From: Art Chakalis (achakali@gcfn.org)
Date: 07/28/03-01:50:24 PM Z
Viscosity and specific gravity measure different things.
Viscosity is a measure of a fluid's resistance to change or a kind of
internal friction. The unit of measure is poise but a more common unit is
a centipoise (one hundredth of a poise). Poise is a measure of force over
a unit of time.
Specific Gravity is a measure of mass density. It is expressed as a ratio to
the mass of an equal volume of water at 4 C. A 14 baume solution equals a
density of 1.107.
The reason gum solutions are measured by their specific gravity is that
the moisture content of dried gum can vary quite a bit. By measuring the
specific gravity of the gum in solution you automatically compensate for
any differences in the dryness of the lumped or powdered gum used to make
your solution.
Sincerely, Art
Art Chakalis
Columbus, Ohio, USA
On Mon, 28 Jul 2003, Gordon J. Holtslander wrote:
>
> Hi:
>
> I'm the one who raised the issue of whether or not specific gravity and
> viscosity are the same thing. From what I've read I think they are not.
>
> There is supposed to be a simple tool for measuring viscosity. Its called
> a Zahn cup. Its small cup with a very small hole in the bottom.
>
> Viscosity is measured by timing how long it takes for the liquid to drain
> out of the cup, but not drain completely - the timimg is stopped when the
> fluid begins to form drops.
>
> There are a number of zahn cups available zahn #1 zahn #2 zahn #3
>
> Zahn cups are using in the paint industry. It may be possible to get a
> zahn cup from your local hardware/paint store.
>
> More info here:
> http://www.paintcenter.org/rj/Dec02m.cfm
>
> Hygrometers are used to measure specific gravity (density of a liquid)
> Hygrometers are often used by aquarium hobbyist as an indirecty (but easy)
> way of measuring the salinity of aquariums.
>
> It may be possible to get a cheap hygrometer from a pet shop/aquarium
> shop. Whether this could be used to measure the specfic gravity of gum -
> I don't know.
>
> I feel guilty having raised these issues and hinting that somebody else
> should to the footwork, and not done anything - my "photo time" has
> evaporated again :(
>
> Gord
>
> On Sun, 27 Jul 2003 stwang1@bellsouth.net wrote:
>
> > Hi Stuart,
> >
> > Very strange - I replied to this message this morning but it never showed up on the list
> > discussions. Did you receive it Stuart?
> >
> > I'll try to recap. No, I have not measured the gum I use, but my guess is that it's not too
> > different from everyone else's, that is, Gum 14 plus equal amount of water. However, I
> > have not used Gum 14 for years - the ones I tried all had too much preservative and were
> > not very soluble after drying. I mix gum granules (tears?) 1 part with 2 parts water, plus
> > very little preservative. In use, I add 1 part water to 1 part gum, add pigment to the right
> > color, and then add dry dichromate. My gum "develops" in very short time, usually in 10
> > minutes or less.
> >
> > I like to compare making gum prints to riding the bicycle: there are no secret ingredients.
> > It's working out variables one at a time. When one hasn't lerned to balance on the bike, it
> > looks so hard. Once he/she learned to ride, it's easy as pie. Talking about it doesn't get
> > you very far. No doubt you would agree, Stuart - I saw a lot of elbow grease on your
> > prints. ;-)
> >
> > Sam
> >
> > >
> > > From: Stuart W Melvin <swmelvin@earthlink.net>
> > > Date: 2003/07/27 Sun AM 10:58:17 EDT
> > > To: alt-photo-process-l@sask.usask.ca
> > > Subject: Sam...jump in here
> > >
> > > oooops......thanks for the info.
> > >
> > > To be more scientific.....we all get the gist......How thick is the stuff
> > > Sam?
> > >
> > > corrections applied:
> > > I'm wondering if you might be able to elaborate on the viscosity of the gum
> > > solution you use. Have you ever measured it with a baume meter? <disregard
> > > baume meter question. What
> > > ratios of water to gum (solution or powder) do you prefer?. Is your
> > > viscosity preference consistent or do you find you vary it for any reason?
> > > How does your viscosity preference relate to your application technique? So
> > > many question marks........I must admire your technique hey.
> > >
> > > Thanks,
> > > Stuart
> > >
> > > -----Original Message-----
> > > From: Clay [mailto:wcharmon@wt.net]
> > >
> > >
> > > Sent: Sunday, July 27, 2003 8:18 AM
> > > To: alt-photo-process-l@sask.usask.ca
> > > Subject: Re: Dry Dichromate and Gum, was Re: News from APIS
> > >
> > >
> > > Industrial viscometers are pretty pricey. But googling "viscosity
> > > measurement" gives you a nifty science project link:
> > >
> > > http://www.science-projects.com/Viscosity.htm
> > >
> > > that makes me think that measuring relative viscosity would be pretty
> > > simple. If one uses the same size tube and bead, then the time it takes
> > > to sink would give an idea of relative viscosity between two fluids,
> > > since gravity is (at least for the precision involved here) the same
> > > everywhere. Maybe at the next gum printers summit, everyone can
> > > standardize on a 1/2 inch diameter 20cm tube and Daisy BB pellet or
> > > whatever. Then when someone asks about your coating viscosity, you can
> > > just say "4 seconds". Nice and low tech.
> > >
> > > Clay
> > > On Sunday, July 27, 2003, at 01:57 AM, Katharine Thayer wrote:
> > >
> > > > Stuart W Melvin wrote:
> > > >>
> > > >> Hey Sam,
> > > >>
> > > >> I'm wondering if you might be able to elaborate on the viscosity of
> > > >> the gum
> > > >> solution you use. Have you ever measured it with a baume meter?
> > > >
> > > > Sorry to jump in here, but just thought I'd interject the comment that
> > > > you're making the same mistake I'd been making all my life until a
> > > > couple of weeks ago, when Gordon enlightened me. A baume meter
> > > > (hydrometer) measures specific gravity, not viscosity, and what's more,
> > > > the two are not related to each other. This information came to me like
> > > > the apple falling on Newton's head, and this is why I asked the other
> > > > day the speculative question about differences in viscosities between
> > > > gums. I don't know if any of us has the equipment to answer this
> > > > question; if someone does I'd sure like to know about it.
> > > > Katharine
> > > >
> > >
> > >
> > >
> >
>
> ---------------------------------------------------------
> Gordon J. Holtslander Dept. of Biology
> holtsg@duke.usask.ca 112 Science Place
> http://duke.usask.ca/~holtsg University of Saskatchewan
> Tel (306) 966-4433 Saskatoon, Saskatchewan
> Fax (306) 966-4461 Canada S7N 5E2
> ---------------------------------------------------------
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