Refractometer calibration...

kona

kona

Member
Hey all,
My friend's mom is a chemist, and she was able to procure a refractometer for me. Problem is, I'm not exactley sure how to go about using it.
From what I have read thus far, I dropped a couple drops of distilled water onto the prism, waited for it to reach a similar temperature, and then turned the screw until the color boundary was set at zero.
The particular refractometer I have has a scale from 0 - 32. Now my question is, without any other constant, other than pure water, how can I make any kind of scale?
Thus far, I have been using a hydrometer and keeping my salt content within a general range of 1.023-1.026.....but, how exactley does this correlate with my refractometer scale of 0-32, and furthermore, how does the number on the hydrometer correlate with ppm or ppt numbers?
I was hoping that on my bucket of salt, it would have some sort of constant. If it said something like, half a cup of salt in 1 gallon equals 1.024, than I would be able to make the solution, and see what number correlates on my refractometer.....say 4. Then I would always know that 1.024 = 4 on my refractometer. I don't know....
Any help would be greatly appreciated.
Thank you.
 
elfdoctors

elfdoctors

Active Member
I believe that your refractometer only measures salinity, not specific gravity. Calibration only involves the 0 setpoint.
Different refractometers are used for different purposes (e.g. concentrating maple syrup). Your refractometer does not sound optimal for saltwater but can probably be used.
Saltwater aquaria tend to do best with salinities between 32-35. Hopefully you can read a little above the end of your scale. Double check with your hydrometer that these readings are in the ballpark. Hydrometers are usually off by a little compared with refractometers.
 
kona

kona

Member
I thought that for every specific gravity reading, there was a correlating salinity reading. I mean....wouldn't a specific gravity of 1.025 equal some sort of salinity, and wouldn't this equality remain constant?
Aside from that....my hydrometer reads about 1.026 on my display tank, and my refractometer reads at 3.8. And as you said, I think you are right about my refractometer. My friend told me that this refractometer is used to measure the amount of anything that is dissolved in the water.
And lastly, lets say I take a measurement of the display with my hydrometer - 1.026. This number is obviously not accurate. Now, my refractometer says 3.8. But this number doesn't necessarily mean a specific grav of 1.026, since the original device (hydrometer) is not accurate. So....where do I get the constant from???????:rolleyes: :rolleyes: :rolleyes: :rolleyes:
 
broomer5

broomer5

Active Member
Salinity in PPT or parts per thousand is just that.
Seawater is normally around 35 parts per thousand.
If you had 3500 pounds of saltwater, and boiled off all the water, you'd be left with 35 pounds of salts.
Specific gravity is not the same at all.
Specific gravity is a comparison of the saltwater to that of pure water.
Pure water having a specific gravity of 1.000
If you take 1 milliliter of pure water ( 1 cubic centimeter or cc ) and weigh it ...... it would weigh exactly 1.000 grams at 68 F.
If you take 1 milliliter of saltwater ( 1 cubic centimeter or cc ) and weigh it ..... it would weigh more than 1.000 grams at 68 F.
Why would it weigh more ? Because there's more stuff in it than just water. Theres' salt in there too, along with the pure water.
So it's going to weigh a little more. It's going to weigh 1.0XX
XX being the weight of the salt in that little cubic centimeter box.
How much more it weighs ? It all depends.
It depends on the actual amount of salt and minerals dissolved in the sample .... and it depends on the temperature. Now we're talking about density. How much stuff is dissolved in the water.
And the density of a given volume of liquid changes as the temperature changes.
So ....... specific gravity is dependent on the temperature, where as salinity as parts per thousand ppt is not.
I don't know what 0-32 means ?
What is your refractometer designed to measure ?
 
razoreqx

razoreqx

Active Member

Originally posted by broomer5
Salinity in PPT or parts per thousand is just that.
Seawater is normally around 35 parts per thousand.
If you had 3500 pounds of saltwater, and boiled off all the water, you'd be left with 35 pounds of salts.
Specific gravity is not the same at all.
Specific gravity is a comparison of the saltwater to that of pure water.
Pure water having a specific gravity of 1.000
If you take 1 milliliter of pure water ( 1 cubic centimeter or cc ) and weigh it ...... it would weigh exactly 1.000 grams at 68 F.
If you take 1 milliliter of saltwater ( 1 cubic centimeter or cc ) and weigh it ..... it would weigh more than 1.000 grams at 68 F.
Why would it weigh more ? Because there's more stuff in it than just water. Theres' salt in there too, along with the pure water.
So it's going to weigh a little more. It's going to weigh 1.0XX
XX being the weight of the salt in that little cubic centimeter box.
How much more it weighs ? It all depends.
It depends on the actual amount of salt and minerals dissolved in the sample .... and it depends on the temperature. Now we're talking about density. How much stuff is dissolved in the water.
And the density of a given volume of liquid changes as the temperature changes.
So ....... specific gravity is dependent on the temperature, where as salinity as parts per thousand ppt is not.
I don't know what 0-32 means ?
What is your refractometer designed to measure ?
Nice writeup on Salinity vs SG.. Mind if i use this on our SW newsletter? I would credit you of course.
 
doublezero

doublezero

Member
RazorEQX,
In one of the books I have on SW topics I have a salinity vs specific gravity table... I could scan it and send it to you so you could also include it in the newsletter..(referenced of course)
If you are interested....
BTW your weekly Letters are outstanding, can't wait for the next installment, they are perfect for the beginner ... keep up the great writing..
#00
Scott
 
kona

kona

Member
Broomer,
When I was given the refractometer, I was told that it was used in a chem lab to measure how much solid was dissolved into a particular solution - any solid, including salt.
The 0-32 is what I see when I look into the refractometer lens. There is a scale that starts on the bottom at 0 and goes up to 32. My display tank reading is 1.026 (with hydrometer).. The refractometer gives me a reading of about 4.
Since people are unfamiliar with this particular refractometer, my only idea for a solution would be to determine the optimal specific gravity, or ppt, make a solution that matches that level exactley (i don't know how I would do this), and then use my refractometer and see what number it displays - then I would always know that that number would be the optimal salt content.
 
D

daniel411

Guest
You get your constant, of zero, by useing pure ro/di water at the right temp. Not all refractometers are auto-temp calibrating.
 
doublezero

doublezero

Member
Kona,
you could also find someone who has a salinity/specific gravity refractometer and compare various salt solutions to determine the values for your particular refractometer ... good luck..
#00
 
kona

kona

Member
DoubleZero,
thats probably the best idea. I'll just take it to my LFS and compare different solutions using their refractometer and mine, and make my own general scale. I can't really think of any other way to approach this problem.
 
kona

kona

Member
Broomer,
sorry, I had one more question.
So, now I understand the difference between ppt and specific gravity. What is it that refractometers measure? I read your info in the FAQs, and it says that you have to wait till the temp reaches the same as the refrac, and since you said that spec grav is affected by temp, I would assume that the refrac measures just the specific grav and not ppt, right?
 
doublezero

doublezero

Member
Kona,
I just had another idea, if you know the company that made the refractometer, you could contact them and see if they have cross referenced that particular scale to salinity or specific gravity, it is worth a shot… we have done this type of thing where I work … saves time and effort..
#00
PS I will email you the table when I get it scanned…
 
donmgicwon

donmgicwon

Member

Originally posted by broomer5
Salinity in PPT or parts per thousand is just that.
Seawater is normally around 35 parts per thousand.
If you had 3500 pounds of saltwater, and boiled off all the water, you'd be left with 35 pounds of salts.

Wouldn't that only be 1000 lbs of saltwater = 35 lbs. of salt.
 
broomer5

broomer5

Active Member
That's correct donmgicwon - it should be 1000 lbs of saltwater = 35 lbs. of salt & minerals.
:notsure: my error
kona - I did a search on the internet - and I think you have a refratometer that measures in 0 to 32% and is called "brix"
Brix is used as a measurement for food & beverage industry, beer making and other liquids that contain sugar.
There may very well be a relationship between the 0 to 32% brix scale and salinity - that you may be able to find or develop.
 
kona

kona

Member
Broomer,
Yes, you are right! When I look into the lens, it does say BRIX on the bottom. Good call....hopefully there is some info on the net about this.
Thanks,
Eric
 
doublezero

doublezero

Member
Kona,
I found this chart also... IMO
The salt solution brix relationship is linear as the g of salt increases so does the brix, this is good you can make a chart of salt verses brix... extrapolating the salt g to 1000g you would look at 3.5 on the salt side and 3.1 on the brix side.. so I would make a chart of salt vers brix (salt from 0, 0.5, 1.0 1.5 etc even could make it 0, 0.25, 0.5, 0.75, 1.0 etc) and plot the 0.5 graduations against the brix 0, 0.65, 1.3, 1.95 etc
this should give you a linear scale and then you can read the brix off the scale and relate it to g of salt per 1000g or ppt...
I could throw it into excel if you have trouble understanding my ramblings.
If anyone sees any flaws please point it out...
#00
Scott
 
kona

kona

Member
DoubleZero,
I understand clearly what you are saying. Thanks for the useful insight. I guess this thing will work afterall.....
 
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