Calibrating a Refractometer

swiseman

swiseman

Premium Member
I started looking in to this topic for a variety of reasons, one of which was my inability to get my alkalinity below 11DKH. There was also this thread located in the GHL/Profilux forum.

http://www.reefcentral.com/forums/showthread.php?t=1738178

The above discussion is hardly a conclusive study, but, it did get me thinking. My next step was to hit the search button and I found this article published in Reef Keeping magazine by Randy Holmes-Farley.

http://reefkeeping.com/issues/2006-12/rhf/index.php

I remember when this article was published there was a major shortage of Pinpoint Calibration fluid for weeks afterword. I actually bought some as well, but, I thought I had a bad sample because the results were off by so much, so, I fired the calibration fluid in the cupboard and never gave it any further thought.......until this weekend.

I pulled out the bottle of calibration fluid and tested my refractometer and it was off by 0.006. I re calibrated the refractometer with room temperature RO/DI and it was dead on 0. I then used the Pinpoint Calibration fluid and got a reading of 1.020. So I said what the heck and re calibrated the refractometer to the Pinpoint fluid. I then did a quick test of my tank water and with the new set point my tank water was registering 1.031. Is this even possible? I will admit, I have been having some issues with the tank since I upgraded last December, however, I have been using this refractometer calibrated with distiled or RO/DI water for the past 5 years. All of the local reefers and LFS have the same readings as me. Can we all be wrong? Unfortunately, we did not have a digital conductivity meter to check it with.

Thoughts?
 
however, I have been using this refractometer calibrated with distiled or RO/DI water for the past 5 years. All of the local reefers and LFS have the same readings as me.

If you all use refractometers calibrated with RO/DI, you will likely all have the same systemic error. It is not generally a random error, unless the device was mismanufactured or broken, but rather a design error they all have.

That said, that design error is not as large as you are getting, so I am concerned that your pinpoint fluid might be off, either by mismanufacture, or by evaporation/condensation, tampering by someone who used some and refilled it with RO/DI and returned it to the store, etc.

I'd get a new bottle or try to make your own. :)
 
I had the same issue with the calibration fluid. It was always off by .005 and even one time it went by .010. I got a new bottle and it ended up being the same thing. Then I went to a fellow reefer and used his calibration fluid and it turned out that my batch of calibration fluid was bad. I ended up using his bottle to calibrate my refractometer. Ever since, my sg has been well on 1.024-1.026.

Do try to get a new bottle, but from another store or something so you won't get another bottle from the same batch like I did.
 
I think the OP"s question may have been answered. I don't want to hijack, but I have a related question, also after reading "Refractometers and Salinity Measurement" by Randy.

You mentioned in your article
Refractometers can lead to incorrect readings in additional ways and, again, these issues abound for reef aquarists. One is that many refractometers are intended to measure sodium chloride solutions, not seawater. These are often called salt or brine refractometers. Despite the scale reading in ppt (‰) or specific gravity, they are not intended to be used for seawater. Unfortunately, many refractometers used by aquarists fall into this category. In fact, very few refractometers used by hobbyists are true seawater refractometers.

Fortunately for aquarists, the differences between a salt refractometer and a seawater refractometer are not too large.
Click to expand...

It sounds like a true "seawater" refractometer is ideal. It doesn't look like most retail shops include a proper description to know if you are getting a "saltwater" or a seawater" refractometer. i.e. The Vital Sine refractometer from Drs. F&S says "This handheld salinity refractometer is exceptionally easy to operate and provides quick, accurate measurements for the salinity level of water." http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=12132

That doesn't clearly state what type it is.

Randy, can you tell me where to buy a true "seawater" refractometer? Assuming that's what you use, can you tell me what you have or recommend one to me. If you don't want to publicly announce what you use, a PM would be nice;)
 
Randy Holmes-Farley said:
It is a salt, not seawater, refractometer. I do not know of any actual seawater refractometers.
Click to expand...

Figure 23 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in salinity.
Click to expand...

Ohhh... I assumed you were the person that actually performed these tests. Were you not? I guess you do not know what "seawater" refractometer was used in the data posted in your article?
 
Huh?

Which data?

It comes from the scientific literature, where folks measure and report refractive index itself and in that case any refractometer with the right range is fine. It is the conversion of refractive index to sg or salinity by the refractometer itself that refractometers mess up when using seawater. :)
 
Randy Holmes-Farley said:
Huh?

Which data?
:)
Click to expand...

Perhaps I made a poor choice of words but I called a bunch of numbers on a graph, like shown in "Table 23" of your article... "data". The table is suppose to be show "the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer." I think I get it now... I incorrectly made the assumption that this was an actual/real measurement/comparison between two real refractometers. Apparently it is a theoretical relationship between the two... if a true seawater refractometer actually existed.
 
Assuming theres nothing wrong with it initially, does the pinpoint solution have a use by period? Or does it need regular replacement?
 
I had a bottle of PinPoint that was 3 years old. It has always been tightly closed after I used it. I bought a new bottle of PinPoint because of how old the other one was. I did a test & it turns out that the old PinPoint was right on the money as the new PinPoint
 
This is a good topic. My refracotmeter says to calibrate with distilled or RO/DI water. I questioned this due to a topic like this one. My LFS says just use the RO/DI water. I have been doing this for several months now. I have problems trying to get my salt mix alk, and mag to the levels mentioned in the guide to salt mixes. I use Tropic Marin Pro Reef and get a result of 400cal, 6.17dkh, 1200mag at 1.026. I brought a sample into my LFS and they came up with the same 1.026. I am thinking of puchasing a cal fluid as well.
 
The LFS is wrong. Your refractometer is designed to be used in salt water, not seawater (assuming it reads in specific gravity). That said, the difference is unlikely to be causing these to be quite as low as they seem, IMO.
 
Randy, typically, how far off will a refractometer be, if calibrated using RO/DI water? I ask because this is how I've always calibrated mine, since this is how the instructions say to calibrate it. What would typically be the harm in continuing to calibrate, using RO/DI water?

Also, it seems I read, probably by you, a recipe for making your own calibration fluid, though I can't find it now. Any leads?

Nevermind on the recipe, found it in the link above, which I've read several times, but seems I need to read it yet again. I believe it'll also answer my second question. For other's benefit, I'll add the link again.
http://reefkeeping.com/issues/2006-12/rhf/index.php
 
I have been having issues with my refractometer staying calibrated. Is it normal to have to calibrate them every month or so? It seems accurate against others when it is calibrated, so should I worry about it?
 
guitar8222 said:
I have been having issues with my refractometer staying calibrated. Is it normal to have to calibrate them every month or so?
Click to expand...

Not for me does yours have a screw or knob? I bought the one with the screw to avoid bumping it. Mine was off .006 with rodi compared to calibration fluid, my friends was off by .008 when using RODI.
 
I made my own calibration fluid at the university's lab. In your article, it states to make a 3.65% by weight solution of NaCl and RO/DI, which I did. However, in the article it states to use iodized salt, but I used non-iodized salt (Morton's). Should this make a difference?

Also, as an interesting aside, the scale I was using was accurate to 0.001 gram, and I could see the water evaporating before my eyes! (the weight was slowly but steadily decreasing). So keep those solutions capped at all times except when you are taking a sample!
 
However, in the article it states to use iodized salt, but I used non-iodized salt (Morton's). Should this make a difference?

No, noniodized is perefect for that use. :)
 
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