View Full Version : Who is wrong, LFS or me?

02/27/2008, 03:03 PM
I bought a refractometer (sp) a few weeks back and also purchased calibration fluid. I calibrated it and tested my water (1.024). I was going to do a water change so I checked the water I picked up at the LFS and it read 1.021. I also tested tap water and it was not 1.000, more like .998.
I had the LFS test and give me a sample of their water, they tested it at 1.023 and it tested on mine at 1.021.

2 possible errors-

1)The LFS is off (and the reading of .998 is due to my refractometer being calibrated at a much higher level)

2)I am .002 low on all my tests because the calibration fluid I bought is wrong.

02/27/2008, 03:09 PM
good question

I have heard that some salinity calibration fluids can be off on refractometers. I'm guessing that not all densities refract the same, and therefore, a calibration fluid that may be good for a hydrometer may not be as accurate for a refractometer.

02/27/2008, 03:44 PM
I would calibrate your refractometer with either some RO water or even better distilled water. After that test again and see if there is a difference. I know when I bought mine it said to calibrate it with distilled water.

02/27/2008, 03:53 PM
I've never used calibration fluid for a refractometer. Just always zeroed it out with RO water. I'd say it looks like your calibration fluid that's out of whack.

02/27/2008, 04:02 PM
I agree... I think your readings are a little bit wacky ... tap water at .998?? Pure water is 1.000 ... so that's some damn pure tap water you have there! Heh.

Tap water will probably read a few points above 1.000 depending on TDS.

02/27/2008, 04:03 PM

Everything you ever wanted to know, but didnt know to ask.

Calibrating with RO/Di water is not the best method to calibrate your refractometer. If you do this, your readings will be slightly off when testing your tank water. The article explains it better than I ever could.

If you used a calibration fluid to set your refractometer, I think the LFS is probably the one with the incorrect readings. Frequent use and unknown calibration methods combine to make a good case for the LFS refractometer being slightly off.

02/27/2008, 04:29 PM
Dont you know by now LFS'S are always right...just ask them!

02/27/2008, 04:48 PM
airinhere ... + 1.

You always calibrate w/ solution, never with RO. despite the instructions saying so. Chances are your LFS calibrated with RO so theirs is reading .002 high at 1.023, the same as your's is .002 low with tap.

02/27/2008, 05:05 PM
I used to calibrate mine with distilled water until I read about using calibration fluid. So I bought some and to my suprise it was way off. I thought all this time my tank water was 1.026 but was actually 1.021. I confirmed this with a pinpoint salinity monitor that was just calibrated and the tank was 1.021.

02/28/2008, 03:41 PM
thanks airinhere, I read the page you linked and have found that the conversion chart I used was wrong!

Never trust anything you read on the internet.

Except here on RC.



02/28/2008, 04:08 PM
Also remember that temperature has alot to do with accurate measurements. It may address it in the link to the article above, im not sure. Perhaps your water was warmer/cooler than the LFS water when you tested it.

Randy Holmes-Farley
02/28/2008, 04:27 PM
Many people find that after appropriate calibration of a refractometer with a seawater standard, that it does not read RO/DI or distilled (or tap) correctly. That is normal. All refractometers we buy are made for sodium chloride solutions,not seawater. So we have to mess with them a bit to give perfect readings with seawater, and that necessitates them being off for RO/DI.

Believe the calibration fluid to be 35 ppt or sg = 1.0264, and if that reads correctly, it is reading tank water near that salinity correctly.

03/02/2008, 12:17 AM
Difference in temperature can effect the outcome

03/02/2008, 03:24 AM
Except in Auto temperature compensated.