Salinity Calibration Question

Gungo · Jan 30, 2025

So I've been using a refractometer for who knows how long, using RODI to set it to 0 and using that as a reference for my salinity. I've also been using a GHL Salinity Probe for too long, so I stopped looking at the values. Recently, I decided to calibrate it, and the tank was showing at 1.030. Reading around the forum, I saw that using RODI for refractometer calibration is a bad practice, but could I use the calibration solution of GHL, APEX, and PINPOINT to calibrate the refractometer? Right now I'm trusting the probe more than the uncalibrated refractometer, when I put a few drops of the salinity solution in the refractometer is showing 1.019 when it is supposed to show 1.026.

KStatefan · Jan 30, 2025

The calibration solution has the be made for the device it is being used on. Some are made for multiple devices but if it is not listed I would not assume you can

Gungo · Jan 30, 2025

KStatefan said:
The calibration solution has the be made for the device it is being used on. Some are made for multiple devices but if it is not listed I would not assume you can

Why not? I guess they're the same chemicals so... I'm using the pinpoint calibration solution too and its around the same value in the refractometer.

KStatefan · Jan 31, 2025

Gungo said:
Why not? I guess they're the same chemicals so... I'm using the pinpoint calibration solution too and its around the same value in the refractometer.

I know the GHL and the pinpoint are different. The GHL is a 50mS standard and the pinpoint is 53mS standard. Standards can be made from different chemicals. If it is labeled a 50mS does not mean it is a 50mS seawater standard and could have a different refractorive index then a 50mS sample of seawater.

Make sure you are comparing the same units. The GHL P4 will display PSU, mS, or kg/L.

Gungo · Jan 31, 2025

KStatefan said:
I know the GHL and the pinpoint are different. The GHL is a 50mS standard and the pinpoint is 53mS standard. Standards can be made from different chemicals. If it is labeled a 50mS does not mean it is a 50mS seawater standard and could have a different refractorive index then a 50mS sample of seawater.

Make sure you are comparing the same units. The GHL P4 will display PSU, mS, or kg/L.

I know each brand has a different potency in the calibration solution, some have 50, others have 53. The GHL controller allows you to set different calibration values, depending on the solution you will use.

I guess my question is, can I use a 53 MS / 1.026 salinity calibration solution to calibrate a refractometer?

exnisstech · Jan 31, 2025

Just mix up some of Randy's diy calibration fluid for the refractometer.

https://reefkeeping.com/issues/2004-06/rhf/index.htm

Randy Holmes-Farley · Jan 31, 2025

Gungo said:
I know each brand has a different potency in the calibration solution, some have 50, others have 53. The GHL controller allows you to set different calibration values, depending on the solution you will use.

I guess my question is, can I use a 53 MS / 1.026 salinity calibration solution to calibrate a refractometer?

Kstate gave the correct answer. A fluid can be made to match a certain salinity of a conductivity probe that will not match that same salinity on a refractometer, and it is cheaper and in some important ways better than making a single fluid to match all devices.

A fluid matching all devices needs to be a seawater type solution, and that can be challenging to make a standard without referring it back to another standard of the single element type. If one is just trying to match a single type of device, a single chemical of high purity can be used, such as sodium chloride or potassium chloride. The latter is used as the definition of the salinity scale, PSU.

So as stated, a fluid can match all devices, or just one. Unless the manufacturer tells you, you should not assume they are interchangeable. The pinpoint is a seawater mimic and can be used on refractometers.