Calibrating refractometer and temperature compensation.

[LouH]

I have a refractometer that states that it compensates for temperature. My calibration fluid, Pinpoint Salinity Monitor, states that the fluid is 53.0mS @77 Deg.F. I assume (possibly a bad assumption), that 53 mS is equivalent to 35ppt. Assuming that it is, should I heat the calibration solution to 77 degrees, allow it to float in my tank until it hits the tank temperature, or just calibrate at room temperature?

Lou

 

[Percula9]

Do not heat the calibration fluid. As stated the refractometer will compensate, 53mS is 35ppt.

 

[LouH]

Great. Thanks

 

[disc1]

Once you put a drop of anything on the refractometer it is going to rapidly come to the same temperature as that crystal. So no matter what temp you have the solution at you are going to be measuring it at the temp of the refractometer.

 

[Randy Holmes-Farley]

I have a refractometer that states that it compensates for temperature. My calibration fluid, Pinpoint Salinity Monitor, states that the fluid is 53.0mS @77 Deg.F. I assume (possibly a bad assumption), that 53 mS is equivalent to 35ppt.

Lou

That would be an incorrect assumption in general (like for a pure sodium chloride or potassium chloride solution), but for the Pinpoint solution (and real seawater) it happens to be true.

If your refractometer has ATC, then it is true for all temperatures that the ATC works properly.

 

[Whitebeam]

This threa has just made me check something that I'd been wondering about for a while. Why (I wondered) do the instructions tell you to calibrate with RO at 20C? Surely if it has ATC then you can calibrate with RO at any temperature within the ATC range and the job will be a good 'un?

It seems not! Based on what I read on the Aquarium Solutions website here http://www.theaquariumsolution.com/d-d-refractometer-calibration, it seems that the ATC will sort out a salinity reading in the 'normal' range for any temperature, but the zero will appear off for any temperature other than 20C.

I hadn't realised this - I assumed that the ATC would pull all points (including 0ppt) onto the correct line at all temperatures. Now I know better!

I wonder if this is why some people get perfect readings with this refracto when calibrating with RO and others get it 1.5ppt out? When I got a perfect reading when calibrating with RO and then testing with a reference solution, perhaps I was just 'lucky' that I had miscalibrated with RO a couple of degrees off 20C and this made things line up perfectly. I fear I detect another round of calibration testing coming up here!

Or perhaps I'll just cal with a 35ppt reference and be done with it! :deadhorse:

Peter

 

[Randy Holmes-Farley]

This threa has just made me check something that I'd been wondering about for a while. Why (I wondered) do the instructions tell you to calibrate with RO at 20C? Surely if it has ATC then you can calibrate with RO at any temperature within the ATC range and the job will be a good 'un?

It seems not! Based on what I read on the Aquarium Solutions website here http://www.theaquariumsolution.com/d-d-refractometer-calibration, it seems that the ATC will sort out a salinity reading in the 'normal' range for any temperature, but the zero will appear off for any temperature other than 20C.

I hadn't realised this - I assumed that the ATC would pull all points (including 0ppt) onto the correct line at all temperatures. Now I know better!

I wonder if this is why some people get perfect readings with this refracto when calibrating with RO and others get it 1.5ppt out? When I got a perfect reading when calibrating with RO and then testing with a reference solution, perhaps I was just 'lucky' that I had miscalibrated with RO a couple of degrees off 20C and this made things line up perfectly. I fear I detect another round of calibration testing coming up here!

Or perhaps I'll just cal with a 35ppt reference and be done with it! :deadhorse:

Peter

I never understood this as well, and I'm not convinced this is accurate. The correction in refractive index for temperature is not that different for fresh water and seawater, so I don't see how this can be true. If true, it also implies the correction is not accurate across the whole range of measurement.

This is my blurb from my refractometer article, but it doesn't address the specific question, just how the ATC works:


Temperature and Refractive Index: ATC
It turns out that refractive index is highly dependent on temperature. When using a refractometer that does not account for this effect, temperature changes can be a large source of errors. Most liquid materials expand slightly when heated and shrink when cooled. For a given material, light can pass through it more easily when it is expanded, so the index of refraction falls when materials are warmed. However, the magnitude of this effect is different for every material, and refractometers must somehow take this into account.

Handheld refractometers account for temperature by employing a bimetal strip inside them. This bimetal strip expands and contracts as the temperature changes. The bimetal strip is attached to the optics inside the refractometer, moving them slightly as the temperature changes. This movement is designed to exactly cancel temperature's effects on refractive index, and generally does a very good job IF the refractometer is designed to cancel out the temperature effects of the specific material being analyzed.

Because many refractometers are designed to use aqueous (water) solutions, the bimetal strip can be designed to account for the change in refractive index of aqueous solutions, although it may not be perfect in some situations because salts and other materials in the water can change temperature's effects on refractive index by a small extent (possibly to a larger extent for very concentrated solutions, like 750% sugar in water, but seawater is not in that category). Other details of this compensation may cause it to be imperfect (for example, the bimetallic strip provides a linear correction while the true temperature effect may be nonlinear), but those issues are beyond the scope of this article, and in general automatic temperature compensation (ATC) is a very useful attribute for aquarists using refractometers.

 

[Plato]

When I calibrate my refractometer with pinpoint solution I use an inferred thermometer to make sure the refractometer and the calibration fluid is at 77. Any one remember the anal retentive chef from Saturday Night Live.

 

[Randy Holmes-Farley]

You would get along well with Peter (Whitebeam).

 

[Whitebeam]

Thanks Randy! :worried:

Incidentally, what are the calibration instructions for the Vital Sine (and the other brand, that I forget just now) refractos that you chaps in the US rave about and we've never heard of? Do they say calibrate with RO at such-and-such temperature, or can you do it at any old temp?

Peter

 

[Randy Holmes-Farley]

I've not seen its instruction manual to know. Its online instructions are not that clear on what it suggests for temperature of calibration.

 

[Bryan]

My refractometer instructions tell me the refractometer must be calibrated at 20 Celsius and it is accurate from 10-30 Celsius once calibrated. Don't think the calibration fluid temperature would matter at all, as it would quickly adjust to the temp of the refractometer quickly as others have pointed out.

Cheers

 

[LouH]

Temperature and Refractive Index: ATC
It turns out that refractive index is highly dependent on temperature. When using a refractometer that does not account for this effect, temperature changes can be a large source of errors. Most liquid materials expand slightly when heated and shrink when cooled. For a given material, light can pass through it more easily when it is expanded, so the index of refraction falls when materials are warmed. However, the magnitude of this effect is different for every material, and refractometers must somehow take this into account.

Handheld refractometers account for temperature by employing a bimetal strip inside them. This bimetal strip expands and contracts as the temperature changes. The bimetal strip is attached to the optics inside the refractometer, moving them slightly as the temperature changes. This movement is designed to exactly cancel temperature's effects on refractive index, and generally does a very good job IF the refractometer is designed to cancel out the temperature effects of the specific material being analyzed.

This piece puzzles me. I just weighed my refractometer, and it weighs 190 g. At the most, the droplet on the glass is 1-2 grams. Now, the laws of thermodynamics tells me that the mass differential is so great that the temperature of the sample drop will very quickly match the temperature of the refractometer (68-70 degrees in most homes). Is the truth more that the refractometers are accurate because the sample temperature is fairly fixed to be around room temperature rather than the refractometter having a very good system of adjusting the monitoring device to the temperature of the sample? I would be curious to see a side by side comparison or a temperature compensating refractometer with one that does not.

As a side note, I often find that my temperature compensating refractometer typically drops its reading by about 0.001 S.G if I dip the tip of the refractometer in the sump for 10 seconds or more as compared to simply putting a drop on the sample glass. That observation seems to agree with this statement: "For a given material, light can pass through it more easily when it is expanded, so the index of refraction falls when materials are warmed." This would tend to support my thermodynamic statement as well.

So, considering all of this, I would like to again propose submersing the test solution and and refractometer in the tank (in waterproof containers of course) so that they match the tank's temperature and then calibrate the device.

Thoughts?

Lou

 

[Randy Holmes-Farley]

It is certainly true that a drop of water will quickly come to the refractometer temperature, which is generally room temp. I would not want to put a refractometer repeatedly into seawater, as they may not be designed for it.

I think much of the ATC effect is relating to different room temperatures, not sample temperatures that are different than the room temperature.

 

[flyfisher2]

Having issues with refractometer vs. Hydrometers. My LFS sold me an RHS-10ATC refractometer(under their own brand) to replace the Coralife hydrometer I was using. I was told to ignore the instructions which called for calibration using distilled water and use the calibration solution which I find is totally off or is dependent on room temp. As it stands two Coralife hydrometers are getting a 1.027 and 1.028 while the refractometer is showing 1.022 with the calibration solution. If i calibrate with distilled and or RO water at 0 I ended up at 1.027. So am I to assume the calibration solution is faulty even though my LFS tells me it's dead on with their refractometer using the same type of solution?

I don't want to be anal but the spread in range seems large enough to affect the life in the aquarium?

Please note room temp is 75 degrees F and calibration fluids read 73 F. Aquarium is at 79F

 

[shifty51008]

if I were you I would order some pinpoint calibration fluid and go by that, not the LFS's

 

[shifty51008]

or you can make your own fluid
http://reefkeeping.com/issues/2004-06/rhf/index.php

 

[Randy Holmes-Farley]

Having issues with refractometer vs. Hydrometers. My LFS sold me an RHS-10ATC refractometer(under their own brand) to replace the Coralife hydrometer I was using. I was told to ignore the instructions which called for calibration using distilled water and use the calibration solution which I find is totally off or is dependent on room temp. As it stands two Coralife hydrometers are getting a 1.027 and 1.028 while the refractometer is showing 1.022 with the calibration solution. If i calibrate with distilled and or RO water at 0 I ended up at 1.027. So am I to assume the calibration solution is faulty even though my LFS tells me it's dead on with their refractometer using the same type of solution?

I don't want to be anal but the spread in range seems large enough to affect the life in the aquarium?

Please note room temp is 75 degrees F and calibration fluids read 73 F. Aquarium is at 79F

Does the calibration fluid indicate what it is or what it is intended for? Not all calibration fluids can be interchanged between different measurement devices.

If it is just an LFS made product, I'd buy one or DIY before risking your tank on it.

 

[flyfisher2]

Dr. Holmes, Calibration solution reads: Refractometer Calibration Solution, calibrate to 35PPT or 1.026 specific Gravity.
I was just reading your findings with the two hydrometers I'm using.
As I recall the deep six read on the lower end of the scale?
I also read elsewhere that if I calibrate with distilled water to a reading of '0' and then added the calibration solution I should be in the neighborhood of 1.024. I tried this twice and ended up at 1.032. That being the case it would explain the reading of 1.022 on refractometer vs 1.027 on the hydrometers. I hope I'm interpreting theses readings correctly and arriving at a correct conclusion?

 

[flyfisher2]

Thanks for the thread shifty 51008 I read it I'm considering making my own solution if not I'll just wait until Saturday and drive down to That Fish Place (two hour drive) in Lancaster and pick some up. Gives me an excuse to go there. lol