Joseph Herbert on mon 7 dec 98
Lois Ruben Aronow Wrote about being confounded by a hydrometer.
Actually, the instructions for this instrument have been distributed in the
form of the tale of Archimedes running through the streets, fresh from a bath,
shouting "Eureka!" As we recall, the famous Greek who invented the screw that
bears his name, was struck by the fact that a body immersed in water displaces
water equal to its volume. This enabled him to determine if the crown of the
king who employed him was made of pure gold or had been debased during
manufacture. The volume of water displaced by the crown and the weight of the
crown can be used to calculate the weight of the crown per unit volume of
crown. If that value is less than the weight of gold per unit volume, the
crown manufacturer is in trouble.
The usual determination is to weigh the object in air and then immersed in
water. The difference gives the weight of the water displaced by the object.
That weight of water is easily converted into volume of water and specific
gravity of the object is calculated.
When the object of interest is lighter than water, it requires force to
submerge it completely. If left to its own devices, the object will float
with part below water and part above. It turns out that the volume of the
part below water is equal to the volume of water that weighs as much as the
object does. So, if you have an object that is of a simple shape, like a
cylinder, you can estimate its weight by the distance water comes up the side
of the object when it floats.
On the other hand, if you have this simple shape OF KNOWN WEIGHT with marks on
the side of the shape, the distance it submerges in a liquid tells the weight
of the liquid per unit volume. A volume that submerges to the 100 mark in
water will submerge further to the 80 mark if floated in gasoline. Similarly,
that volume will not submerge as far, perhaps to the 150 mark when floated in
honey. By selecting the volume and weight of the shape and the distance
between the marks, the scale can be arranged to read in Grams per Cubic
Centimeter or in some other value. Hydrometers read in amount of sugar per
volume of fluid, alcohol per unit volume, blood weight, and a host of other
values convenient to a particular measuring job.
In the case of a pottery slurry, the fluids are always heavier than water and
they are not the kind of fluids hydrometers work well in. When the materials
in the fluid are dissolved - present in molecule size pieces - the interaction
between the hydrometer and the fluid is consistent if not convenient. When
the materials in the fluid are particles of some size, the interactions
between the fluid and the hydrometer are more complex and subject to
influences that make accuracy difficult. Other correspondents have discussed
the procedures for trying to get a consistent reading in a slurry.
Some pottery operators use a very simple form of the hydrometer - a weighted
stick. The stick is placed in a fresh batch of a glaze and the level of
submergence is noted. When the glaze is used later, the same stick is
inserted and the observed level is used to determine the need for added water.
The problems with this are the same as with a more sophisticated model of the
instrument, the viscosity of the fluid works to distort the reading, the
thixothropy of a clay bearing fluid prevents a true reading, the changes in
the electrolyte content of the fluid influences the thixothropy effects of the
clay in the fluid. All these things, and others, work against even a
moderately consistent comparison of the specific gravity of a fluid from one
measurement to the next.
I have previously suggested the use of a timed funnel emptying as an
alternative to a hydrometer. The oil drilling industry deals with slurry
content and weight all the time. The drilling mud is a thixothropic fluid
that contains bentonite clay and (sometimes) barium sulfide. Weights of 17
pounds per gallon are possible, but not common. Given the difficulties of
hydrometer use, the standard practice is to weigh a standard volume of the
liquid and to time the flow of the standard volume through a funnel with a
relatively small opening. I would suggest this to our glaze making
correspondents. Most glaze makers have a gram scale. Weighing a container,
like a yogurt container, of fluid and recording that weight for fresh glaze
gives a standard to use to control the water content of the glaze. This is
certainly less subject to error than is an immersion hydrometer. I suspect
that the timed funnel test would produce similar results, without weighing,
but have not actually tested it in pottery glazes.
Now we can resurface.
Joseph Herbert
JJHerb@aol.com
Cameron Harman on tue 8 dec 98
Joseph Herbert's posting was beautiful: clear, concise and
correct.
I would propose a minor addition: it is common practice in
industry to make two separate measurements of a slurry (either
glaze or casting slip). One measure is the timed flow through a
Zahn Cup and the other is the weight of a specific volume of
slurry. The purpose is to get both the viscosity and the density.
It is possible to change the viscosity without changing the
density and sometimes, this is a very important thing to do. The
viscosity measures how well, or how fast, it flows and the density
tells you how much material you actually have in a given volume of
slurry. Since the same volume is often used each time a casting is
made, it is important to have the same density so that the wall
thickness doesn't vary and the set up time doesn't vary.
A Zahn cup is any one of a series of cups of a specific size with
a single hole of a specific size. You do not need to buy a Zahn
cup, you can use your own. A typical example is a cup 2" diameter
by 2" deep with a 1/8" hole. Fill to the top without running over,
then hold your finger over the hole until you release. Start a
stop watch when you release and stop the watch at the same volume
of flow each time. It is best not to wait until the cup is empty,
time until you reach a score mark on a glass container, for
example, so you can clearly see it each time.
I often use a 50 mm beaker to fill for the density test. It is
small, but large enough to get a representative sample and you
know its volume (50 cc). It is also light so that it can be set on
a scale and the scale set at zero with the empty glass on it. Of
course, anything will do as long as you can measure everything OK.
Cameron Harman
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