MEASUREMENT OF THE PERCENTAGE CONTENT OF ANTIMONY IN THE LEAD SHEATH OF A CABLE BY THE TEMPERATURE OF THE ONSET OF CRYSTALLIZATION OF THE ALLOY

V. K. ZHUKOV, R. M. ZAKIROV, V. A. NIKULIN

(Presented by the scientific seminar of the Faculty of Automation and Computer Engineering)

It is shown in [1] that between the percentage content of antimony (Sb) in a lead–antimony alloy (Pb—Sb) and its temperature of onset of crystallization there exists a functional dependence, which lies in the temperature range from 327°C, corresponding to the end of crystallization of pure lead, to 321.1°C, corresponding to the onset of crystallization of the alloy (Pb—Sb) with an antimony content of 0.9%. When measuring the temperature of the alloy with a chromel–copel thermocouple, its thermoe.m.f. in the indicated temperature range will vary from 23.978 mV to 23.406 mV.

The dependence found was taken as the basis for an installation developed at the Department of Information and Measuring Technology and intended for measuring the percentage content of antimony from 0 to 0.9% in a lead–antimony alloy. The installation consists of an electric furnace in which the alloy under investigation is melted, a chromel–copel thermocouple, a PS1-02 recording instrument, and a device for automatic control of the heating and cooling process.

The design of the electric furnace is shown in Fig. 1. The main element of the furnace is heater 3, which is a vitrified resistance of type PE-25 rated at 300 ohms, with a metal cup 4 of conical shape placed inside it and intended for the alloy 5 under investigation. A bushing 6 with a chromel–copel thermocouple is screwed into the bottom of the cup; the thermocouple is clamped in it in such a way that the upper part of the hot junction of the thermocouple has mechanical and, consequently, thermal contact with the alloy under investigation.

The heater is installed inside housing 1 of the furnace and is fastened there by means of cover 2. In order to reduce heat transfer from the heater to the surrounding medium, the space between the heater and the housing is filled with crushed asbestos. The heater is supplied with a voltage of 127 V from the secondary winding of the installation’s power transformer.

For measuring the thermoe.m.f. of the thermocouple, an automatic potentiometer PS1-02 was used. The potentiometer is manufactured for measuring temperatures of 0–150°C with a chromel–copel thermocouple. Since in the present case it was necessary to measure, with the potentiometer, temperatures from 321.1 to 327°C, this made it necessary to introduce certain changes into its measuring circuit. The changes concerned primarily those arms of the bridge by which the position of the beginning of the scale and the measuring range are determined.

Fig. 1. Design of the electric furnace

… measurement (Fig. 2), as well as the arm compensating for the effect of ambient temperature on the cold ends of the thermocouple.

The measuring limit of the potentiometer is determined by the resistances \(R_{\mathrm{ш}}\) and \(R_{\mathrm{п}}\). The values of these resistances were calculated from the condition of obtaining a compensating voltage varying by \(0.572\ \mathrm{mV}\).

Fig. 2. Schematic circuit diagram of the automatic potentiometer

The beginning of the potentiometer scale is usually set by the arm of the bridge in which the resistance \(R_{\text{n}}\) is included. In the present case the beginning of the scale, i.e., the extreme left position of the carriage, corresponds to a voltage at the bridge input of 23.406 mV. This voltage must compensate the emf of the thermocouple developed in it at an ambient temperature of \(20^\circ\)C and a hot-junction temperature of \(321.1^\circ\)C. The resistances \(R_{\text{n}}\) and \(R_{\text{n}}'\) are calculated so that the left position of the carriage corresponds to a bridge voltage of 23.406. The resistance \(R_{\text{n}}'\) serves for precise adjustment of the scale.

All resistances of the potentiometer measuring circuit are made of manganin, with the exception of resistance \(R_{\text{m}}\), which is made of copper. The main purpose of this resistance is to compensate fluctuations of the thermocouple emf caused by changes in ambient temperature. The value of the resistance \(R_{\text{m}}\) is determined by the sensitivity of the thermocouple in the specified range of measured temperatures and by the current flowing in that branch of the bridge into which the resistance is connected.

Since the measuring range of the instrument has been considerably narrowed in comparison with what it was, its error associated with instability of the bridge power source (IPS-020) and with the temperature instability of the resistances has increased. To reduce these errors, a calibration operation has been introduced into the potentiometer. Calibration is carried out by pressing the button \(K_{\text{n}}\). When this button is pressed, the thermocouple and the copper resistance are short-circuited, and the resistance \(R\) is shunted by the resistance \(R_2\), the value of which is selected so as to bring the carriage to scale mark 70. If, during calibration in the course of operation, the carriage does not reach the required mark, this can be achieved by means of the trimming resistance \(R_{\text{n}}'\), made in the form of a rheochord and brought out to the front panel.

To simplify operation of the instrument, a special attachment has been added to it, which automatically melts the alloy and subsequently cools it. The schematic diagram of the attachment is shown in Fig. 3.

After loading the alloy into the furnace, the controller presses the “start” button located on the front panel of the attachment. Relay \(P_1\) then operates. The normally open contact \(P_1^1\), when relay \(P_1\) operates, closes and causes relay \(P_4\) to operate. If the “start” button is now released, relays \(P_1\) and \(P_4\) will be latched and will not change their state.

Relay \(P_1\), by means of its other contact \(P_1^3\), switches on the furnace heater \(R_6\), as will be indicated by the lighting of signal lamp \(L_1\), also located on the front panel of the attachment. The break contact of this same relay opens the circuit of the SD-09 electric motor; therefore, although the PS1-02 instrument, through contact \(P_4^2\), is connected to the mains, the chart paper will remain stationary. Heating of the alloy begins. During heating, the temperature in the furnace rises and, finally, reaches the point corresponding to the melting temperature of the alloy with 0.9% Sb; the carriage of the instrument, which until then had been in the extreme left position, will begin to move to the right. In its motion the carriage will reach the extreme right position and close the limit switch KVP; at this, relay \(P_2\) will operate, which by its contact \(P_2^1\) will cause relay \(P_1\) to release and, consequently, the heater to be switched off. The temperature in the furnace will continue to rise for some time and then will begin to fall. When it falls below \(327^\circ\)C (the melting temperature of pure lead), the carriage will come off the stop and will move to the left, as a result of which relay \(P_2\) will release. From this moment on, all contacts connected in series with one of...

Fig. 3. Schematic circuit diagram of the automatic control unit for operating the instrument.

Labels appearing in the schematic: \(-127\ \mathrm{V}\); Start; Stop; Manual; Automatic; KVL; \(\sim 220\ \mathrm{V}\); PSL-02; SD-09; \(P\); \(P_1\); \(P_2\); \(P_3\); \(P_4\); \(P_1^1\); \(P_2^1\); \(P_3^1\); \(P_4^1\); \(P_1^2\); \(P_2^2\); \(P_3^2\); \(P_3^3\); \(P_4^3\); \(C_1\); \(C_2\); \(C_3\); \(C_4\); \(R_1\); \(R_2\); \(R_3\); \(R_4\); \(R_5\); \(R_6\); \(A_1\); \(A_2\); \(A_3\); \(A_4\); \(L_1\).

of the windings of the SD-09 motor are closed, the motor will begin to rotate, the chart paper will begin to move, and the cooling curve of the alloy will be recorded on it.

As the temperature decreases, the carriage continues to move to the left and at some point reaches the left stop, causing another limit switch, KVL, to close. The limit switch switches on relay \(R_3\), which, by means of its contact \(R_3^1\), blocks KVL, and by another contact, \(R_3^2\), switches off the chart-paper drive, thereby stopping the recording of the cooling curve. Yet another contact, \(R_3^3\), of the same relay switches the furnace heater on again; as a result, after several minutes the alloy is melted and remains in this state for as long as desired, provided that toggle switch P is in the “Manual” position, corresponding to single-cycle operation. When the toggle switch is moved to the “Auto” position, the carriage of the instrument, in its extreme position, will, by means of limit switch KVP, switch on relay \(R_2\), and through it relay \(R_3\) and the heater. Thus, in this mode the instrument will operate in repeated cycles, making it possible to automatically record several cooling curves and thereby reduce the random error of measurement.

The readings of the instrument are taken from the position of the horizontal portion of the cooling curve on a scale graduated in \(\% \mathrm{Sb}\).

The tests carried out showed that, with the time for one measurement not exceeding 15 min, the error of the instrument is \(\pm 0.05\% \mathrm{Sb}\). In the tests, a chemical method of analysis was used as the “standard” method.

REFERENCES

1. V. K. Zhukov, R. M. Zakirov. Studies on determining the possibility of monitoring the percentage content of antimony in the lead sheath of a cable by electrical methods. Present collection.