Calibration of the new seawater refractometer

Question:

Hi,

I am after some advice regarding the above refractometer that I recently purchased.

To begin with, to be absolutely sure of the calibration, I purchased a fauna marin reference solution to calibrate it to read 35ppt salinity.

Having done this, I cannot seem to get a "stable" calibration.

Initially, I think I was focussing on washing off thoroughly with RO water to prevent any carryover, and I was using too much RO, and this was in fact cooling the refractometer.

I was finding it impossible to get reproducible readings of standard - each time rinsing off with RO water. I was getting fluctuations of up to 2ppt from the same solution.

Yesterday I tried something different, thinking I was cooling the refractometer by rinsing - instead of rinsing, I used a lens cloth to wipe clean the lens and cover surface, and then a seperate lens cloth to clean again. I recalibrated to 35 ppt and carried out a series of tests of standard and tank water and I seemed to get consitent results - at last!!

Thinking I had solved my problems, today I have checked the standard, and to my surprise, its reading about 2ppt low!!! I have tried a few times in sucession, and despite results being consitent - each was low by the same amount (I am waiting about 30 secs for the temp of solution to equilabrate on the refractometer). The refractomer was stored overnight in a cupboard in the room where I take the measurements.

The only thing that I can think of that could cause this is the ATC. I have just tried leaving the refractometer out in the open in the room I take measurement in for an hour to see if reads any different, and its still reading low.

This is really frustrating me.

My only thought now is that the instrument is faulty - could this be the case?? Have you had any reported problems similar??

Can you PLEASE HELP ME??

regards...

Daryl

Answer:

Daryl

I do not know if the fauna marine calibration solution will give you a true 35ppt as I do not know the chemistry behind it.
Salinity is not temperature dependant however as you are measuring the salinity indirectly with a refractometer which is temperature dependant then the salinity will be temperature dependant too.

I wonder what was used to set the original salinity of the standard at 35ppt - hydrometer, conductivity, SALTWATER refractometer. All of these measurements are temperature dependant.

Regardless of this if you use the same solution with the refractometer at the same temperature each time then the reading should be the same.
The refractometer should be at 20C when you calibrate the unit and if you are testing with the instrument at different temperatures then the calibration will shift from test to test.

If it is a true seawater standard and you are using our new SEAWATER refractometer then it should calibrate both at 35ppt and zero too.

The other sources of error are salt on the plate and flap and residues in the dropper pipette or residual RO in the dropper pipette.

Have a play with these parameters and let me know what happens. Have you read this article
http://www.theaquariumsolution.com/specific-gravity-salinity-and-its-measurement

All the best
Stuart

Response:

Stuart,

Just to let you know, I believe I have resolved this issue.

Firstly, The past 3 days I have had the refrac stood in a room at approx 20.3C and checked zero calibration - each time has been spot on.

I then moved the refrac to a room that I take the measurement in and allow to equilibrate to that room temp for 8 hours. I can then get a stable reading each day at that room temp. With the Fauna Marin standard, as you say it is not a seawater standard and so the reading comes out less than 35 ppt.

Its taken a while, but I am now happy with the measurements.

I believe the following points should be highlighted to customers with this device:

1. You will NOT get reproducible results if you use too much RO water to clean between each measurement (also only need 3- 4 drops of solution, no more). I found it is best to use 2 lens cloths - one to wipe sample away and the other to final clean no rinsing with RO at all.

2. You will get apparent drift in calibration if you DO NOT leave the refrac out in the open in the room that measurement will be taken for some time for temperature of refrac to equilibrate. I was storing refrac in its box in a cupboard in the room where I was taking readings, and this was sufficient to cause daily discrepancies in readings.

regards...

Daryl.