and couldn't find them. I'm really just curious how their calculations arrive at a different value.

 

[Randy Holmes-Farley Verified Community Expert]

Thanks. I'll read up on that. I'm more curious for the sake of learning now than anything. Here's what the Milwaukee manual says:

In the MA887 refractometer, light from an LED passes through a prism in contact with the sample. An image sensor determines the critical angle at which the light is no longer refracted through the sample. Specialized algorithms then apply temperature compensation to the measurement and convert the refractive index to: PSU (Practical Salinity Units), ppt (part per thousand) or S.G. (Specific Gravity) (20/20). 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). Specific Gravity (20/20) ia based upon the published relationship between density at 20 °C and the mass of dissolved salts in the seawater sample (CRC Handbook of Chemistry and Physics, 87th) Edition).

I tried to look up those references and couldn't find them. I'm really just curious how their calculations arrive at a different value.

I do not disagree with any of that, but there's no significant calculation difference between ppt and PSU from refractive index that the Milwaukee measures.

 

[Randy Holmes-Farley Verified Community Expert]

If you really want to get into the weeds, there's a tiny (0.47%) conversion discussed here when discussing PSS, PSU, and PPT:

Does anyone have experience with practical salinity scale and...

i'm confused on the differences between PSS and PSU for ocan salinity measurements. Im wondering are there any differences between them or almost similar?? Can anyone give me suggestion on how can...

www.researchgate.net

 

[BradVol]

Got the hydrometer. If floats still in my tank when I turn everything off for a few minutes. I'm only intending to use it to sanity check my other instruments so I think that will work fine.

I got 1.02565 on the hydrometer at 78.6 degrees F. I'm assuming you read from below the water line and not where the surface tension pulls it up the tube on top of the water. The temperature-corrected SG is 1.0259, which is 34.38 ppt. The MA887 showed 34 for the tank water and 35 for the Fritz calibration solution. Sounds to me like I can toss the old Brightwell calibration solution and I have three methods of measuring that seem to agree. I'm happy and I've learned a bunch, so well worth it.

 

[Randy Holmes-Farley Verified Community Expert]

I got 1.02565 on the hydrometer at 78.6 degrees F. I'm assuming you read from below the water line and not where the surface tension pulls it up the tube on top of the water.

correct.

Chemistry and the Aquarium: Specific Gravity: Oh How Complicated!

This month, Randy explains and reviews specific gravity.

reefs.com

Figure 4. A Tropic Marin hydrometer showing the meniscus rising to about 1.0260, but the actual reading is about 1.0265.