Refractometer temp parameters

[NWDiver]

I was wondering how accurate the temperature correction is for the chinese refractometers. I have two tanks one at ~55F and one at 77F. My room temp is usually around 60F. If I let the water sit for a minute or so on the prism will it give an accurate measurement? How much does it matter what the temperature is when you do the calibration?

 

[lvsuckerfish]

there is a chart to show what the salinty is for different water temps the tru reading would be all of the above as long as the tank is stable at that temp I believe the higher the water temp the higher the salnity will raise. i will search for the chart and post a link if I can find it.

 

[cyberdeth]

there is a chart to show what the salinty is for different water temps the tru reading would be all of the above as long as the tank is stable at that temp I believe the higher the water temp the higher the salnity will raise. i will search for the chart and post a link if I can find it.

http://www.americanmarineusa.com/salinityconversion.html

 

[NWDiver]

Thanks guys, I am aware of the effect of temp on salinity, I wondered how accurate the refractometers are with reguard to temp. I can see that if I test the water quickly and the refractometer is cold, the readings are whacked, my question is more along the lines of "If I let the water cool to the temperature of the refractometer will I get the correct reading?". If so does it work reasonably accurate both up and down the temperature scale, and lastly does it matter at what temperature I do the calibration?
I suppose I could get a non-correcting refractometer and do the conversion...

 

[Boomer]

This table http://www.americanmarineusa.com/sal...onversion.html is way off and should not be used.

I have two tanks one at ~55F and one at 77F

Is this a ATC refract, if so they are good 10 ( 50F) to 30°C ( 86F). So, you are pushing it IMHO with that 55 F

You are best off to start here

Refractometers and Salinity Measurement
http://reefkeeping.com/issues/2006-12/rhf/index.php


For converting go here
http://www.hamzasreef.com/Contents/Calculators/SalinityConversion.php

 

[lvsuckerfish]

sorry thats along the lines I was getting at if you test cold your measurements aren't tech. out of wack thats what the 55 degree tank is at if you warm the water up then it will be off. what refractometer do you have I have a marine depot brand and also a digital which is still not hooked up. and on my handheld I believe it says room temp. NOT 100% on that thought.

 

[Boomer]

I need to know Who's you have got a link ?

It you measure either of the two tanks and let the water sit on the refract for a couple of min so the refract glass and water sample on refract are the same you will be fine. You do not want to take 55 F water and put it on a refract that may have a room temp of ~70 F and try of an instant reading

 

[NWDiver]

Thanks guys!
I have a marine depot ATC refractometer.
Thanks for the reply Boomer, I grew up out your way (Grand Rapids MN).
I read through Randy's paper but the following is not what I have experienced.
Randy says:
"This calibration should ordinarily be carried out at room temperature using an ATC refractometer. The directions with some ATC refractometers insist that the calibration be carried out at a specific temperature, but I've never understood how that could matter and I would not worry about it. If the refractometer is not an ATC refractometer, then careful temperature control or correction is necessary, and such corrections are beyond the scope of this article."

And here lies my problem. with the room temp at ~60F I read 40ppt. When I warm the refractometer up to ~70F I get 35ppt which is very near what the POS float reads.
Is the ATC refractometer that I have junk? User error? It did not come with directions that called for a specific room temperature.
I will gladly purchase any refractometer that you suggest, though I would rather not have to heat it up to get a reasonably accurate measurement.
I would rather not heat the room to 70ish as it makes the coldwater tank chiller work too hard, it seems to be more cost effective (so far) to warm the reef.
If it helps, I can bring a calibrated surface temp meter home and get more exact measurements.
I understand that I might not be able to get an accurate measurement on the cold tank. I do 50% water changes every two weeks with local water, it shouldn't have drifted much. the livestock in the tank is pretty hardy in any case, so close is fine with me.

 

[lvsuckerfish]

Boomer is the specific gravity different @ 55 then 70 degrees?

 

[Boomer]

Yes it is but a refract DOES NOT measure Sg. It just converts a RI value into Sg for std NaCl Salinty (not seawater, see below). Meaning, if the temp of the sample and refract are the same the Sg value seen in the window will be or should be the same. If it reads 1.026 at 77F it should read 1.026 at 55 F. True Sg, where it is very affected by temp is in floating hydrometers. If a floating hydrometer reads 1.0264 at 77 F = 35 ppt, the water will still be 35 ppt @ 80F but the Sg will be 1.025.

http://reefkeeping.com/issues/2004-07/rhf/conversion.htm


Hit the calculate button as is and the Salinity = 35 ppt

Now;
Leave this as is "Hydrometer calibration temperature" 77F and 77F

In the next box type in 80 F and the next type in 1.02582 = 35 ppt

So, if you are using a floating hydrometer calibrated to 77 F and are using it in 80 F water and you want NSW salinity @ 35 ppt, it needs to read 1.025, which is equal to 1.0264 when the water is 77F.


NW

And here lies my problem. with the room temp at ~60F I read 40ppt. When I warm the refractometer up to ~70F I get 35ppt which is very near what the POS float reads

What do you mean warm it up ? Are you warming up the refract to 70F and then testing the water which is 60 F or are you warming up the refract with the water sample on it, so both 70 F ? And what are you calling a POS float exactly ? I want a name. On a proper ATC they should read the same at 60 F and 70F with ATC. But you need to wait a coupling of min to make sure the water temp and refract glass temp are the same. If you put 55F water on a refract that is 70F it will be off if you try to read it immediately.

Did you calibrate this refract at 25 C and did you use a PinPoint 53mS to calibrate it with. You should, NOT being using RO/DI water to cal it with, as it will be off. Refracts are set to measure salt water and NOT seawater. The 53mS will allow it to read correctly in seawater accurately, for a narrow range of Salinity from ~28 ppt - 42 ppt.

This is the best one for us

http://thefilterguys.biz/refractometers.htm

 

[NWDiver]

My refractometer is here : http://www.marinedepot.com/Captive_Purity_Refractometer_with_Calibration_Dial_Refractometers_for_Saltwater_Aquariums-Captive_Purity-CP2111-FITEOPRF-CP2113-vi.html
For the sake of keeping things easy let's just deal with the reef tank for now. I agree with what you suggest Boomer, that if I were to let the refractometer and water match temperatures the salinity should read good. I tested this theory last night, and it was not the case with my refractometer. I had no way to accurately measure room temperature, but it was ~59F by the cheap thermometer in there. The water in the reef is quite accurately measured between 77F and 78F. I droppered water onto the refractometer and waited for 1 minute with a result of 40ppt. I cleaned the refract and tested the same way with the same results. I then droppered another sample and waited 5 minutes with similar results. I say similar because I did not closely read this measurement (40ppt +or-1), it seemed unimportant at the time. I then brought the refractometer into a warm room ~71F and left it there for about 30 minutes. I again tested the 77. 7F water with the refractometer at ~71F I got 35ppt.
After reading a bunch and with what you have explained, I have to believe that the refractometer is to blame.
To answer your questions directly:
I was warming the refract to ~70F and letting the water sit for ~1 min.
POS Float = my marineland hydrometer
I did indeed calibrate the refract with American marine pinpoint salinity calibration fluid 53 mS. BUT I did not pay any attention to the room temperature. It would likely have been ~60F.
I should order some new filters anyway, so I will get the refractometer from the filterguys next week.
Given what has happened so far, how would you suggest I test further to get accurate results?
How would I apply that to the cold water tank?
I can use lab grade equipment if need be, to set an accurate experiment (My office is in the university chemistry building )

 

[Boomer]

Then my conclusion is the refract is not doing its ATC job or has a poor ATC range. It seems to be working at ATC for 71 F refract and 77 F water, as you gt 35 ppt. I said earlier you may be pushing it with that 55 F. I talked to a refract expert on the phone and he agrees with me something is wrong. He said it is either the refract is not working right for ATC or it is the user doing something. I do not think it is you

Given what has happened so far, how would you suggest I test further to get accurate results?

That is easy Measure the salinity that you want for the 55 F tank in the 71 F room. Lets say you want the 55 F @ 1.026 Sg = 35 ppt. Make up a 1 l sample of salt mix so it reads 1.026 @ 71F in that 71 F room. Then take a cup of that sample, that is all that is needed and put it and the refract in the 55 F room and let both sit for 1/2 hr, then test the Sg. That will be the Sg you want that refract to read for that 55 F tank when the refract is also ~ 55 F. That means when you use this refract for the 55 F tank or the 77 F tank you are going to have to leave the refract in that room until it cools down or warms up i.e., 1/2 hr or so before testing.

 

[NWDiver]

Thanks Boomer!
I can do that.

 

[NWDiver]

With a new refractometer of course

 

[NWDiver]

You nailed it again Boomer
The Refractometer was toast. I ordered the Vee from the Filter Guys and all is fine.
Speaking of the filter guys- Great service!
I rather miss northern MN. In the summer anyway...
Calibrated both at 68deg F with pinpoint 53 mS.
Side by side at ~60deg F again using pinpoint, after 20 minutes new refract reads dead on, the "old" refract reads 1.030.
I can't quite figure how my cold water tank is not creeping up in salinity. I don't top off as evap in near nil on it and I change 40% every two weeks or so. Still, after a year and a half I would not be expecting to see 1.024.
The reef tank was a little low but I'm bringing it up slowly and nothing seems bothered so far.
Thanks again for all that you do!
-Todd

 

[Boomer]

Glad to hear things turned out OK Todd