Judith Enright on tue 3 nov 98
I received the following message. Can anyone help with this? Please respond
directly to firstname.lastname@example.org.
Judith Enright @ Black Leopard Clayware
Date: 11/1/98 10:01 PM
Hi, I am a student currently working on a project. My project requires that I
research and make a hydrometer. I would appreciate it if you would send me any
information on how hydrometers work and any other information you have on
hydrometers. You can contact me at email@example.com with you reply.
Hi, I am a student currently working on a
project. My project requires that I research and make a hydrometer. I would
appreciate it if you would send me any information on how hydrometers work and
any other information you have on hydrometers. You can contact me at href="mailto:firstname.lastname@example.org">email@example.com with you
This email may contain confidential and privileged material for the sole use of
Gavin Stairs on wed 4 nov 98
At 10:12 AM 11/3/98 Judith Enright wrote:
>I received the following message. Can anyone help with this? Please respond
>directly to firstname.lastname@example.org.
>Hi, I am a student currently working on a project. My project requires that I
>research and make a hydrometer. I would appreciate it if you would send me
>information on how hydrometers work and any other information you have on
Ordinarily, research means a bit more work than simply asking someone to
dump the whole load on you, but this seems to be a topic that more than one
person on the list might be interested in, so here goes.
Hydrometery is the measurement of the density, or specific gravity, of
liquids. Hydro means water, so the word root is water measure. If you
like, the ratio of the density of the liquid in question to the density of
water. That's the definition of specific gravity, the density of water
being 1g/cc. So, numerically, the density in g/cc and the specific gravity
are the same.
A hydrometer is an instrument for making this measurement. An immersion
hydrometer consists of a sealed bulb with a well defined mass shaped and
weighted so as to float upright in liquid with a projecting scale on top.
The measurement is taken by reading the scale where the surface of the
liquid contacts the bulb neck.
The bulb is really a sort of inverted cup with a built in weigh scale.
This may be clearer if I describe another kind of hydrometry first.
Take a cup and weigh it dry (Wd). Put into it a known volume of liquid
(V). Weigh it again (Ww). The density of the liquid is (Ww-Wd)/V. If you
weigh in grams and cubic centimeters, the result is in g/cc, or without
units, it is the specific gravity of the liquid. If you weigh in other
units you must convert to get the specific gravity. This method, by the
way, works for any mixture, especially glazes and clays. However, if you
use it with dry material, be aware that one of the mixture components is
air, so you don't get the density of the solid material, but something
somewhat less, and somewhat imprecise.
The difficulty in making a precise measurement this way lies in
establishing the volume. There are instruments to help with this by
defining and enforcing a standard, fixed volume (e.g., 100cc), which helps
with the arithmetic as well as with the volume. Two kinds are the overflow
cup, with a spout that dribbles off the excess, and the picnometer, which
has a tightly fitting lid with a fine hole in it, through which the excess
liquid squirts until the lid is tight. The first one is chiefly found in
student labs, and works best with watery liquids. The picnometer is more
precise, but may have trouble with suspensions like glazes. Other methods
involve the use of metering delivery systems, like burettes and pipettes.
All of these methods might be called the constant volume method, since the
volume is a parameter of the measurement, while the mass is the variable to
The immersion method is a variable volume method. The weighing is done by
the fixed mass of the instrument, while the measurement is done by the
displacement of a variable volume of liquid by the bulb and stem of the
instrument while floating freely in the liquid. If the bulb floats freely
in a Newtonian fluid ( I'll get to this), then it will displace just enough
liquid such that the weight of the displaced volume of liquid equals the
weight of the instrument. There is your balance, which fixes the mass (M).
All you need now is to establish the volume of the liquid displaced (Vm).
Then the result is M/Vm. You can calibrate your instrument and write the
result directly on a scale attached to the stem. Calibration is difficult,
but you have as much time as you need to do this, since you only do it once.
Most such hydrometers have a long, narrow stem, so that a small change in
displaced volume will represent a longer distance on the scale. This
increases the sensitivity of the instrument. It also increases its
fragility: Don't dry a hydrometer by shaking it while holding on to its neck.
Now, I mentioned Newtonian liquids above. This means that the fluid will
support no stress without yielding. In other words, a normal liquid, like
water, beer or wine. Not a liquid with thixotropy, or the ability to hold
its shape. Such a non-Newtonian liquid will support some stress before
yielding, like paint, or wine must, or jelly, or clay suspensions. Like
most glazes and slips. These non-Newtonian liquids, or sols or gels, will
not permit the bulb to reach its neutral buoyancy position without some
error introduced by the residual support of the stress in the fluid, which
may be either positive or negative. So immersion hydrometry is not a
particularly good choice for measurement of the density of glazes and
slips, unless they are very runny.
The easiest studio method for measuring density of glazes and slips is
probably some variant on the following method:
Take a small pot of about 100-250cc -- about a cup is ok. It should have a
level rim, and a mouth as small as possible. It should also be non-porous.
A small glass, porcelain or plastic bottle or jar will do. Dry it and
weigh it carefully (Mp). Mark the weight on it somewhere. Fill it with
water, just to the top, so it overflows. Take a straight edge, and strike
across the rim of the pot so that the meniscus of water is removed, or
carefully reduce the water until the surface is perfectly flat. Then dry
off the outside,and weigh it again (Mpw). The difference these two weights
is the water weight (Mw). Mark this on it as well. Then, to measure the
density of any liquid, simply fill the pot, strike off the excess so the
liquid is flush to the brim, wipe off the overflow and weigh (Mpl). The
specific gravity is (Mpl-Mp)/Mw.
If your most accurate scale will measure a larger pot, this will increase
the accuracy. For ordinary potting liquids, the specific gravity will be
between 1 and 2, so the mass you have to weigh is less than the volume
contained in cc times 2, plus the weight of the pot, in grams. So, for a
250cc pot this will be about 600g, and about 2250g for a 1-liter pot.
There are, of course, many fine points, but I see that most of the list
people have bailed out already, so that's it for now.
Stairs Small Systems (S3)
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1