With a refractometer calibrated to RO water,and a temp of 78,and a reading of 1.022 on refractometer would it really be 1.022? Or do I have to do any math?
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Question for the scientists
- Thread starter btodd43
- Start date
NOTE: I am NOT a scientist nor did I stay at a Holiday Inn last night 
but...it depends on the refractometer. The one I have automatically adjusts for temp (which is why I bought it). Do you have the instructions that came with the unit as maybe it'll tell you what's what.
What brand do you have?
but...it depends on the refractometer. The one I have automatically adjusts for temp (which is why I bought it). Do you have the instructions that came with the unit as maybe it'll tell you what's what.What brand do you have?
btodd43":1q9fmdnx said:
The S.G. of an ATC refractometer will not be accurate except for the temp that it was calibrated at (77F, 60F, etc.).
This is immaterial in the hobby because it is Salinity that is important when keeping saltwater organisms. Just ignore the Specific Gravity scale and use the Salinity scale. Salinity is all that matters for us. 35 - 37ppt is the normal Salinity for tropical reef organisms.
Once again - If you have an ATC refractometer, ignore the S.G. scale, use the Salinity scale. Salinity matters, S.G. is irrelevant.
I thought ATC stood for Automatic Temperature Compensation, which means that the temperature wouldn't effect the reading?
And not to be difficult, but as far as I know the only actual measurement a refractometer makes is of Specific Gravity. Salinity, I assume, is then extrapolated from that reading.
And not to be difficult, but as far as I know the only actual measurement a refractometer makes is of Specific Gravity. Salinity, I assume, is then extrapolated from that reading.
Guy":2pbqnhon said:
what utter nonsense...
SG is a perfectly acceptable measurment of suitability of aquarium water...
How many people do you know that blend their own saltmix... yes, about NONE
as a measure of dissolved ionic salts when making up pre blended commercial sea water, a good quality hydrometer will work just fine, and when the reading is temp adjusted then it will be very accurare
try a real reading in a good quallity calibrated hydrometer that is thermally adjusted and use this http://www.reefs.org/survey/salinity_conversion.html and compare with your refractometer (which probably isnt calibrated) the difference in results will be , at best neglegable
Wait....Salinity IS what matters.
And yes, SG is a perfectly acceptable way to measure it. But probably only until something better comes along. SG measures total dissolved solids, so is accurate as long as all you have mixed is water and salt.
I'd bet if someone came out with an affordable way for to measure actual salt ions in their water, that would soon become the gold standard.
I would also have to say that I do not doubt that a good quality calibrated hydrometer that is thermally adjusted is very comparable to a refractometer. But I highly doubt it compares to the $6 version available to the home hobbiest.
And yes, SG is a perfectly acceptable way to measure it. But probably only until something better comes along. SG measures total dissolved solids, so is accurate as long as all you have mixed is water and salt.
I'd bet if someone came out with an affordable way for to measure actual salt ions in their water, that would soon become the gold standard.
I would also have to say that I do not doubt that a good quality calibrated hydrometer that is thermally adjusted is very comparable to a refractometer. But I highly doubt it compares to the $6 version available to the home hobbiest.
Yikes- Whoa, Now I am getting confused..... It's a cheapo refractometer probably not calibrated for temp. I keep my tank at 78 (as best as I can) and just put the water on the refractometer and the dark line is at 1.022 so I'm thinking is it where I want to be? Would it be safe to say don't go higher due to the fact that it's not calibrated? My tank inhabitants seem to be very happy-..... I had a red sea Hydrometer and was keeping it at 1.024/25 but one day I came home after a water change the day before and half my tank was dead so the only thing I thought could be wrong was a incorrect reading from my hydrometer when I mixed the water change water ( I was getting different readings from time to time) So I went out and bought the Refactometer and checked the water it was at 1.028! NICE! So from then on I use the refractometer and keep it at 1.022 To be safe.... I guess I just can't trust things anymore..... ??
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To be honest, I can't believe the temperature correction part is all that big a deal. When I first used a refractometer, it was in a clinical chemistry lab (many, many years ago) and we never did any temperature corrections that I recall. The only calibrations we performed were to verify that pure deionized water tested out at 1.000.
The real issue is that the refractometer is a much more reliable way of measuring than your hydrometer, because you will get a much more consistent result with the refractometer.
FWIW I keep my SG at 1.025.
The real issue is that the refractometer is a much more reliable way of measuring than your hydrometer, because you will get a much more consistent result with the refractometer.
FWIW I keep my SG at 1.025.
ickypimp":1vy68auf said:
The discussion was about refractometers and that is what my comment was in reference to. I'm not sure why you spun off on mixing salt and hydrometers. The S.G. scale on a refractometer won't normally be accurate, the Salinity scale will be. My point was to ignore the S.G. scale and use the Salinity side.
OK, I don't want to get into the above debate, but here is where you can get all the scientific equations that you could ever want about the relationship between salinity, density, specific gravity, etc.... Which is what I think you were after in the beginning...
http://www.cwr.uwa.edu.au/cwr/teaching/ ... uation.pdf
HTH
http://www.cwr.uwa.edu.au/cwr/teaching/ ... uation.pdf
HTH
My refractometer's instructions say to let the water set on the refractometer for 30 seconds before taking a reading. This ensures that the water and the prism are at the same temperature.
But, from day to day, the temperature of the refractometer itself could change. Let's say its 78F in your kitchen on Monday and you calibrate it using distilled water. Then, on the next day, you take a reading in your kitchen, but this time it is 82F. Won't both scales (SG and SC) be off because they were calibrated @ 78F?
Seems to me that when they say its temperature correcting, they may be asusming that the temperature of the refractometer itself stays constant across readings.
Thus requiring a mathematical adjustment taking into account the gradient between the refractometer's calibrated tempearture and the temperature at the time of the reading...
Another approach would be to recalibrate the refractometer every time you use it.
Or maybe I'm missing something?
-ash
But, from day to day, the temperature of the refractometer itself could change. Let's say its 78F in your kitchen on Monday and you calibrate it using distilled water. Then, on the next day, you take a reading in your kitchen, but this time it is 82F. Won't both scales (SG and SC) be off because they were calibrated @ 78F?
Seems to me that when they say its temperature correcting, they may be asusming that the temperature of the refractometer itself stays constant across readings.
Thus requiring a mathematical adjustment taking into account the gradient between the refractometer's calibrated tempearture and the temperature at the time of the reading...
Another approach would be to recalibrate the refractometer every time you use it.
Or maybe I'm missing something?
-ash
This is, understandably, a confusing topic... ...here is my take on it... ....I'm sure someone smarter than me will correct me if I screw any of this up... :wink:
Our primary interest is to measure salinity levels of our saltwater. A refractometer gives you a salinity reading based on the refractive index of salt dissolved in water. The hydrometer does it by measuring specific gravity.
With many hydrometers you'll have to compensate to get an accurate indication of salinity via the SG measurement as outlined in this article by Randy Holmes-Farley: http://www.reefkeeping.com/issues/2004-07/rhf/index.php This temp correction procedure (based on SG-temp relationship) would not directly apply to refractometers (based on refractive index).
The refractive index and SG are both temperature dependent, but not likely to the same extent, which is why I think Guy (correct me if I'm wrong here Guy) is saying to pay attention to the salinity scale and not so much the SG scale. The ATC will automatically adjust for the sample/ambient temps giving you the correct salinity reading (based on refractivity), but not necessarily the correct SG reading.
So, when using an ATC refractometer, just take your sample, let it stand on the prism for at least 30 seconds to let temps equalize, and then read the salinity (we are generally shooting for S=35 in a reef tank). That's it.
If you want to check your refractometer to ensure it's giving you an accurate reading, here's an article by Randy Holmes-Farley that discusses a DIY calibration solution:
http://www.reefkeeping.com/issues/2004-06/rhf/index.php
HTH
Our primary interest is to measure salinity levels of our saltwater. A refractometer gives you a salinity reading based on the refractive index of salt dissolved in water. The hydrometer does it by measuring specific gravity.
With many hydrometers you'll have to compensate to get an accurate indication of salinity via the SG measurement as outlined in this article by Randy Holmes-Farley: http://www.reefkeeping.com/issues/2004-07/rhf/index.php This temp correction procedure (based on SG-temp relationship) would not directly apply to refractometers (based on refractive index).
The refractive index and SG are both temperature dependent, but not likely to the same extent, which is why I think Guy (correct me if I'm wrong here Guy) is saying to pay attention to the salinity scale and not so much the SG scale. The ATC will automatically adjust for the sample/ambient temps giving you the correct salinity reading (based on refractivity), but not necessarily the correct SG reading.
So, when using an ATC refractometer, just take your sample, let it stand on the prism for at least 30 seconds to let temps equalize, and then read the salinity (we are generally shooting for S=35 in a reef tank). That's it.
If you want to check your refractometer to ensure it's giving you an accurate reading, here's an article by Randy Holmes-Farley that discusses a DIY calibration solution:
http://www.reefkeeping.com/issues/2004-06/rhf/index.php
HTH
ChrisRD":22epkpdo said:
My bad.. sorry, wasnt meant to be un civil
Guy":22epkpdo said:
once again sorry... misinterpreted from the offing..just heard so much "dont buy a hydrometer, buy a refractometer..."
you made a comment to the effect of"SG is not relevent salinity is..." thats the bit i picked up on, as most peeps using refratometers (that i know) tend to talk salinity i guess i added 2+2 and came up with 6
I've been known to get 6 or 7 from that as well. I agree with you that S.G. + temp is a perfectly acceptable way to determine the salt content of the water.
Hydrometers measure S.G. and Refractometers derive Salinity from the refractive index. Adding a Salinity scale to a hydrometer and adding a S.G. scale to a refractometer are equivalently deceptive IMO.
btodd43":1bj1cpe9 said:
Close. :wink:
Ideally the salinity should read about 35. A salinity of 35 could coorespond with a variety of SG values depending on temperature of the sample (including 1.025, 1.026, etc.). Don't worry too much about the SG side of the scale, just go by the salinity side when using your refractometer.
HTH
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