Marine Depot Refractometer Calibration

[mathias999us]

Hi Randy,

I just received one of these cheapy marine-depot refractometers:

http://www.marinedepot.com/md_viewItem.asp?idProduct=MD2101

It actually "seems" like a decent unit.

So, I want to calibrate it. The instructions just say to use distilled water to zero it out, but I've seen your comments in other threads in here about how this may not be the best way to calibrate it. Presumably, the idea here, is that the scale/readout may introduce some linear or curved error as a function of the salinity that's being measured, so you want to calibrate with a solution as close as possible to the target salinity you will be using the instrument to measure most of the time. This would cause the error to be ~0 AT the target salinity, with error increasing as the measured values stray away from the target...

This is my hypothesis, does this sound correct?

So, I'd like to do a couple of things. I'd like to first calibrate it with distilled (RO/DI with 0TDS) to zero it out. Then, I'd like to calibrate it using one of your calibration standards, and see if the calibration with the RO/DI water was off. This should show me the amount of error that my particular unit introduces when you're 0.0264 away from your target (or at least -0.0264 away from target).

For making the calibration solution, I'm a little confused about the difference between refractive index and spg. In your article, the refractometer solution table uses refractive index, but the scale on my refractometer is labled as sg. Here's a pic of it actually:
http://www.marinedepot.com/IMD/refractometer_md_logo_large.jpg

So, am I supposed to calibrate using the Refractive Index standard or the SPG standard from your article? I suppose I could just use the PSU scale?

Also, Do you know what that d20/20 symbol on my meter is?

One more thing. I assume these calibration formulas are linear, so I can multiply the amounts by a fixed constant. For instance, I don't have a scale, but I do have a 100mL graduated cylinder. So, for the salt, my best option is to use the teaspoon measurement of ~6.20g NaCl.

I bring this up, because the formula for the refractive index standard of 35PSU(1.3394 refractive index) calls for 3.65g NaCl and 96.35mL of water. The constant to get 6.20g (1tsp) of NaCl is 1.69863, which gives us 163.6mL. But, your formula for 35PSU(1.0266 spg) calls for 6.20g of NaCL and 161mL water... Acutally, now that I notice this, I see that in the %NaCl column in table 2 (refractive index) and table 5 (spg) from your article have different values for solutions of 35.0PSU. What am I missing here?

For reference, here's the article:
http://www.reefkeeping.com/issues/2004-06/rhf/index.php

Thanks so much for your patience and teachings,
Mathias

 

[mathias999us]

Just an update on this

Just an update on this

First, I calibrated the unit using RO/DI water. Straight from the vendor, it was showing about 1.002, so a moved it back down to 1.000.

For now, I went ahead and made the specific gravity calibration standard using 6.20g NaCl and 161mL of RO/DI water. With the refractometer that was calibrated to 1.000 with RO/DI water, it was showing 1.032 spg - way off. For sanity, I checked the same batch of solution in my 2 hydrometers. My brand new hydrometer showed it as 1.0260, just about right on. My old old old hydrometer showed it at about 1.0282, which is too high (the inaccuracy of the old hydrometer is what set me on this quest for accurate salinity measurement if you recall).

So, I believe I made the solution correctly. I used a graduated cylinder from a chemistry set (and I've taken a few college chemistry courses, I know all about the miniscus and such), and a perfectly level teaspon of morton's iodized salt. I also think my new hydrometer is quite accurate (based on the expected Ca levels of a fresh batch of IO).

So, I adjusted the refractometer to show ~1.0265 spg with the spg calibration standard. Now, when it has RO/DI in it, it shows ~-0.006 This leads me to believe that the error is linear, like, the scale graduations in the refractometer aren't properly spaced, since a known delta of 0.026 spg creates an apparent movement of 0.032 units of the indicator... OR, maybe the scale is incorrectly labled, and it actually IS measuring refractive index instead of spg?

Is this thing a piece of junk? What is a reefer to trust...

Mathias

 

[Randy Holmes-Farley]

For a refractometer, you'd use a refractive index standard, regardless of what units the device actually reports in. The refractometer standard can be matched to read 35 ppt or sg =1.0264 on the refractometer.