A Vacuum Tube Time-Delay Relay d
d
EARL J. SERFASS
Lehigh University, Bethlehem, Penna.
A
NECESSARY auxiliary for many laboratory appliances and industrial devices is a time-delay relay or switch.
The control of compressors, refrigerators, and other motordriven devices which are likely to be injured by the “chattering” or the too frequent operation of a sensitive relay, is greatly facilitated by the use of a time-delay switch. The automatic application of plate voltage to thyratron rectifiers or x-ray tubes at a given interval after the filament current has been turned on is a convenience afforded only by an automatic time-delay relay. Although the device presented here was designed primarily for use in conjunction with a sensitive vacuum tube relay (8) for the operation of a thermostatically controlled refrigerator, many other uses may be suggested. Very short time-delay action, from a small fraction of a second to a few seconds, may be obtained from a vacuum tube relay by placing a high-capacity condenser across the terminals of the magnetic relay (I, 3). Although this procedure may limit chattering, the time-delay feature is a function of the rate of charge and discharge of the condenser and consequently its flexibility is limited to the comparatively narrow range afforded by condensers of practical size. Several commercial types of relays are available in which the timedelay action is obtained by a clockwork mechanism which trips a switch. Although a wide range of timedelay intsrvals is made available by this type of mechanism, the complexity of the clockwork trip unit makes its cost prohibitive in many instances. Other time-delay relays based upon the principle of the thermal inertia of a n electrically heated bimetallic element are commercially available. These units offer difficulties of construction for the average laboratory technician. Le Van (2) has utilized the cathode heating time of a vacuum tube as the basis of a timedelay switch. The unit
described below likewise obtains time-delay action from the heating time necessary to allow electrons to flow from the cathode to plate of a vacuum tube rectifier. The circuit provides a simple, inexpensive, reliable, and easily constructed alternative time-delay device, which can be operated by a bimetallic thermoregulator of the type whose contacts open when the temperature setting is exceeded.
A schematic diagram of a single-tube relay unit is shown in Figure 1 A . When contact is made across contacts X X by means of a switch or relay, heater current begins to flow in the vacuum tube rectifier, T-1. When the cathodes of this tube reach a temperature sufficient to allow electrons to flow to the plates, the relay magnet becomes energized. The relay closes when T-1 has reached a temperature sufficiently high to allow the passage of the minimum amount of current which will offset the relay spring tension, and remains closed as long as the cathodes of T-I exceed this minimum temperature. If the circuit across the contacts X X is broken, the relay does not immediate1 release. Instead, the heater of T-1slowly cools, causing a graiual decrease in the pmsage of current from the cathodes to the plates and through the relay coil. When the flow of current is no longer sufficient to offset the relay spring tension, the load circuit is opened. Large variations in time interval may be obtained by increasing or decreasing the number of rectifier tubes used in cascade. Thus, one or more rectifier tubes connected as shown in Figure 1 B may be inserted in the relay unit shown in A at the lettered points. When contact is made across contacts X X in a two-tube circuit of the type described above, heater current begins to flow in rectifier T-1. When the cathodes of this tube reach a temperature sufficient to allow electrons to flow, the heater of rectifier T-2 starts to heat, since the direct current load of the first rectifier is the heater of rectifier of T-2. As the second rectifier begins to pass current, the relay magnet becomes energized, since it is now directly connected across the output of the second rectifier. Starting time delays up to 3.5 minutes have been obtained by using a series of five tubes connected in such a fashion that the output of each tube is used to s u p ply the heater of the following rectifier. Changes in time interval may be accomplished by by-passing one or several A tubes in the cmcade. Rapid changeover may be facilitated by simply removing one or more tubes and replacing them with dummy plugs, certain terminals of which have been connected as shown in Figure 2. Minor @anges in the magnitude of the time-delay interval may be obtained by the adjustment of the spring tension on the relay contacts. An increase of spring tension will require a greater relay-actuating current, which will necessitate a longer heating period for the final rectifier tube. Although an increase in spring tension will cause an increase in the starting time interval, a corresponding decrease in the cooling or open contact delay will be produced. Since maximum reliability of operation is obtained at one definite adjustment of spring tension, only slight changes in time delay action may be made by 11726GT this method. A third method of producing variations in the time-delay interval is realized by changing the capacity of A.C.-OR D.C. condenser C-1. Although the main purpose of this condenser is to prevent DIAGRAM OF TIME-DELAY RELAY FIGURE 1. SCHEMATIC chattering of the relay when an alterRelay operated by direct current, 116 to 136 volts t o energize at 60 milliamperes or less nating current power supply is used, C-I. 8-mfd. electrolytic condenser, 250-volt working voltage slight time-delay variations may be T-1 T - 2 . 117Z6GT vacuum tube, National Union Co. 352
ANALYTICAL EDITION
May 15, 1941 gained by changing its capacity within practical limits (1 to 16 microfarads). If a highcapacity C-1 is used when 115-volt alternating current is applied, the direct current potential across the relay coil may reach as high as 160 volts. Danger of injury to the relay coil mny result unless a current-limit ing resistor is placed in series with the coil.
FIGURE 2
The time-delay relay described above offers a number of advantages over many of the commercially available devices, despite itssimplicity of'construction and 106 cost. Since standard radio replacement parts were u3ed throughout the construction, the total cost of parts for the
353
two-tube unit did not exceed $6.50. The use of rectifier tubes which require 117 volts for heater operation eliminates the necessity for filament currenblimiting resistors or transformers and consequently permits the use of several tubes in cascade. Furthermore, the design permits the operation of the relay unit from either alternating or direct current power supplies. When a direct current power supply is used, proper polarity must be observed, as shown in Figure 1.
Literature Cited Hawes, R. c., ENCt. CHEM., Ana,. Ed,, 11, 222 (1939). (2) Leven, p. s., QST,20, N,,. 5, 57 (1936). (3) Serfass, E. J., IND.ENO.CHEM.,And. Ed., 13, 262 (1941).
Calibration of Existing Gum-Stability Test Bombs In Terms of the New A. S. T. M. Bomb D . L. YABROFF AND E. L. WALTERS Shell Development Company, Emeryville, Calif.
A method is presented whereby induction period bombs of non-A. S. T. M. type may be so calibrated as to yield "A. S. T. M. induction periods". This permits comparable results without the large expense of replacing useful equipment now in existence.
I
T IS now well known that the deterioration of gasoline in
storage, leading to the formation of gum, is a manifestation of atmospheric oxidation. It is accordingly reasonable that some form of an accelerated oxidation test employing air or oxygen a t elevated temperatures will be required as a measure of stability. I n 1930, Hunn, Fischer, and Blackwood (3)introducxl a bomb test for this purpose, employing a temperature of 100" C. and an oxygen pressure of 7 kg. per sq. cm. (100 pounds per sq. inch) gage. Since that time, the bomb test has been rather widely adopted and used in one form or another-i. e., the length of the induction period or the gum formed after a given oxidation time. It has been rather difficult, however, to compare the bomb results obtained in different laboratories because of important differences in both the equipment and the testing procedure employed (6). Recently the American Society for Testing Materials has met this need by standardizing the bomb equipment and the testing procedure. I n this laboratory, there are already available twenty bombs of a type different from that specified by the A. S. T. M., and it would be advantageous to be able to calibrate them so that they would yield induction periods equal to those obtained by the A. S. T. M. bomb and procedure. Such a calibration would obviate the necessity for replacing useful and expensive equipment in order to obtain results comparable to those from other laboratories. This situation undoubtedly holds also for many other laboratories. An investigation was accordingly undertaken with the above objective in mind. The A. S. T. M. bomb would, of course, be used for referee purposes.
Equipment The A. S.T. M. bomb setup was purchased complete from one of the equipment manufacturers, and unless otherwise noted waa operated according to A. S.T. M. specifications. The bombs already available in this laboratory are of the Universal Oil Products type (9). They were operated "in place" starting with the bath at room temperature, and they used a 100-ml.gasoline sample contained in a 240-ml. (8-ounce) oil sample bottle. The induction periods obtained with the U. 0. P. bombs were selected according to A. S. T. M. specifications.
Temperature Correction The variation of the induction period with the temperature can be expressed by the equation (1): log induction period = A
+ B/T
where A and B are constants depending on the particular gasoline, and T is the absolute temperature. The value of the slope, B, of this equation, when the temperature is expressed in degrees Kelvin, falls between the limits of about 5000 to 6.500, averaging close to 5500, on the basis of a very large number of tests made in this laboratory with different gasolines. Over a fairly narrow temperature range (* about 2" C.), a simplified correction factor can be introduced which deviates only slightly from the rigorous equation given above. Considering now induction period determinations made in the range of 98" to 102"C., we can correct these to 100" C. by use of the simple factor:
F
=1
+ 0.1 AT
(1) Here, AT is the difference between the reference temperature, 100" C., and the operating temperature (in the range 98' to 102" C.), and is always positive, so that F is always larger than 1. The observed induction period is multiplied by this factor, F , when the operating temperature is about 100" C. and divided by it when below 100" C. I n Table I are given induction periods determined a t 98" and 102", corrected to 100" by the exact equation and by the simplified factor, choosing materials having an extreme difference in temperature susceptibility characteristics (B = 5000
