Salinity low according to ICP

abecker · Nov 7, 2017

I sent a water sample to ATI-Aquaristik for ICP testing. I just received the results and it shows my salinity being 30.46 PSU. From a quick search online it looks like PSU would be the same as PPT. A conversion to SG puts it around 1.022. According to my refractometer, I am at 1.025. I have calibrated based on my RO/DI water. The TDS meter going in to my RO/DI storage bucket shows 0 TDS, however the test from ATI shows I have 36.93 µg/l of silicon. Would this silicon throw off my refractometer reading if it is in the water I calibrate the refractometer from?

chipmunkofdoom2 · Nov 7, 2017

abecker said:
I sent a water sample to ATI-Aquaristik for ICP testing. I just received the results and it shows my salinity being 30.46 PSU. From a quick search online it looks like PSU would be the same as PPT. A conversion to SG puts it around 1.022. According to my refractometer, I am at 1.025. I have calibrated based on my RO/DI water. The TDS meter going in to my RO/DI storage bucket shows 0 TDS, however the test from ATI shows I have 36.93 µg/l of silicon. Would this silicon throw off my refractometer reading if it is in the water I calibrate the refractometer from?

I'm not sure about the silicon. I do know that calibrating a refractometer with freshwater does not always produce accurate results. Randy's article on the subject goes over the specifics. Figure 17 shows the exact scenario you're describing. If you're reading water that's supposed to be S=35ppt, then it's best to use a calibration standard specifically designed to mimic seawater at 35ppt.

Randy Holmes-Farley · Nov 7, 2017

No, nothing throws off refractometers in this sense.

Randy Holmes-Farley · Nov 7, 2017

30.46 psu seawater has a specific gravity of 1.023. I’d check/calibrate the refractometer with a 35 ppt standard.

Reef Able · Nov 7, 2017

This is exactly what I am dealing with now. My salinity showed 1.025 and I use to recalibrate with ro/di however I took my refractometer to my LFS who recalibrated it with the 35ppt solution. I found out my refractometer was 6 points below so I was running my tank at 1.019 now I am gradually increasing it. Lesson learnt do not use ro/di to recalibrate your refractometer.

lolmatt · Nov 7, 2017

+1, just buy the 35ppt solution from brs and it will last forever.

Newb73 · Nov 7, 2017

This is why it's a actually good to spend the money on a PM2 and conductivity probe.

I cross check my refractometer with my Digital reader with my conductivity probe...all calibrated.

If one drifts I'll figure out which one fairly quick.

For example i just mixed up some 33ppt fritz but my conductivity probe shows a slow steady drift from 34.2 to 34.6 in my display and i was expecting it to drift down a bit.

That could be my dos is out of calibration, that my conductivity probe cable is picking up a bit of interference from all the cords around it, evaporation from my salt storage or one of my measuring devices is off...i may have to pull my optical as the tie breaker and do a comparison.

Be that as it may at least i am aware that what appeared on one device as a slightly hypotonic mix is giving me readings on another device that it may be hypertonic.

Point is when you have multiple ways to measure you can self QA without needing to run the the lfs or use somone elses stuff thats also questionably calibrated.

Just be careful not to contaminate your calibration fluid and its probably smart to recalibrate the probe & buy a new bottle every year or two depending on how often you use it as insurance.

abecker · Nov 8, 2017

Newb73 said:
This is why it's a actually good to spend the money on a PM2 and conductivity probe.

I cross check my refractometer with my Digital reader with my conductivity probe...all calibrated.

If one drifts I'll figure out which one fairly quick.

For example i just mixed up some 33ppt fritz but my conductivity probe shows a slow steady drift from 34.2 to 34.6 in my display and i was expecting it to drift down a bit.

That could be my dos is out of calibration, that my conductivity probe cable is picking up a bit of interference from all the cords around it, evaporation from my salt storage or one of my measuring devices is off...i may have to pull my optical as the tie breaker and do a comparison.

Be that as it may at least i am aware that what appeared on one device as a slightly hypotonic mix is giving me readings on another device that it may be hypertonic.

Point is when you have multiple ways to measure you can self QA without needing to run the the lfs or use somone elses stuff thats also questionably calibrated.

Just be careful not to contaminate your calibration fluid and its probably smart to recalibrate the probe & buy a new bottle every year or two depending on how often you use it as insurance.