Measuring Salinity

WVNed · Jun 21, 2018

I have always used a swing arm hydrometer. 1 single hydrometer I bought a long time ago.

I would like to have some way to check it.

I set up a 75 gallon tank Sunday. No water was added to or removed from the tank, no ATO or skimmate produced. Went to install the ATO yesterday

I tested the SG to add water for evaporation and it was 1 full point lower. Scared me. 1.024
So I either had an error when I mixed the water Sunday or yesterday.
I have 2 swing arm hydrometers and they agreed yesterday. So the problem had to be Sunday.
Not sure how a swing arm hydrometer could read too low though. Bubbles dragged the arm down?

Is the
Milwaukee MA887 Digital Salinity Refractometer with Automatic Temperature Compensation

the answer I seek.

Randy Holmes-Farley · Jun 21, 2018

There are lots of good ways to measure salinity, some more or less expensive. My preference happens to be conductivity, but other methods can work well too.

You can also make a standard to check your swing arm hydrometer.

WVNed · Jun 21, 2018

I have pharmacy grade scales and masses at my disposal. I understand conductivity. Let's throw out expense and consider easy and repeatable. I just want an infallible backup I can go to to test myself and my results. I don't want to kill things in my care.

cloak · Jun 21, 2018

I just use a floating glass hydrometer. No problem. My parents used to use the same thing on their tank back in the late 70's early 80's too. Here it is 2018 and they look identical to me, so I'm assuming the science behind them was pretty good.

Randy Holmes-Farley · Jun 21, 2018

WVNed said:
I have pharmacy grade scales and masses at my disposal. I understand conductivity. Let's throw out expense and consider easy and repeatable. I just want an infallible backup I can go to to test myself and my results. I don't want to kill things in my care.

With some new sodium chloride (not seawater mix) and a scale, you can make good standards to test any of the devices you'd likely use for salinity, but make sure you make the appropriate solution for the device type:

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2004-06/rhf/index.htm

miPapareef · Jun 21, 2018

I bought the Milwaukee MA887 and haven’t looked back. Simple and accurate and I’d highly recommend it. Calibrates with your RODI water and yes it’s temp calibrated.

My room temp varies 67 to 70 throughout the year, when I put 3 drops of 78 degree water it settles out between 69 and 71 degrees but always reads 1.026 or maybe a few times 1.027.

It’s way easier and simpler than my refractometer and the Apex conductivity probe.

Randy Holmes-Farley · Jun 22, 2018

miPapareef said:
I bought the Milwaukee MA887 and haven’t looked back. Simple and accurate and I’d highly recommend it. Calibrates with your RODI water and yes it’s temp calibrated.

My room temp varies 67 to 70 throughout the year, when I put 3 drops of 78 degree water it settles out between 69 and 71 degrees but always reads 1.026 or maybe a few times 1.027.

It’s way easier and simpler than my refractometer and the Apex conductivity probe.

How do you know it is accurate? Quite a few people have commented that they have them and think it is off.

Mr. Roboto · Jun 22, 2018

Save yourself some trouble and just get a good refractometer and calibration solution maybe?

chipmunkofdoom2 · Jun 22, 2018

I personally don't like the Milwaukee digital refractometer because It doesn't allow you to calibrate with a salinity standard. While calibrating with freshwater will likely work okay in most situations, some people have reported inaccuracies when measuring samples of a known refractive index with the Milwaukee. If you're spending digital refractometer money, I would go with a conductivity salinity measurement tool instead. I have the American Marine Pinpoint conductivity meter and it works great. I've only had to adjust the calibration once in over a year and a half of use, and it was only off by 0.2 mS/cm.

WVNed · Jun 22, 2018

chipmunkofdoom2 said:
I personally don't like the Milwaukee digital refractometer because It doesn't allow you to calibrate with a salinity standard. While calibrating with freshwater will likely work okay in most situations, some people have reported inaccuracies when measuring samples of a known refractive index with the Milwaukee. If you're spending digital refractometer money, I would go with a conductivity salinity measurement tool instead. I have the American Marine Pinpoint conductivity meter and it works great. I've only had to adjust the calibration once in over a year and a half of use, and it was only off by 0.2 mS/cm.

What is the difference between the Pinpoint salinity monitor and the conductivity monitor?

Randy Holmes-Farley · Jun 22, 2018

WVNed said:
What is the difference between the Pinpoint salinity monitor and the conductivity monitor?

The range. Both are conductivity monitors, but the salinity one is designed to measure values close to seawater while the conductivity one is designed for lower values, like ro/di water.

WVNed · Jun 23, 2018

I ordered the Pinpoint salinity monitor from BRS.
According to my hydrometer the SG has gone down again to 1.0245. My PH and alkalinity have gone down too. Same conditions I found testing on Thursday. The volume of water in the tank/sump is not changing. Calcium tests at 500. I dosed Reef Fusion 2 again.

This makes no sense to me.

Randy Holmes-Farley · Jun 23, 2018

Are you sure the water level in the tank is not changing?

Are you using an ATO that may not control the level especially precisely?

WVNed · Jun 23, 2018

It's a Tunze 3155. My ATO reservoir is a graduated 5 gallon bucket. It has gone down a reasonable amount for a 75 gallon open top since Thursday and doesn't need refilled yet.
Looks to be about 1/2 a gallon used. The level on my overflow and the ATO sensor in the return section of the sump is constant.

??? I dont know.

Randy Holmes-Farley · Jun 23, 2018

If you are sure no salt water is leaving the system somehow, I would not keep boosting the salinity. I'd watch it longer term to see if it is just measurement fluctuation.

There's really no way for salinity to keep declining without removing salt water.

WVNed · Jun 23, 2018

I boosted it Thursday to 1.0250 assuming I had mixed it wrong. I only corrected the PH and Alk today. I filled the ATO Thursday and have added no more water since then. It has used about 1/2 gallon between then and now. I just put the tank up Sunday so I didn't do my normal water change Wednesday.
The water would have to be going straight out the bottom into the crawl space.

Rich Klein · Jun 23, 2018

miPapareef said:
I bought the Milwaukee MA887 and haven’t looked back. Simple and accurate and I’d highly recommend it. Calibrates with your RODI water and yes it’s temp calibrated.

My room temp varies 67 to 70 throughout the year, when I put 3 drops of 78 degree water it settles out between 69 and 71 degrees but always reads 1.026 or maybe a few times 1.027.

It’s way easier and simpler than my refractometer and the Apex conductivity probe.

+1, but Milwaukee insists that we use steamed distilled water for calibration.

Randy Holmes-Farley · Jun 24, 2018

Rich Klein said:
+1, but Milwaukee insists that we use steamed distilled water for calibration.

Which only makes sense if they do not trust you to properly make RO/Di water. lol

0-1 ppm TDS RO/DI water will also work perfectly well, and 35 ppt would only be off by about 1 ppm at most due to that issue, or read 35 ppt as, say, 34.999 ppt