[Clayart] Clayart Digest, Vol 8, Issue 76

L TURNER magnolia.mud.list at gmail.com
Wed Jul 27 20:36:39 EDT 2016

Yes.  you have the correct concept of SG in mind.  My point is that the
technique implied by Jeff's post has a built in systematic "dead volume"
that must be accounted for or the calculated ratio will be in error.  The
importance of the error will depend on the situation, and as Mel has told
us many times, some situations are less forgiving of errors that others.

Work backwards from a 100 CC syringe filled with glaze:

Step 1:  weigh the weight of the syringe filled to 100 cc mark with
glaze and record as the FULL WEIGHT.
Step 2:  remove 100 cc of glaze
Step 3:   weight the syringe and record that weight as the TARE WEIGHT.
Step 4:   remove the remaining glaze from the syringe, wash the syringe,
dry the syringe, reassemble the syringe and weight the clean dry syringe.
record that weight as the EMPTY SYRINGE WEIGHT.
Step 5:  do the math:
The value of the FULL WEIGHT  minus the TARE WEIGHT will be the NET WEIGHT
of the 100 cc of glaze.  Divide NET WEIGHT by 100 and the result is the SG
for the glaze that was in the syringe.
The value of the TARE WEIGHT minus the EMPTY SYRINGE WEIGHT will be the
weight of the glaze in the "dead volume".

Jeff's description implies that JEFF'S Tare Weight is the weight of the
empty syringe.  JEFF'S Tare Weight will be equal to the TARE WEIGHT minus
the weight of glaze in the "dead volume".

One way to account for the "dead volume" would be to fill the syringe with
water to the 100 cc mark and weight the syringe and water.  subtract the
weight of the dry empty syringe.  and calculate the total volume of water
in the syringe form the weight differences.  calculate the mark to fill to
so that the total volume will equal 100 cc.  This is a way to calibrate the
syringe to fit the situation.  I'm sure there are other ways to deal with

I used  Jeff's method years ago to check the densities of several lab
slurries and got results that were inconsistent with known standards.
Trouble shooting why the results were off lead to the discovery of the
"dead volume" which is vital to syringes.  The effect of the "dead volume"
is more likely to be relatively small on a 100 cc syringe, but on a 50 cc
syringe my eyeball memory is that it could be significant.  It certainly
was on 10 cc syringes I was using when I encountered calibration errors.

Thanks asking me to be more clear.


On Wed, Jul 27, 2016 at 12:49 PM, Michael McDowell <
michael at mcdowellpottery.com> wrote:

> On 7/27/2016 4:13 AM, Lou Turner wrote:
>> All the syringes I have used or seen have been designed to deliver an
>> amount determined by moving the plunger from "this mark" to "that mark" on
>> the body of the syringe; when the plunger was pushed all the way to the
>> zero mark, there was still some liquid in the syringe.  The point being
>> that a 100 cc syringe holds more than 100 cc of liquid. Yes this might
>> only
>> be a cc or two on a 100 cc rated syringe, but it means that the measured
>> weight is for a volume greater than 100 cc, and the result is that the SG
>> of the glaze is actually less than the result of the gross weight of the
>> full syringe minus the tare weight of the empty syringe divided by 100.
> Actually Lou, I think you need to hold on a minute as well. As I
> understand it, Specific gravity is the ratio of the weight of a given
> volume of one substance to the weight of that same volume of pure water.
> One could pick any convenient volume on which to make the comparison.
> Michael McDowell
> Whatcom County Washington
> www.McDowellPottery.com
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