Salinity refractometer or meter, which to believe

[Randy Holmes-Farley Verified Community Expert]

yes you are correct in all your given facts but to a new person it's a bit overboard.

Not going to keep arguing with you.

If you think dissolving table salt in Ro/DI and using it the same way you already use Ro/DI is too complicated, then don't use it.

 

[Randy Holmes-Farley Verified Community Expert]

Holy smokes! That’s a thing? What kind of a jerk would mess with calibration fluid? Some people…

I figured there might be differences in calibration fluids, and I wondered if they drifted between bottles/instruments/time. But I guess I always figured it would be close enough. 34-36 anything would work as long as it’s consistent. I guess I should pay just a little bit more attention to my salinity then I have been. Thanks for the help to all of us

I've heard from vendors that's it's a thing, sadly.

 

[BoxKing]

Sorry, I'm not following. If you are just adding salt and water to weight each time, why bother with the conductivity meter that is just "calibrated" to some unknown salinity that matches the 2.5 cups per gallon?

You do know that 35 ppt seawater cannot be made from 35 grams of salt mix plus 965 grams of RO/DI, right?

I’m not calibrating my probe by simply testing my weighed measurements, I use the Hanna Solution. I referenced this bc ppl think you need to calibrate every time you test your water. I guess you can, but thats not necessary, and if you feel like you have to do this every time to be accurate, then my belief is those ppl should change their testing method to something they can trust to be accurate without calibrating on every use. My “Method” is my redundancy to see if my sanity probe needs to be calibrated i.e.: if I get a result of 32ppt when mixing, its going to raise an eyebrow. I’d then check my calibration and retest the same water; if I get the same 32ppt reading, then I would determine that my weighing method isn’t 100% (which it has been 98% so far), and the cause would be the salt mix variation, which is not 100% each mix. Now would I change the scale on the next batch… nope, bc it was simply the mixture of the elements used when dissolved, not the practice itself. I bother with the meter bc I use it to check temp…I would still check even without the temp option on the probe, bc it would be dang lazy of me not to use the HANNA meter due to its simplicity.

Lol, yes, I am aware that 35 grams doesn’t equal 35ppt- that weight would almost be untraceable.

I’m just a Fan Boy of the HANNA or Milwaukee meters, and I don’t understand why ppl choose refractometers/hydrometers in this day and age, which is why I give my vote to go digital whenever the option is available.

 

[Randy Holmes-Farley Verified Community Expert]

I’m not calibrating my probe by simply testing my weighed measurements, I use the Hanna Solution. I referenced this bc ppl think you need to calibrate every time you test your water. I guess you can, but thats not necessary,

I certainly agree. My conductivity meter could not be calibrated, and in 10 years it never deviated when put into the reference solution.

 

[Randy Holmes-Farley Verified Community Expert]

Lol, yes, I am aware that 35 grams doesn’t equal 35ppt- that weight would almost be untraceable.

OK, just wanted to be sure, since a few folks made their own standards this way.

The main reason one cannot is that there's a lot of water in "dry" salt mix, and so putting 35 grams salt mix into 965 grams of water ends up with salinity well below 35 ppt due to much of that 35 grams added being water.

 

[jgsolo32]

Hi,
I'm new to the forum, but not new to saltwater/reef. Been away from saltwater for about 15 years.
I have 2 refractometers, a Hanna salinity meter and an old school float hydrometer. Both refractometers have been calibrated with 1.026 solution and distilled water. The meter has been calibrated with the proper cal solution.
Both refractometers are showing the salinity @ 1.027. The meter is showing 1.024 and 1.025 at times. The hydrometer shows just a hair over 1.025.

Which would you believe is correct?

I'm thinking that the 2 refractometers are correct.

i would get a Tropic Marin Hydrometer. Accurate! Had to send back my Milwaukee MA 887 due to reading error of 1.008 after 2 months if use

 

[Darren in Tacoma]

Taste it. If it's too salty, add water. Not salty enough, add salt. This method is more art than science

 

[Randy Holmes-Farley Verified Community Expert]

Taste it. If it's too salty, add water. Not salty enough, add salt. This method is more art than science

I understand that is a joke, but just to be clear, it is not safe (IMO) to randomly taste reef tank water for risk of bacterial issues.

 

[ClownWrangler]

Refractometers measuring salinity by how much light bends travelling through it.

Conductivity meters measure salinity by how fast ions move in it in an electric field.

Neither is particularly intuitive.

What's even less intuitive is "Automatic Temperature Correction". In my mind this could mean different things.

For an optical refractometer, which measures refractive index, which varies directly with density (SG), but for some reason has a salinity scale as well, I presume this means that its compensating and showing what your SG would be at 20c? Its the only way the SG and salinity scales would line up.

For a digital meter, which effectively reads electrical conductivity, which varies directly with salinity, but displays SG, I presume it means that its calculating SG based on temperature and measured salinity?

Reality (Physics 101):
Temperature increase results in decreased SG, but no change in salinity.

As measured by my refractometer:
Single drop on slide takes on ambient temperature quickly, so temperature has no real effect if calibrated at that temperature. Change in ambient temperature throws off calibration, so no real temperature compensation going on. End result is very consistent readings at room temperature.

As measured by my digital meter (same sample at different temperatures), (Seaside Aquatics SM 01)

14.4c 26.7PPT 1.023 SG
18.8c 27.6PPT 1.023 SG
24.7c 28.8PPT 1.022 SG


-This is very counterintuitive for a meter said to have "Temperature compensation". I presume its calculating salinity from EC readings then calculating SG from temperature and salinity, but why would the salinity increase with temperature? Crappy meter? Looks like its back to using the refractometer.

 

[ClownWrangler]

I understand that is a joke, but just to be clear, it is not safe (IMO) to randomly taste reef tank water for risk of bacterial issues.

Makes me wonder how people ever survive swimming in the ocean and getting random tastes by accident. Id be more worried about palytoxins.

 

[Randy Holmes-Farley Verified Community Expert]

Makes me wonder how people ever survive swimming in the ocean and getting random tastes by accident. Id be more worried about palytoxins.

How many people get diarrhea and do not know where it came from? lol

 

[Randy Holmes-Farley Verified Community Expert]

What's even less intuitive is "Automatic Temperature Correction". In my mind this could mean different things.

For an optical refractometer, which measures refractive index, which varies directly with density (SG), but for some reason has a salinity scale as well, I presume this means that its compensating and showing what your SG would be at 20c? Its the only way the SG and salinity scales would line up.

For a digital meter, which effectively reads electrical conductivity, which varies directly with salinity, but displays SG, I presume it means that its calculating SG based on temperature and measured salinity?

Reality (Physics 101):
Temperature increase results in decreased SG, but no change in salinity.

As measured by my refractometer:
Single drop on slide takes on ambient temperature quickly, so temperature has no real effect if calibrated at that temperature. Change in ambient temperature throws off calibration, so no real temperature compensation going on. End result is very consistent readings at room temperature.

As measured by my digital meter (same sample at different temperatures), (Seaside Aquatics SM 01)

14.4c 26.7PPT 1.023 SG
18.8c 27.6PPT 1.023 SG
24.7c 28.8PPT 1.022 SG


-This is very counterintuitive for a meter said to have "Temperature compensation". I presume its calculating salinity from EC readings then calculating SG from temperature and salinity, but why would the salinity increase with temperature? Crappy meter? Looks like its back to using the refractometer.

The two primary corrections that these devices are concerned with are:

1. Ions always move faster as temperature increases. Thus, the raw conductivity rises as temperature rises. If one wants to get a measure of the total ions present from a raw measure of conductivity, one needs to "correct" the measurement for this difference. The standard for temperature measurement/correction of conductivity, regardless of what you are measuring is 25 deg C.

2. The ATC aspect of a refractometer is designed to offset the change in refractive index of water (or seawater) as a function of temperature. In fact, water and seawater change very slightly differently in refractive index vs temperature, but it is close enough given the inherent low precision of typical refractometers used by reef keepers.


Neither of these devices are trying to correct for changes in actual SG that are caused purely by temperature. These changes are small, but are not zero. SG is defined as the density of the seawater divided by the density of fresh water at the same temperature. The density of both move similarly with temperature, but not identically, so there is a very small change in SG with temperature.

How well any particular device makes such corrections can be assessed as you did by measuring the same solution at different temperatures. They are not perfect, but are typically pretty good in the range reefers are concerned with.

 

[Darren in Tacoma]

How many people get diarrhea and do not know where it came from? lol

I think we all know where diarrhea comes from! Ha! (sorry, it's early)

 

[ClownWrangler]

How many people get diarrhea and do not know where it came from? lol

Surfers in southern CA know. Thanks to Tijuana sewage spilling in the ocean.

 

[ClownWrangler]

Glad you asked.

I have been "offering" a perfect DIY calibration solution for any type of conductivity device (including the Tropic Marin hydrometer) for many years:

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com

Thanks, this sort of answered some questions I had but brings up some new ones. I recall from chemistry classes and basic dimensional analysis that PPT, being dimensionless mass/mass ratio x 10^3, is not the same as Mass/vol. ratio, yet I have seen salinity (PSU) defined as "g/l", which is not the same as PPT and is not a dimensionless measure. This is confusing. Also, I'm assuming that PPT is the mass ratio for solvent/solute rather than solvent/solution, is this correct? I guess this would make g/l close enough to PPT at 20c.

 

[bnord]

the old Japanese proverb............. Man with two or more salinity meter, never sure what salinity really is, man with just one is always certain.

is there a similar proverb for Mg levels?

 

[ClownWrangler]

is there a similar proverb for Mg levels?

That one goes, "He who doses Seachem Marine Buffer adds Ca, Mg and carbonate at a fixed ratio and has no Mg meter"

 

[bnord]

that's amazing... just what my fortune cookie said last night

 

[attiland]

Eeny, meeny, miny, moe…. Use the refractometer

 

[ClownWrangler]

I understand there are many ways to get to ones target salinity. I also understand that our aquaria are very forgiving of the exact salinity level.

But the "method" described by BoxKing seems problematic to me, and hence why I was asking how he decides how much salt to use. I don't understand how his measurement adds anything to the process.

It doesn't add anything. Sea salt is hygroscopic, very hygroscopic, I assume mainly due to the Ka and Mg salts. I did an experiment last night. I set out 35.00g of dry Instant Ocean salt and weighed it again this morning and it measured 36.33g. It absorbed 1.33g of water out of the atmosphere just over night. That's already a 3.8% error and I know it can absorb much more. Every time you open a salt bucket, you let more moisture in and your salt gains more mass, so mass measurement of sea salt is useless unless completely dried at high temperature first.

This is why I like what you published about using NaCl for calibration solution. It is not hygroscopic, so mass measurements can be consistent without special storage.