Refractometer or hydrometers, shocking results

[Bramzor]

I saw the BRS video where Lou Ekus mentions that a hydrometer is more precise than a refractometer since it does not require calibration. I can understand this logic so I wanted to test this theory.
So I had one glass hydrometer laying around that I used before and wanted to test this out. But the results where shocking.

Refractometer showed 35 ( 1026 ), before and after calibration.
Glass floating hydrometer showed 1020
Plastic hydrometer (no bubbles) showed something similar 1021
(everything at 25°C, so 77°F)

Ok, lets assume my I did bad calibration for the refractometer and my salinity of the refractometer is off. So how did I make sure this was not the case.
I got some corals today from a german coral farm and still had some shipping water.
Result: I also tested the corals shipping water (so an external source) and it was almost exactly in line with the results above. Although water was slightly colder.

So I'm not sure what to do now. Pretty sure refractometer is measuring 35 salinity correctly since it showed this result before and after calibration and also for the coral farm water.
But it's hard to believe that both types of hydrometers are off by almost the same value.

 

[redfishbluefish]

What is the reference temperature on your glass hydrometer? Temperature effects readings.

 

[Bramzor]

What is the reference temperature on your glass hydrometer? Temperature effects readings.

Was 23°C so 73°F. It does indeed effect the reading but only by .5 so its measuring 28.0 vs 28.5, which is still a way of from 35.

 

[Nano_Man]

I think Randy has a thread on this subject. Could be wrong

 

[bushdoc]

I would say that not just any hydrometer is more precise than refractometer, but high quality one is. For example Tropic Marin Hydrometer.
Also, you need to use reference table with different temp than hydrometer was calibrated. There are tables for Tropic Marin Hydrometer.

 

[Bramzor]

I would say that not just any hydrometer is more precise than refractometer, but high quality one is. For example Tropic Marin Hydrometer.
Also, you need to use reference table with different temp than hydrometer was calibrated. There are tables for Tropic Marin Hydrometer.

This temperature correction is not the problem since that should only cause a small difference and not measure 28 salinity instead of 35. A lower quality could also cause a small difference but not again, it should not measure 28 anyway.

 

[KStatefan]

What is your hydrometer measuring? There are two Tropic Marin models one measures Specific Gravity and the other measures density. The density model is sold in Germany.

 

[Bramzor]

What is your hydrometer measuring? There are two Tropic Marin models one measures Specific Gravity and the other measures density. The density model is sold in Germany.

Now we are getting somewhere. FYI the ones I’m using are not from Tropic Marin, have them for a long time but only as a backup. But if there are differences in what it measures, I suppose that might be causing these differences.

 

[KStatefan]

Now we are getting somewhere. FYI the ones I’m using are not from Tropic Marin, have them for a long time but only as a backup. But if there are differences in what it measures, I suppose that might be causing these differences.

It would be worth verifying. I just remember a thread that someone bought a Hydrometer (i do not think that is the correct name when it measures density) while they were in Europe on vacation and did not know it was different.

 

[GARRIGA]

Hydrometer floated in tank water or external vessel? Seems unless tank water deep enough and can be made to stay still post turning off all pumps the hydrometer might present different values although I doubt at this level of discrepancy.

Buying the TM item might be a better start. Those seem to be highly accurate based on reviews but no personal experience. Just talking out loud

 

[BeanAnimal]

Now we are getting somewhere. FYI the ones I’m using are not from Tropic Marin, have them for a long time but only as a backup. But if there are differences in what it measures, I suppose that might be causing these differences.

What does the scale label on the hydrometer say? This is extremely important.
It should tell you the reference temperature.

 

[bushdoc]

TM hydrometer sold in US, which I have been using has got sg scale. It is calibrated at 25C or 77F. If your tank temp is different then you are supposed to use table for corrections.
It is my personal opinion, but backed by many reefers that TM hydrometer is highly accurate device if used properly.

 

[BeanAnimal]

Also - if memory serves me - the temp conversion chart at BRS is not correct.

 

[bushdoc]

Also - if memory serves me - the temp conversion chart at BRS is not correct.

I got table directly from TM

 

[BeanAnimal]

I got table directly from TM

Can you post here or a link please?

 

[redfishbluefish]

Can you post here or a link please?

I believe the table in Randy's article HERE is TM's table.

 

[BeanAnimal]

I believe the table in Randy's article HERE is TM's table.

I am pretty sure Randy's table matches the TM table, but wanted to make sure.
I have seen a lot of confusion recently on the TM hydrometer temp compensation and people using the wrong table.

 

[bushdoc]

Can you post here or a link please?

Sorry, I was wrong. I have and I am using Randy’s table, not TM and it seem to be the right one in my opinion.

 

[Bramzor]

FYI: The cheap hydrometer that I used has a range of 1,000 to 1,040 (and is a lot smaller in size) while the Tropic Marin Hydrometer has a range of 1,021 to 1,031 which is way better. I guess that's the "High Precision" in the Tropic Marin product.

So I guess not every salinity hydrometer is good "enough". Still a bit shocked that both of them where off by that amount.

 

[KStatefan]

I just use 0.00019 per 1ºF from Randy's Article to convert mine from the 77°F calibration temp.

I think this is the formula that is used in Hamza's Reef online calculator

corrected-reading = r * ((1.00130346 - (0.000134722124 * t) + (0.00000204052596 * t^2) - (0.00000000232820948 * t^3)) / (1.00130346 - (0.000134722124 * c) + (0.00000204052596 * c^2) - (0.00000000232820948 * c^3)))

r=S G reading
t=sample temp °F
c-calibration temp °F

I was wrong when I used the online calculator and compare to the formula there are a couple conversions that are different.