The Model SP-3A is a unique, patented system that was designed as a low cost instrument for quickly and easily measuring the Littleton Softening Point of glasses without pulling fibers. The tests are performed on a small blocks of glass samples.
It takes 20 minutes to measure the Littleton Softening Point of a glass sample. The SP-3A is used for a quick, simple, and reproducible QC checks on batch chemistry changes.
The Model SP-3A was conceived, developed and patented by Dominick Labino, an internationally renowned glass scientist and artisan, who recognized the glass industry's need for a simple, rapid softening point determination. This simple, low cost device is a modified parallel plate viscometer whose results correlate very well with Littleton Softening Point Temperatures that are determined according to ASTM C-338. Solid glass samples are used instead of glass fibers. Consequently, sample preparation is easier, particularly for glass compositions that are difficult or impossible to form fibers.
The precut sample is placed in the stainless steel sample holder at room temperature. A thin layer of a high purity alumina prevents the glass sample from sticking to the sample holder. The 1/8" diameter stainless steel ball bearing is placed on the sample top surface, and the penetration rod is lowered to hold the ball bearing in place. The sample holder is lowered into the room temperature furnace, the operator positions the microswitch with the micrometer, pushes the start button, and walks away to perform other tasks. The instrument does the rest. The operator returns in about 30 minutes and records the softening point temperature shown on the controller display. After the correction factor is applied by a simple addition, the approximate Softening Point Temperature is determined.
When the operator presses the start button, the furnace begins to heat at 25°C per minute. The glass sample correspondingly heats and begins to soften. As the the glass softens, the static weight of the penetration rod and ball bearing forces the ball bearing to penetrate the surface of the glass sample. Once the ball bearing has penetrated the sample by a predetermined amount, the microswitch reaches the limit position and disconnects the power to the heating element. The controller stores the temperature at which the power was terminated, and displays it for later viewing.
The SP-3A measures the temperature that corresponds to a certain AMOUNT of deformation. The Littleton method measures the temperature that corresponds to a certain RATE of deformation (as elongation). These two temperatures are not always the same.
Correction Factor and Calibration
The SP-3A glass sample heats and softens as a function of the heating rate, the sample size, and the applied load. So glasses of the same composition (the same viso-elastic characteristics) will have different "softening point temperatures" if the heating rates, the sample sizes, and the applied loads are different. The Model SP-3A controls 2 of those factors by using a single heating rate and a uniformly applied load (fixed weight and fixed geometry of the ball bearing contact tip). The operator controls the third factor by preparing sample sizes that are uniform.
The SP-3A generates reproducible readings for samples that are the same size (length, width, and thickness - thickness is the most important) and from the same composition. By testing a series of samples cut from the same composition, the SP-3A will generate a consistent temperature value. It most cases, this temperature will be different from the Littleton Softening Point Temperature as measured by the fiber elongation method, but this difference will be consistent. This consistent difference is the "correction factor".
This "correction factor" is determined by measuring samples of a similar glass composition with a known softening point, such as a NIST glass composition. Sample from this standard batch are cut to the same size and tested in the SP-3A. The difference between the SP-3A temperature and the Littleton temperate is the "correction factor". This correction factor is added to the SP-3A temperature from the test glass sample, and the Littleton Softening Point Temperature is approximated.
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