Refractometer
 

Good posting Eric,
In my work we've used the terms resolution capability versus accuracy,
but they mean exactly what you have defined as precise versus accuracy,
Mike
"Eric" wrote in message
...
Pszemol wrote:
Looking at the scale of mine refractometer with divisions for 34 and 35
ppt so close
to each other and comparing wide scale of swing arm with a centimeter
between 34 and 36
is it hard to believe refractometers are "much more accurate"... But I
will take your word.

Also, would you agree that Coralife statement of +/-0.001 SG accuracy of
their
DEEP SIX is not true if you say +/- 1ppt refractometer is "much more
accurate"?

A measurement tool is more PRECISE if it can measure to a greater number
of significant digits. The Deep Six is a more precise measuring device.

A measurement tool is more ACCURATE if it can come closer to the true
value of whatever it is you're measuring. The refractometer is more
accurate.

From this, hopefully it's clear that it does no good to be more precise
if the instrument is not accurate. We use refractometers because they
are fairly accurate. Unfortunately, they are not very precise.

Eric

Refractometer
 

"Eric" wrote in message ...
A measurement tool is more PRECISE if it can measure to a greater number
of significant digits. The Deep Six is a more precise measuring device.

A measurement tool is more ACCURATE if it can come closer to the true
value of whatever it is you're measuring. The refractometer is more
accurate.

From this, hopefully it's clear that it does no good to be more precise
if the instrument is not accurate. We use refractometers because they
are fairly accurate. Unfortunately, they are not very precise.

:-)
I guess I understand the difference between precision and accuracy, and it
looks to me, a good accuracy is not worth anything without a good precision...

A meter cannot be more accurate than 1ppt if it is not precise to a single ppt.
Highly compressed scale on the refractometer hardly guarantee 1 ppt
precission, which means the accuracy is not better than 1ppt as well...

What refractometer gains on accuracy it looses on precission to swing arm.
I would risk the statement a nicely calibrated (checked with the standard)
swing arm could be more accurate than refractometer because of
its greater precision :-)

Refractometer
 

Terminology quibles aside, you're quite right. Unfortunately most swing
arm devices don't seem to be well calibrated, and are not easily
calibrated after leaving the factory (at least the one I bought wasn't).
I will take a less precise instrument over one that is not accurate
any day. (Being able to measure that you're at 35-36ppt accurately is
better than thinking you're at 35.2 - 35.3 ppt when you're actually at
38ppt or worse because the instrument isn't calibrated properly.)

That said I do think my refractometer is difficult to read.. (The
"white" part is only a slightly lighter shade of blue than the blue
part. Maybe I just didn't get a very good one. :-) )

Eric
http://www.atreis.com/


Pszemol wrote:

"Eric" wrote in message ...

A measurement tool is more PRECISE if it can measure to a greater number
of significant digits. The Deep Six is a more precise measuring device.

A measurement tool is more ACCURATE if it can come closer to the true
value of whatever it is you're measuring. The refractometer is more
accurate.

From this, hopefully it's clear that it does no good to be more precise
if the instrument is not accurate. We use refractometers because they
are fairly accurate. Unfortunately, they are not very precise.


:-)
I guess I understand the difference between precision and accuracy, and it
looks to me, a good accuracy is not worth anything without a good precision...

A meter cannot be more accurate than 1ppt if it is not precise to a single ppt.
Highly compressed scale on the refractometer hardly guarantee 1 ppt
precission, which means the accuracy is not better than 1ppt as well...

What refractometer gains on accuracy it looses on precission to swing arm.
I would risk the statement a nicely calibrated (checked with the standard)
swing arm could be more accurate than refractometer because of
its greater precision :-)

Refractometer
 

"Eric" wrote in message ...
Terminology quibles aside, you're quite right. Unfortunately most swing
arm devices don't seem to be well calibrated, and are not easily
calibrated after leaving the factory (at least the one I bought wasn't).
I will take a less precise instrument over one that is not accurate
any day. (Being able to measure that you're at 35-36ppt accurately is
better than thinking you're at 35.2 - 35.3 ppt when you're actually at
38ppt or worse because the instrument isn't calibrated properly.)

And this is the reason for the standards Randy was talking
on ReefCentral - you could use these standards to check
your swing arm and find out it gives you too high or too low
reading for 1ppt - later you can use this correction when
measuring water from your tank and periodicaly check the
swing arm for accuracy.

That said I do think my refractometer is difficult to read.. (The
"white" part is only a slightly lighter shade of blue than the blue
part. Maybe I just didn't get a very good one. :-) )

Exactly my point. The scale is so compressed and the border
line not clear, it is difficult to get high accuracy with these guys.

Refractometer
 

I havent read the discussion on ReefCentral so I hope this isnt a repeat. Why
not use a common substance such as vinegar to test your hydrometer? I found
this on a webpage:
Malt vinegar is sold in four strengths designated 18, 20, 22, 24, the last
being proof vinegar, containing 6 % of acetic acid and having a specific
gravity of 1.019.
You could clean and check calibration in one step :)

Also, here's a chart of specific gravities of other different substances.
http://www.csgnetwork.com/sgvisc.html
~John

Refractometer
 

Try pointing the unit at a very bright light. I think you'll find the
white and blue are much more striking that way.

Marc


Eric wrote:

That said I do think my refractometer is difficult to read.. (The
"white" part is only a slightly lighter shade of blue than the blue
part. Maybe I just didn't get a very good one. :-) )

Eric
http://www.atreis.com/


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Refractometer
 

Yes, it helps a little. Also, I tried to point the meter dirrectly at the bulb.
Anyway, the scale is so tight I can barelly see what ppt reading is it.
Maybe I need to check my vision ;-)

I have a dilema right now in my tank...
I have two different deep sixs showing 34ppt in my tank, but the
refractometer RHS-10ATC is at 39-40ppt mark. What to do now? :-)
I am not sure if my animals would feel healthy in 40ppt.

Ok, because refract is calibrated to NaCl not sea water, I should
subtract maybe 1.5 ppt from the reading = this gives me 38ppt...

But still, what to do with the rest, 4 ppt difference? :-)))
I know... I will start playing with the NaCl salt in my kitchen.

"Marc Levenson" wrote in message m...
Try pointing the unit at a very bright light. I think you'll find the
white and blue are much more striking that way.

Marc


Eric wrote:

That said I do think my refractometer is difficult to read.. (The
"white" part is only a slightly lighter shade of blue than the blue
part. Maybe I just didn't get a very good one. :-) )

Eric
http://www.atreis.com/


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Refractometer
 

Did you calibrate your unit with distilled water yet?

What is the sg reading on the opposite side of the scale?

Marc


Pszemol wrote:

Yes, it helps a little. Also, I tried to point the meter dirrectly at the bulb.
Anyway, the scale is so tight I can barelly see what ppt reading is it.
Maybe I need to check my vision ;-)

I have a dilema right now in my tank...
I have two different deep sixs showing 34ppt in my tank, but the
refractometer RHS-10ATC is at 39-40ppt mark. What to do now? :-)
I am not sure if my animals would feel healthy in 40ppt.

Ok, because refract is calibrated to NaCl not sea water, I should
subtract maybe 1.5 ppt from the reading = this gives me 38ppt...

But still, what to do with the rest, 4 ppt difference? :-)))
I know... I will start playing with the NaCl salt in my kitchen.



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Personal Page: http://www.sparklingfloorservice.com/oanda/index.html
Business Page: http://www.sparklingfloorservice.com
Marine Hobbyist: http://www.melevsreef.com

Refractometer
 

"Marc Levenson" wrote in message m...
Did you calibrate your unit with distilled water yet?

Yes, I did.
It was not at zero, so I used a screwdriver to make it point zero.

What is the sg reading on the opposite side of the scale?

More than 1.030 sg.

Good my animals do not know that...
They probably panic and die if they knew how much salt they swim in ;-)

Refractometer
 

Well, I'm sure others like Boomer will have more to say, but basically I
just use the sg scale with my refractometer. And listen, when my sg is
1.l028 my zoanthids shrivel up and die... so take that 'with a grain of
salt'. :)

Marc


Pszemol wrote:
"Marc Levenson" wrote in message m...

Did you calibrate your unit with distilled water yet?


Yes, I did.
It was not at zero, so I used a screwdriver to make it point zero.


What is the sg reading on the opposite side of the scale?


More than 1.030 sg.

Good my animals do not know that...
They probably panic and die if they knew how much salt they swim in ;-)

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