Hi,
Please can you explain how to then calibrate the refracto using the 35ppt solution. I have an ATC Refracto how do I ensure its calibrated at the correct temp. Do I need to ensure the stock solution is at the same temp the refrato is caliberated at? What happens if my tank temprature at testing is higher than the calibration of the Refracto?
Yes this is a question that have debated on other forums a number of times. We must understand that refractometers do not measure Salinity or Specific Gravity. They measure refractive index and that temperature influences refractive index and Specific gravity. It does not influence Salinity.
In the case of ATC refractometers there is a mechanism that compensates for temperature change. So what in effect happens is that the scale moves up and down depending on the temperature of the instrument.
The later saltwater and seawater ATC refractometers have been designed to be calibrated at normal room temperature. In fact even the high end hydrometers. So we have some that have a calibration scale calibrated to 20C and some calibrated to 25C. Some manufactures feel that the average room temperature is 20C and others 25C. This could be a smoke screen but technically a SG of 1.0264 @20C is S=35 and a SG of 1.025 @ 25C is S=35. However, if we check out the scales of the 25C instruments we note that the 1.026 relates to S=35 this is a technical error. But that does not mean the instrument is not correct reading Salinity when calibrated with a S=35 solution.
The whole purpose of having a 35 ppt calibration solution is so that we can calibrate the instrument to this reference solution and be confident that no matter what the temperature the instrument will read 35pp.
So depending on the instrument calibration temperature. It is best to calibrate your instrument at that temperature. But it is not a must as the ATC will compensate by moving the scale. It must be remembered that the temperature of the instrument is important, not the temperature of the testing solution. The testing solution, due to the small sample, temperature will quickly equilibrate to that of the instrument.
Sorry for the brief explanation and I hope I did not confuse you. The process is to simply calibrate your refractometer with the S=35 calibration solution so that the refractometer reads S=35 or 3,5% depending upon the scale you have. The ATC will compensate for differences in instrument temperature and the testing water will equilibrate to the instrument temperature.
However it is important to calibrate quite quickly as evaporation does take place on the prism. I recommend adding the reference solution and doing a rough calibration. Then clean the prism and add a second sample and fine adjusting. After calibration check again with reference solution. This way you calibration will have no evaporation affecting the calibrating.