Salinity. What to trust?

[jason2459]

Originally Posted by bertoni

You got the seawater version, I presume. I'm glad you like it. It does seem fairly pricey, but nice toys are what makes life worth living.

Basically, with the PSU scale which is what my reference solution is measured to via Randy's recipe. It is nice that it is easy to read, seems just as accurate as my veegee, and much much faster to come to an answer then the pinpoint salinity probe that typically needs to settle in for a bit.

[bertoni]

I remember the PinPoint unit taking quite a while to settle. It's good to know that there are alternatives, even if they are pricey.

[Ssteve]

My issue ended up being interference from my t5 lights to the salinity prob. Once I switched to leds my apex read spot on what my refractometer was reading and I haven't had any issues since.

[bertoni]

I'm not surprised. I had that same issue with a pH meter.

[jason2459]

Mine did that as well. I had a retro kit from BRS that came with a ballast that interfered with my conductivity probe and it wasn't even close to each other.

T5 and ballast on my first floor above my tank and my conductivity probe in my basement in the sump. Totally different electrical circuits too. The EMF from that ballast had to of been a lot.

The readings dropped every time the ballast kicked on.


I changed out that retro kit for a Hamilton lighting one and problem solved.

[five.five-six]

I just use a hydrometer, they aren't all that precise but they are plenty accurate for our needs.

[jason2459]

Originally Posted by five.five-six

I just use a hydrometer, they aren't all that precise but they are plenty accurate for our needs.

You have that flipped around. They are generally not accurate at all but can be precise as long as nothing is weighing down, sticking, or pushing up the arm and is done at close to the same temps all the time.

Otherwise one could get a reference solution, fill it up and mark where it points to and use that "calibration" mark.

[ReefsandGeeks]

A bit off topic, but I've noticed I had issues with numbers varying with my refractometer, even right after calibrating it. Then I realized my error...when I use the refractometer, I'd point it down and gently shake it to get most of the water off of it before drying. This seems to have been throwing the calibration off without me knowing it. I've since just used a cotton cloth (t-shirt) to gently soak up the couple drips of water, then giving a quick wipe with the provided cloth. This seems much more consistent now. I want to get a digital refractometer, but is cost prohibitive at the moment with a baby on the way.

I've also heard of different variations that are taken for granted, like sample temperature, temperature of the meter itself...etc that could possibly cause error. It's easy to forget just how hard it is to measure salinity accurately. I've even gone as far as directly measuring specific gravity with a +-0.01mg scale, and even then there's a confidence level in the results. And don't forget to make any changes to salinity slowly in the tank itself. Stability>spot on #s.

[five.five-six]

Originally Posted by jason2459

You have that flipped around. They are generally not accurate at all but can be precise as long as nothing is weighing down, sticking, or pushing up the arm and is done at close to the same temps all the time.

Otherwise one could get a reference solution, fill it up and mark where it points to and use that "calibration" mark.

I dissagree, most hydrometers only have resolution to 1ppt, not very precise but so long as your temp is in range and no bubbles on the swing arm they are pleanty accurate.

[jason2459]

Originally Posted by five.five-six

I dissagree, most hydrometers only have resolution to 1ppt, not very precise but so long as your temp is in range and no bubbles on the swing arm they are pleanty accurate.

You still have the definitions flipped. Accurate and Precise.

[five.five-six]

No, you may need glasses. Precise correlates to a high degree of granularity. Accurate means centers on the target. Look at your picture again.

https://youtu.be/Yii1u2Lz-II

[jason2459]

Originally Posted by five.five-six

No, you may need glasses. Precise correlates to a high degree of granularity. Accurate means centers on the target. Look at your picture again.

b]

A Hydrometer is not very accurate and usually off. Yet can be precise in that they can get the same readings repeatedly.

[five.five-six]

You seem to have confused consistent with precise.

[jason2459]

Originally Posted by five.five-six

You seem to have confused consistent with precise.

joy.


SMH

Precision is consistency of hitting the same location

Accuracy is how close you get to hitting the center of the target

A swing arm hydrometer is usually not accurate in that it will not land at 1.026 with a PSU solution of 35. But it will consistently, with out interference, hit the same number which is precise. So, someone could then mark that spot that a reference solution shows on that hydrometer and use that mark to test their water to a salinity 35 or know how off their water sample is.


https://www.ncsu.edu/labwrite/Experi...yprecision.htm

"Accuracy and Precision:


Accuracy refers to the closeness of a measured value to a standard or known value. For example, if in lab you obtain a weight measurement of 3.2 kg for a given substance, but the actual or known weight is 10 kg, then your measurement is not accurate. In this case, your measurement is not close to the known value.

Precision refers to the closeness of two or more measurements to each other. Using the example above, if you weigh a given substance five times, and get 3.2 kg each time, then your measurement is very precise. Precision is independent of accuracy. You can be very precise but inaccurate, as described above. You can also be accurate but imprecise.

For example, if on average, your measurements for a given substance are close to the known value, but the measurements are far from each other, then you have accuracy without precision.

A good analogy for understanding accuracy and precision is to imagine a basketball player shooting baskets. If the player shoots with accuracy, his aim will always take the ball close to or into the basket. If the player shoots with precision, his aim will always take the ball to the same location which may or may not be close to the basket. A good player will be both accurate and precise by shooting the ball the same way each time and each time making it in the basket. "


The target image above I thought would be easier to get a good understand of the difference and correlation between accuracy and precision.

[five.five-six]

Not in the picture you posted

[jason2459]

Originally Posted by five.five-six

Not in the picture you posted

Image shows exactly that.

[bertoni]

All right, let's end the personal attacks. I have edited some posts.


Here at Reef Central, we believe that dialogs between participants should be conducted in a friendly and helpful manner. If you disagree with a posting, please express yourself in a way that is conducive to further constructive dialog. Conversely, when you post on any given subject, you must be willing to accept constructive criticism without posting a hostile or inflammatory response. Personal attacks of any kind will not be tolerated. Please work to insure that Reef Central remains a friendly and flame free site where everyone, especially newcomers, can feel free to post questions without fear of being unfairly criticized. Thank you for your cooperation.

Hydrometers can be accurate and precise enough for our purposes if they are maintained and used properly. Checking the reading against a known standard tends to be important, though, and they are temperature-sensitive. I agree that keeping them clean and being careful with bubbles are two issues to keep in mind.

A hobbyist hydrometer generally won't be very precise as compared to a salinity meter, on average. I would rate the hydrometers that I have used as somewhat less precise than the refractometer I used, but there are a lot of hydrometers on the market.

[five.five-six]

Originally Posted by jason2459

joy.


SMH

Precision is consistency of hitting the same location

So, a gas gauge that always reads about 1/2 tank precise? No, it isn't.

"The sun is big" that is an accurate but not precise statment

"The sun is 15.432 km in diameter" that is a precise but not accurate statement

"The sun is 1.3914 million km in diameter" is both accurate and precise.


"Precision is consistency of hitting the same location" is neither accurate nor precise.

[Mishri]

jason is correct. In chemistry precision is hitting the same spot, accuracy is near your target.

it is not like precision in mathematical terms of .0000000001. being more precise than .01.. in chemistry they instead have what is called significant digits.

-There are videos and websites that explain this, just google chemistry precision and accuracy.

if you keep getting the same results with every test you have high precision. if you get different results with every test you have low precision. If you are near where it's supposed to be (such as calibration solution) you have high accuracy. Most chemists who are reef keepers i've talked to believe precision is more important than accuracy... since we have a range of acceptable parameters and we aim for the middle of acceptable parameters, and we want it to be consistent.

example: if your calcium test is +/- 50 you are fine as long as you keep seeing 400-precision, it doesn't matter if it's actually 450. With poor precision you'd wonder why your calcium is bouncing all over the place if you got 450 one time then 350 the next... having both is ideal but accuracy isn't overly important.

[five.five-six]

There is a basic misunderstanding here. The "target" in testing isn't necessarily 35 PPT or 1.026, the target is the actual salt content.

As described in several posts on the first page, there are several test methods which while haveing a high degree of mathematical precision, they may be wildly inaccurate. Meaning that the reading may be pretty far off from the actual salt content

A deep 6 hydrometer won't give you much precision, all you'll know is that it's somewhat about 35ppt but so long as you get all the bubbles off the swing arm, that's plenty accurate for our needs. If it says it's about 35ppt, it's about 35ppt. Close enough not to hurt your animals anyway.

[DivingTheWorld]

I went through this same exercise a few years ago and determined the most accurate (and affordable) device for me was the Tropic Marin High Precision Hydrometer. It has resolution down to 0.0001 and I find it dead accurate.

I initially found it a pain to test at 77 degrees, but have since created my own temperature adjustment table so it's a quick and easy test.

http://www.reefcentral.com/forums/sh....php?t=2461263

[Martin Kuhn]

Originally Posted by DivingTheWorld

I went through this same exercise a few years ago and determined the most accurate (and affordable) device for me was the Tropic Marin High Precision Hydrometer. It has resolution down to 0.0001 and I find it dead accurate.

Very good, when not even the best, choice.
Also, you don't need to keep your water at a certain temperature.
Measure density + water temperature and convert this to salinity.
So you have a very precise/accurate measurement. Also with very Little Chance if making any mistakes.


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[five.five-six]

Originally Posted by DivingTheWorld

I went through this same exercise a few years ago and determined the most accurate (and affordable) device for me was the Tropic Marin High Precision Hydrometer. It has resolution down to 0.0001 and I find it dead accurate.

I initially found it a pain to test at 77 degrees, but have since created my own temperature adjustment table so it's a quick and easy test.

http://www.reefcentral.com/forums/sh....php?t=2461263

Is this the one?

https://www.amazon.com/Tropic-Marin-.../dp/B0002DK1YU

One of the big problems with refractometers is that magnesium sulfate has a much higher refractive index than sodium chloride and it is a common practice to run magnesium at elevated levels to avoid precipitation on and in pumps and valves and other things. It's a lot like setting your finger on the scale when weighing produce.