bad readings with refractometer at cold temps?

[astroboy]

Greeting WAMAS comrades,

 

Over the winter months I mixed and kept my salt water in an unheated shed. I kept my refractometer outside at the same temperature as the water and mixed in enough salt and Mg (to 1600) to give a reading of 35. I normally do 30% water changes. A few days after the water change a few corals were looking a little droopy so I checked the salinity the the same refractometer, which was at room temperature. After waiting a while for the water and refractometer changes to equalized, I got a salinity of 30. This has happened three times now. I have my limitations, but I cant believe I misread the salinity in the mixing shed three times in a row. I'd calibrated the refractor meter using good RO/DI water.....

 

Just this morning I checked the salinity of a batch I mixed up six weeks ago when it was still pretty good, and I'd swear the refractometer read 35 when I mixed it up, but today, at around 70 degrees, it read only 30.

 

I was wondering, do these portable refractors only work for temperatures around 70-80? After all, the metals the instrument itself is made of will contract with cold to differing degrees, and I'm wondering if once you're below say, 45 degrees, if you're simply going to get inaccurate (low) measures of salinity.

 

Does anyone have any insights/experience on this?

 

Thanks,

 

Mark

[Origami]

Mark, first, unless you have an NSW refractometer, you probably have a saline refractometer that should be calibrated using a 35 PSU (ppt) salinity standard, not RO/DI.

 

This seems to indicate that a 10 degree C temp change can change the index of refraction of salt water by as much as a 5 ppt salinity change.

 

I thought that many of our refractometers were ATC (automatic temperature compensating), but this may either not be the case or possibly intentional mislabeling.

 

Have you tried bringing the refractometer and sample indoors and measuring again at the new temperature?

[treesprite]

Mine is supposed to be ATC, but I read something about it's limitations... I can't remember what it said now - it's either only for atmospheric temp OR water temp - I can't remember which of them it is. I would make sure the air temp and water temp for both the tank and the mix is the same to prevent discrepancies.

[Coral Hind]

Most refractometers are desinged to take readings at around 70F, that is the units temperature and not the sample temperature. The ATC function of the refractometer is a very basic principle of thermal capacitance. The mass of the glass prism and the units aluminum frame help to equalize the temperature when the test sample, just two drops, are added to the prism.

 

Reefers must remember that the ATC function does have limitations. The units only have an average capabality of raising or lowering the temperature by about 20F and that is if adding only two drops. I have seen reefers dunk the meter into the tank to get a water sample. That could slightly raise the temperature of the unit which was calibrated at room to temperature to the warmer water temperature of the tank. Also remember that bringing a refractometer that was stored in the garage in say winter time into the house to quickly take a reading will also not be accurate as the units cooler mass will affect the samples temperature. Keeping the unit at the same average room temp as when it was calibrated and close to the design temperature is key.

[treesprite]

Thanks for the info. I was seeing others just dumping theirs in the tank and decided that if they were able to do it, I could be lazy and do it that way too. Guess I'll go back to dropping.

[astroboy]

Mark, first, unless you have an NSW refractometer, you probably have a saline refractometer that should be calibrated using a 35 PSU (ppt) salinity standard, not RO/DI.

 

This seems to indicate that a 10 degree C temp change can change the index of refraction of salt water by as much as a 5 ppt salinity change.

 

I thought that many of our refractometers were ATC (automatic temperature compensating), but this may either not be the case or possibly intentional mislabeling.

 

Have you tried bringing the refractometer and sample indoors and measuring again at the new temperature?

 

My first post was a little ambiguous. Here's the short version:

 

A couple months ago I mixed up a batch of saltwater at 35 ppm, in a shed outdoors when the temperature was 40-45 degrees. I had kept the refractometer (which has ATC) in the shed, so it was at the same temperature as the water: about 45 degrees. I had waited a couple minutes for the sample and instrument to reach the same temperature.

 

A few days ago I measured a sample from the same batch, when the water and refractometer were at 75-80 degrees. I waited a few minutes to make sure the temperatures of the sample and refractometer had reached equilibrium. The reading was now 30 ppm.

 

Apparently refractometers only work when the instrument is within a certain temperature range, say 60-90 degrees.... In fact, I was just now looking at drsfostersmith, which I think is where I bought it from, and they claim a temperature range from 50-86.

 

However, as origami2547 has pointed out, a 10 degree Celsius change in temperature changes the index of refraction of water about as much as a 5 ppm change. In view of that, it seems my refractometer was giving fairly accurate results: the cold water temperature was the main problem!

 

Thanks everyone, for the information!

Edited May 22, 2011 by astroboy