Lyophilization of pharmaceuticals II. High-sensitivity resistance bridge for low-conductivity measurements at eutectic temperatures
✍ Scribed by Leon Lachman; Patrick P. Deluca; Ronald Withnell
- Publisher
- John Wiley and Sons
- Year
- 1965
- Tongue
- English
- Weight
- 587 KB
- Volume
- 54
- Category
- Article
- ISSN
- 0022-3549
No coin nor oath required. For personal study only.
✦ Synopsis
A description is presented of the circuitry, design, and o erating characteristics of a temperature-conductivity bridge es ecially fabricated &r eutectic and freezingpoint temperature measurements. TLe resistance bridge is capable of measuring accurately from lo3 to 1 0 1 2 ohms over a temperature range from -50' to 4-50'.
A d.c. applied potential of 1, 10, or 100 v. is used as a power supply, and a solidstate operational amplifier is employed as the detecting system. By employing a high-gain amplifier with an input impedance in excess of 108 ohms, stable power supplies, and special insulation design, it is possible to cover the range from lo5 to 10'2 ohms in narrowor wide-range decade steps. The circuitry of the temperatureconductivity plotter permits temperature measurement by either thermocouple or thermistor circuits. The information obtained is displayed on a X-Y recorder with variable sensitivity.
NOWLEDGE of the eutectic temperature of a pharmaceutical substance to be lyophilized is essential for the design of accurate and optimal freeze-drying cycles. Of the several methods available for the determination of eutectic temperatures, electrical resistivity measurement is the most accurate (1). This method consists of measuring the resistance of a frozen sample as a function of temperature. The resistance of a frozen mass below the eutectic temperature is very high. Consequently, for accurate estimations of eutectic temperatures through resistivity measurements, a sensitive low-conductivity bridge capable of measuring resistance to 1 0 1 2 ohms is required. Since the normal conductivity bridges available on the market are not capable of precise measurements of resistance in the range of 10" to 10I2 ohms, a temperature-conductivity bridge of unique capabilities was designed. This bridge is capable of measuring resistance as high as 10I2 ohms. It is composed of four wide-range decades ranging from 0-106 ohms to O-lOIa ohms and eight narrow-range decades from 10C106 ohms to 1011-1012 ohms. Calibration resistors are provided for use as internal standards. A thermistor or thermocouple probe is used to sense changes in the temperature of the material in the conductivity cell. An X-Y recorder is used for plotting the resistivity as a function of tem-