floating hydrometer

tanhql · Post by **tanhql** » Thu Jan 25, 2007 9:18 am

today, i 'calibrated' my floating hydrometer with tap water to find that it gives a reading of 0.008(0.002 less than 1.000), which is impossible. the TDS of my tap water shouldn't affect the SG (should be undetectable). so, for future readings, should i add 0.002 to the measured SG to make up for the difference? anyway, i'll be buying bottled distilled water tml to make another calibration.

Nick · Post by **Nick** » Thu Jan 25, 2007 1:43 pm

I'd say it's off a bit, most floating type are off even more than the swing arm type, which are often quite off. Check with the distilled water, your tap water might have oddities in it unknown to you.

Phaedrus · Post by **Phaedrus** » Thu Jan 25, 2007 3:23 pm

I'd say it's time to start looking at getting your hands on a refractometer instead.

Peregrine · Post by **Peregrine** » Thu Jan 25, 2007 3:55 pm

What temp is the water you are using to calibrate vs the temperature the hydrometer was corrected to

Salinity is salinity...SG is an approximation to determine salinity but the results will vary with temperature...or there could be an air bubble on it somewhere

I agree with with Phaedrus...get a refractometer with ATC much easier

Boxermom · Post by **Boxermom** » Thu Jan 25, 2007 4:30 pm

Cheapo on Ebay. Got a second one yesterday for $24. He shipped it really fast, too. It took 9 days from auction end to delivery.

And when I tested it last night, I didn't even have to adjust it, it was calibrated perfectly.

http://stores.ebay.com/OPTICS-MART

Gomez · Post by **Gomez** » Thu Jan 25, 2007 6:51 pm

what is the process of calibration on a refractometer?

Did 24$ include shipping?

Phaedrus · Post by **Phaedrus** » Thu Jan 25, 2007 9:42 pm

It varies, but generally it involves using a few drop of ultra pure water at 20c to get an accurate baseline reading of 1.000. Most if not all refractometers should come pre-calibrated if they are new from the factory, and would only need to be re-calibrated every couple months with steady use.

tanhql · Post by **tanhql** » Thu Jan 25, 2007 10:45 pm

Nick wrote:I'd say it's off a bit, most floating type are off even more than the swing arm type, which are often quite off. Check with the distilled water, your tap water might have oddities in it unknown to you.

sad to say, my swing arm hydrometer is even more off, measuring 0.006 for tap water. as it is for brackish only, refractometer is only my last (and hopefully, never need to be used) resort.

Boxermom · Post by **Boxermom** » Thu Jan 25, 2007 10:48 pm

Gomez wrote:Did 24$ include shipping?

No, that was an additional $9.95, still cheaper than I've found anywhere else.

tanhql · Post by **tanhql** » Fri Jan 26, 2007 12:14 am

so if i can find a floating hydrometer that says 1.000 when put in distilled water, it's accurate?

Boxermom · Post by **Boxermom** » Fri Jan 26, 2007 12:21 am

tanhql wrote:... as it is for brackish only, refractometer is only my last (and hopefully, never need to be used) resort.

Not sure I understand this. Why would you consider it a never need to be used resort? They work great for low end brackish to full marine.

tanhql · Post by **tanhql** » Fri Jan 26, 2007 5:35 am

Boxermom wrote:
tanhql wrote:... as it is for brackish only, refractometer is only my last (and hopefully, never need to be used) resort.
Not sure I understand this. Why would you consider it a never need to be used resort? They work great for low end brackish to full marine.

i'm not exactly flushed with cash right now, which is why i'm trying to make-do with the present equipment by 'calibrating' the hydrometer against something which i know the SG, like distilled water, then i can do the necessary adjustments to the readings. now all i'm asking is can it be done? because i read somewhere that the floating hydrometer, even if inaccurate, is consistent with the SG, ie the difference in SG between two lines is always 0.002.

Nick · Post by **Nick** » Fri Jan 26, 2007 5:57 am

I'm not sure if that goes for the floating type as much as it goes for the swingarm type, since the floating type can be inconsistent in many other ways, being shaped glass. Be sure to calibrate it at several salinity points, and see if you can use someone's refractometer or known to be accurate hydrometer aloneside yours when calibrating.

Phaedrus · Post by **Phaedrus** » Fri Jan 26, 2007 2:59 pm

Since you're strapped for cash I can understand wanting to make do with what you have, but a refractometer is certainly worth saving up for in the near future. As you can see by your current troubles, hydrometers are inaccurate and not to be trusted.

That said, you may be able to clean the hydrometers up a bit by using a very acidic solution to dissolve the mineral deposits that attach themselves to the hydrometers surface. To do this, I've found using a 2:1 mixture of hydrogen peroxide and vinegar to soak the hydrometer in for 24 hours or so works well. Be sure to rinse it very thoroughly afterwards.

This may help, but it may also be that your hydrometers are physically damaged in some way, or simply naturally inaccurate.

Boxermom · Post by **Boxermom** » Fri Jan 26, 2007 3:02 pm

Also, are you floating it in the tank or in a separate container like a glass filled with the tank water? I believe Neale says floating it in the tank leads to incorrect readings and it should be floated in a glass or something. I'll see if I can find his note about it.

Here you go: http://www.thepufferforum.com/forum/vie ... php?t=6339

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