INSTRUMENTATION by Ralph H. Müller
Automatic recording vacuum thermobalance records w e i g h t changes as function of temperature or time T H I S mid-November day we leave O NNew York City for the Southwest after a stimulating and enlightening visit to the Eastern Analytical Symposium a n d Instrument Exhibit and a session on Automatic Chemical Analysis at the New York Academy of Sciences. Among the several new and interesting instruments which we saw, one of particular interest to us was the automatic recording vacuum thermobalance, designated as t h e "Thermo-Grav" by its manufacturer—The American I n strument Co., Inc., 8030 Georgia Ave., Silver Spring, Md. This instrument records thermogravimetric curves of samples in vacuum or controlled atmospheres a t temperatures u p t o 1000° C. The instrument can record changes in weight as a function of temperature, programmed for a selected heating rate, a,nd also changes in weight as a function of time, at a constant temperature. This instrument was developed at the American Instrument Co. with the assistance of consultants Saul Gordon and Clement Campbell, both of t h e Picatinny Arsenal, Dover, N . J. A general view of the instrument is shown in Figure 1 and a schematic of the operating principle in Figure 2. The instrument consists primarily of a highly precise spring balance enclosed in a glass vessel and can be operated a t controlled temperatures under a vacuum or a controlled gaseous atmosphere, at or below ambient pressure. Spring deflections, proportional t o changes in sample weight, are converted into electrical signals b y movement of the core in a linear differential transformer. These signals, after passing through the amplifier and demodulator, are presented as weight changes on the Y axis of an XY recorder. T h e X axis of the recorder receives a n input signal which corresponds either to furnace temperature or to elapsed time. The suspension system carries a weight calibrator pan, an oil-dashpot, damper, a n d a t t h e bottom, fused quartz rings to hold the sample crucible and tare crucible. Two furnaces are designed t o allow consecutive determination without delay due to "cool-
ing-down" time. These can be rotated and elevated into position with ease and precision. The lower quartz jacket is easily removed and is fitted with a, lateral port for rapid evacuation and subsequent purging or filling with any desired gaseous atmosphere. The system is shown in greater detail in Figure 3. The furnace control and programmer can be set for any one of 16 temperature spans with a choice of seven rates of temperature rise. Temperatures and elapsed time are indicated digitally on the control panel as well as presented on the recorder. For temperature control computer
techniques are used. Three signals are fed into a summing junction, amplified by a high gain amplifier, the output of which is fed to a relay. T h e signals consist of ihe measuring thermocouple output, t h e output from a motor-driven potentiometer, and a small sweep voltage. The thermocouple, in series with a fixed resistor, generates a current proportional to its temperature. The voltage across the potentiometer is maintained constant b y means of a Zener diode. Since t h e potentiometer is driven at a fixed rate, its output voltage will be proportional to time. A resistor in series with t h e . arm of this
Figure I . Thermo-Grav balance—general view VOL. 3 2 , N O . 1 , JANUARY 1960
·
77 A
INSTRUMENTATION
Spring
Transducer Armature
Demodulator
Transducer Coil Weight Calibrator Damper
X-Y Recorder
Sample Chamber
Vacuum Pump
Sample Crucible Tare -Crucible Sample Thermocouple
Furnace Control and Programmer
Controlling Thermocouple Furnace
Figure 2. Thermo-Grav balance— schematic view
5:10 P.M.: Beginning
of a difficult, time-consuming
analysis:
Cary Spectrophotometers Record Spectra Around the Clock...unattended ...with the Program Control Accessory This useful accessory enables CARY SPECTROPHOTOMETERS to automatically repeat spectral scans a t predetermined time intervals. Scans m a y be repeated a t intervals ranging from as little as 10 minutes to 24 hours, and may be spaced in any desired program. Some laboratories almost double the usefulness of their CARY SPECTROPHOTOMETERS b y using them during t h e day on routine work, then setting them up with the P r o g r a m Control for automatic analysis during the night. However, automatic programming is just one feature which d e m o n s t r a t e s t h e u n m a t c h e d v e r s a t i l i t y of CARY SPECTROPHOTOMETERS. The wide wavelength range, high resolution, low stray light, plus t h e availability of a number of useful accessories for other measurement techniques, enable Cary Instruments to serve a wide analytical field. Complete information on these versatile instruments is yours for the asking. Write for Data File A13-10.
Cary
RECORDING SPECTROPHOTOMETERS
APPLIED
78 A
·
PHYSICS
CORPORATION
· 2724 SOUTH PECK ROAD · MONROVIA, CALIFORNIA
Circle No. 44 on Readers' Service Card ANALYTICAL CHEMISTRY
potentiometer can be varied by means of a switch, and the output is therefore a function of this switch position. I t is calibrated in degrees centigrade per minute and the potentiometer arm is calibrated in minutes. A third input is a small sweep current. This is driven at 7 r.p.m. and provides a small modulating signal to the amplifier. Thus the furnace is switched on and off seven times a minute, the length of "on" time being determined by the error signal between the thermocouple and the potentiometer outputs. A proportional control is used to adjust the stability of the system and prevent temperature cycling. This is similar to the gain control in any servo loop. A blocking diode prevents the relay from being energized when the summing junction is positive (indicating "too hot") but allows it to be energized and apply power to the furnace when the summing junction is negative. In order to read the temperature of the sample in digital form and to be able to drive recorders linearly with respect to temperature, a nonlinear converter is required, because a thermocouple is not sufficiently linear for the purpose. The converter is a standard servo loop with a nonlinear feedback potentiometer. This requires a potentiometer, the output of which matches the thermocouple. Linear potentiometers with proper taps and shunts are used. Thus the shaft rotation is a linear function, not of voltage, but of temperature. The readout dial counter is coupled to this shaft and other linear
NALGON flexible plastic tubing dimensional stability water-clear transparency high chemical resistance withstands autoclaving
W non-toxic economical absolute safety complete neutrality lasting flexibility heat resistance 4 0 different sizes
PRODUCT
Our informative Catalog H-459 and a sample of NALGON will be sent on request . . . Write Dept. 22 W O R L D ' S
L A R G E S T
The
potentiometers are also coupled t o it t o drive t h e recorders. T h e voltage across t h e potentiometer as well as across t h e cold junction com pensation network is regulated with Zener diodes. These various control devices a r e illustrated in Figure 1. Some other characteristics of this elegant instrument a r e as follows: Changes in sample weight from 0 t o 200 mg. can be measured with an a c curacy of 1% of full scale with sample weights u p t o 10 grams. Controls permit t h e linear heating rate of t h e furnace t o be set a t 5 ° , 1 0 ° , 2 0 ° , 5 0 ° , 100°, 200°, 500°, or 1000° C . per h o u r ; or for maintaining furnace a t constant temperatures. Recorder adjustment permits X-axis spans (full scale deflection) for tem perature increments of 200°, 500°, and 1000° C. T h e T h e r m o - G r a v is a most versa tile instrument a n d can be expected t o contribute greatly t o a variety of in vestigations.
N A L G E Co. I n c . •
P R O D U C E R
INSTRUMENTATION
"
"
OF
"
'
—
•
"
PLASTIC
"
•
"
'
L A B - W A R E
Circle No. 95 on Readers' Service Card
A N e w Concept in Moisture Testing!
OHAUS MOISTURE DETERMINATION BALANCE A new type of balance for accurate determination of mois ture percent of any granular solid, semi solid or liquid which can be dried safely by heat. A combination dryins unit and precision balance for meas uring the moisture content o f 1 0 gram samples o f a w i d e vari ety of materials w i t h a sensitivity of ± 0 . 1 % . Can also be used as direct reading balance, sensitivity 0.01 sram. Balance is of the substitution type. Loss of weight caused by evaporation of moisture from the sample is compensated for by the addition of chain to one arm of the beam by turning the graduated dial on front of the instrument. Knife edges are of hardened, polished steel. Bearing surfaces are of agate. Beam is provided w i t h zero adjustment and is magnetically damped. Shadow-type indicating vane gives immediate indication of balance position.
For Moisture Per Cent in:
The 6 5 0 watt infrared heater provides rapid, even evaporation and is adjustable both in percentage output and in distance above the sample. Heating chamber has heat-resistant handle and a Pyrex glass w i n d o w for observation of sample. Timer shuts off heat at any period from 1 to 0 0 minutes.
Cements, Chemicals, Detergents, Fertilizers, Flour, Food, Grain, Leather,
Cat. No. A 16080 Balance, Moisture Determination—Overall dimensions: 12'/ 2 χ 10 χ 15 inches high. With 25 disposable pan liners. For 115 volts, 60 cycles A. C $295.00 Cat. No. A 16081 Pan Liners, Disposable—25 per box box $2.00
Paper, Pharmaceuticals, Seeds, Soils, Tobacco
STANDARD SCIENTIFIC
Supply Corp.
8 0 8 B R O A D W A Y NEW YORK 3, N Y .
Circle No. 113 on Readers' Service Card
80 A
·
ANALYTICAL CHEMISTRY
lABCtHATOIY APPARATUS •EACENTS AND CHEMICALS
ι j I
Figure 3. Heating system for vacuum thermobalance
