Cold room, refractomiter readings?

[HandofThanatos]

We had a cold front in FL and my room temp hit 65. I have a heater in the tank keeping at 79.

When I did a salinity test it read 1.030, but I let it sit for a bit and it went down to 1.025.

I guess my question is how do I know when I have let it sit to long or not long enough? If my room temp is 70 for example and the test is supposed to be reading it at 78-79 degrees am I functionally getting a false reading?

I have had no major issues recently, but my torches have not been fully expanding, and this morning it looked like I found the tip of one of their polyps in the sand bed so I just want to make sure salinity is not the issue.

Nitrates are 5, phosphate is .05 if that helps

 

[Ryan115]

Room temperature should not affect the corals as long as the tank temperature is not fluctuating wildly. What has the tank temperature been?
Letting the water sample sit should not really change anything as the tiny bit of water being used in a refractometer would equilibrate with the refractometer temperature rapidly, and it is the temperature of the refractometer that would mostly determine the temperature of the test.
Regardless, the temperature difference should not account for that much of a change in density. At most you should see a difference of ~0.002 with that change in temp. Here is a chart that shows you the temperate vs density relationship for different salinity water.

Is it possible that there was some residual salt on the optics that raised the salinity of the sample?
Also just to clarify, this was tested using an optical refractometer and not a hydrometer correct?

 

[HandofThanatos]

Correct optical refractometer.

But yeah, the tank has been a stable 79 degrees Fahrenheit. It was the temp of the refractometer that I was worried about. According to that chart the 1.030 reading at approximately 18 c would be equivalent to 1.027, but I was getting 1.025 after it warmed up in my hand.

The issue is the small sample cooling down on the tester and reading higher than it is I guess, but do I need a laser thermometer to check the temp of my refractometer lol.

I was more worried about too high or too low salt content in the tank effecting the corals than I was the temp itself though yes. I just want to make sure it isn't the salt because everything else seems in line.

 

[Randy Holmes-Farley]

An ATC (automatic temperature compensation) refractometer should read correctly at all the temps you mention. Is it an ATC type?

 

[Randy Holmes-Farley]

Room temperature should not affect the corals as long as the tank temperature is not fluctuating wildly. What has the tank temperature been?
Letting the water sample sit should not really change anything as the tiny bit of water being used in a refractometer would equilibrate with the refractometer temperature rapidly, and it is the temperature of the refractometer that would mostly determine the temperature of the test.
Regardless, the temperature difference should not account for that much of a change in density. At most you should see a difference of ~0.002 with that change in temp. Here is a chart that shows you the temperate vs density relationship for different salinity water.

Is it possible that there was some residual salt on the optics that raised the salinity of the sample?
Also just to clarify, this was tested using an optical refractometer and not a hydrometer correct?

Do not use such a chart this way. It is not appropriate if one is measuring specific gravity rather than density. Specific gravity changes far less .

 

[HandofThanatos]

An ATC (automatic temperature compensation) refractometer should read correctly at all the temps you mention. Is it an ATC type?

It says that on the case yes. So does that mean it should be the 1.025 that it finally settles at after a couple minutes? The instructions say to wait 15 seconds and it still reads 1.030 after that long and there is no way that can be right.

I appreciate the help.

 

[Randy Holmes-Farley]

How do you know it can’t be right?

Have you recently calibrated it?

 

[HandofThanatos]

How do you know it can’t be right?

Have you recently calibrated it?

Yes, I calibrated it, and I checked the salinity of the water I used for the water change before the cold snap and it was 1.025 as was the water in the tank the last time I had checked it.

Doubt it would increase 1.005 in 3 days especially after a water change that would bring it down.

 

[Ryan115]

Do not use such a chart this way. It is not appropriate if one is measuring specific gravity rather than density. Specific gravity changes far less .

Not to question you in any way. I may not have explained it very well originally.
But I was trying to use that chart to illustrate how the density changes depending on temp (though not to the extent of what was being questioned).

Could the temperature at which the refractometer was calibrated cause some of the issue? I know most houses in FL aren't kept ~20degC. So if it was calibrated when warmer and read when colder, it could be off. But that would be a difference in accuracy, not in precision between the multiple tests, so may not be relevant here.

 

[Randy Holmes-Farley]

Not to question you in any way. I may not have explained it very well originally.
But I was trying to use that chart to illustrate how the density changes depending on temp (though not to the extent of what was being questioned).

Could the temperature at which the refractometer was calibrated cause some of the issue? I know most houses in FL aren't kept ~20degC. So if it was calibrated when warmer and read when colder, it could be off. But that would be a difference in accuracy, not in precision between the multiple tests, so may not be relevant here.

I just did not want anyone to think that the posted graph reflects changes in specific gravity. While sg of seawater does change with temp, it looks nothing like that graph of density changes. It is a far smaller change since sg is seawater density divided by freshwater density at the same temp. Thus to a large extent those two cancel each other out as temp changes.

 

[Randy Holmes-Farley]

Despite manufacturer claims otherwise, an ATC refractometer should be able to be calibrated at any temp within its ATC range.

 

[Randy Holmes-Farley]

Yes, I calibrated it, and I checked the salinity of the water I used for the water change before the cold snap and it was 1.025 as was the water in the tank the last time I had checked it.

Doubt it would increase 1.005 in 3 days especially after a water change that would bring it down.

well, the refractometer may be defective, may need recalibrating again, may have been off for the first measurement, etc. lots of possible explanations. Temperature should not be one of them if the ATC is working correctly, but it may not be.

Try calibrating and measuring at the same temp.