DIY Salinity calibration solutions

[nereefpat]

Ok, thanks. I'll need a bigger scale I guess. The small digital scale I'm using was throwing errors when I tared it with the plastic mason hard and added the 963.5g of water. It's only good for 100g total. So the water plus the plastic jar must have been exceeding the limit.

Just use the scale for the salt, then add it to something that measures volume. Graduated cylinder, or measuring cup.

 

[DeputyDog95]

Just use the scale for the salt, then add it to something that measures volume. Graduated cylinder, or measuring cup.

I don't have any graduated cylinders or anything that measures liquids w that level of accuracy. I'll grab a slightly bigger scale, they're cheap and a good investment.

What is the difference between RHF's DIY solution and something like "Refracto Juice" from BRS? It's made by Brightwell and distributed by BRS.

I did crank the air down to 68 in the office after everyone left and calibrated the VeeGee. Tried anyway... It wasn't holding a "zero" using distilled water. It kept creeping up. So I think something is wrong it with it. As mentioned, it was shipped in a plastic bag in it's own box with no padding in the bag or the factory box. Just a thin plastic sleeve over the device. Probably got a little beat up.

When I tested a tiny batch of Randy's solution it came in at 35ppt, even at 68 degrees. Is Randy's temp dependent?

The reason I ask is that the BRS solution, at 68 degrees, was coming in at 33ppt, but the bottle says it's supposed to be 35ppt at 25 degrees C.

Are some of these solutions only accurate at the prescribed temp?
And is refracto juice a completely different type of salinity reference solution as compared to Randy's?

 

[Randy Holmes-Farley]

When I tested a tiny batch of Randy's solution it came in at 35ppt, even at 68 degrees. Is Randy's temp dependent?

This is a complicated question, but the short answer is that if you are using a device that compensates for temperature changes (and it does that compensation correctly), then the temperature is not important as long as it is within the range that it accurately compensates for.

ATC refractometers, most conductivity meters, and swing arm hydrometers fit that description, although they may each have their own recipe depending on how it is designed.

 

[nereefpat]

I don't have any graduated cylinders or anything that measures liquids w that level of accuracy. I'll grab a slightly bigger scale, they're cheap and a good investment.

You don't have a liquid measuring cup that measures in mLs? like this?

 

[DeputyDog95]

You don't have a liquid measuring cup that measures in mLs? like this?

I do, but I would have to "eyeball" 96.35ml of water. Which for me anyway, would be difficult. I'd rather just weigh it and have a precise measurement to ensure the mixture is spot on. Not saying you couldn't use a measuring cup and get it pretty close, but it's just easier to use a scale. Although a 100ml graduated cylinder would probably get you really really close. But I could use the scale for other things, so I'm just going to grab one.

 

[DeputyDog95]

This is a complicated question, but the short answer is that if you are using a device that compensates for temperature changes (and it does that compensation correctly), then the temperature is not important as long as it is within the range that it accurately compensates for.

ATC refractometers, most conductivity meters, and swing arm hydrometers fit that description, although they may each have their own recipe depending on how it is designed.

It was interesting... I made a new batch of your refractometer reference solution (using scales for the best precision) and tried checking it at 80 degrees ambient. I inadvertently left the back door to my office cracked open (where the tank is) and it got hot in there.

The solution was reading 1ppt high (36ppt) on two different devices. Even after letting them sit for a couple of mins.

I tried it again yesterday when the office was 77 degrees, and it was spot on. Both devices have ATC.

Can we assume reference solutions are engineered to have a specific reading at a specific temperature? I know the refracto juice from BRS/Brightwell references their solution to be 35ppt at 25c.

I know when my office is 75 and I take a sample of 79 degree tank water, it takes a min before the BRS refractometer and the Milwaukee digital will show a stable reading and both fluctuate a little until that happens.

I just recently bought a VeeGee refractometer and the directions are implicit about calibrating with distilled water at specifically 68 degrees F. I'll have to wait until it cools down a bit in FL before I can get the office to that temp using the AC.


Is it plausible that the reference solutions should be read at a particular temp for highest accuracy?

 

[Randy Holmes-Farley]

It was interesting... I made a new batch of your refractometer reference solution (using scales for the best precision) and tried checking it at 80 degrees ambient. I inadvertently left the back door to my office cracked open (where the tank is) and it got hot in there.

The solution was reading 1ppt high (36ppt) on two different devices. Even after letting them sit for a couple of mins.

I tried it again yesterday when the office was 77 degrees, and it was spot on. Both devices have ATC.

Can we assume reference solutions are engineered to have a specific reading at a specific temperature? I know the refracto juice from BRS/Brightwell references their solution to be 35ppt at 25c.

I know when my office is 75 and I take a sample of 79 degree tank water, it takes a min before the BRS refractometer and the Milwaukee digital will show a stable reading and both fluctuate a little until that happens.

I just recently bought a VeeGee refractometer and the directions are implicit about calibrating with distilled water at specifically 68 degrees F. I'll have to wait until it cools down a bit in FL before I can get the office to that temp using the AC.


Is it plausible that the reference solutions should be read at a particular temp for highest accuracy?

If the results vary with temperature, it is most likely because the ATC functionality is not perfect, not because the solution itself is only good at a certain temp (which is always true if it is a non ATC refractometer).

You might try the same tank water at widely different room and solution temps to see if it changes.

 

[DeputyDog95]

If the results vary with temperature, it is most likely because the ATC functionality is not perfect, not because the solution itself is only good at a certain temp (which is always true if it is a non ATC refractometer).

You might try the same tank water at widely different room and solution temps to see if it changes.

I definitely see it change when I take the 79 degree tank water out and immediately take readings in the 76 degree ambient office air. Usually takes about a min, but there is a noticeable change. Reads a tiny bit higher than expected and then comes down.

I guess I was just thinking that if someone created a reference solution at a particular temp specifically for calibration purposes, you would need to do it at that temp for your baseline and then let the ATC adjust for your samples moving fwd.

That being said, I have no idea what I'm talking about Appreciate all the info.

 

[Randy Holmes-Farley]

Think of it this way. If the calibration solution IS 35 ppt seawater, it is 35 ppt seawater at all temperatures, and should read 35 ppt on a properly functioning ATC refractometer regardless of temp (assuming the temp is within the ATC temp range).

They only way that deviates is if the ATC function is not perfect. That seems to be what you find.

if the standard is a different chemical than 35 ppt seawater, like my diy standards using NaCl, then the only other way that happens is if the change in refractive index with temp are different than the change in refractive index with temp for seawater. Seawater is mostly NaCl and differences in temp effects for smallish temp changes will be quite small and are not likely detectable against the other errors in use, especially near the center of the atc range in the 25 deg C area.

 

[DeputyDog95]

Think of it this way. If the calibration solution IS 35 ppt seawater, it is 35 ppt seawater at all temperatures, and should read 35 ppt on a properly functioning ATC refractometer regardless of temp (assuming the temp is within the ATC temp range).

They only way that deviates is if the ATC function is not perfect. That seems to be what you find.

if the standard is a different chemical than 35 ppt seawater, like my diy standards using NaCl, then the only other way that happens is if the change in refractive index with temp are different than the change in refractive index with temp for seawater. Seawater is mostly NaCl and differences in temp effects for smallish temp changes will be quite small and are not likely detectable against the other errors in use, especially near the center of the atc range in the 25 deg C area.

Makes sense. Appreciate the explanation.

I was making some salt water (Red Sea blue bucket) yesterday, so I let it mix fully for an hour or so and tested the Alk using my Hanna device.

Red Sea says on the bucket that the Alk (at 25c) should be 8dkh at 35ppt, 7.5dkh at 33ppt, and 7dkh at 30ppt.

I came in at 8.6 dkh using my measured salinity of 35ppt, but my Hanna seems to read about 0.3-0.4 dkh higher than other standards.

So it would seem I'm getting reasonably accurate results with both of my salinity devices. Just another data point to give me confidence that my equipment seems to be working and that my recent ICP's showing that I'm 32-33ppt are probably not correct.