IF YOU have ever hankered to have the outdoor temperature, relative humidity, wind speed and wind direction displayed on a panel in your living room, it has been neatly worked out for you. David A. Sankey, an Air Force officer, has built a weather station that displays information of this kind in his home in Victorville, Calif. A similar station can be made by any amateur.

The sensing devices of Sankey's station are on a 20-foot mast about 300 feet from his house. The measurements are transmitted electrically to an instrument panel in the house by means of a cable. The cost of a station comparable to Sankey's will vary with the resourcefulness of the builder and the length of the cable, but it should not exceed $50. Sankey describes the construction and operation of his station as follows:

"The exterior sensing devices are mounted on a horizontal crossarm of wood that is bolted at its center to the top of a vertical pipe about 20 feet long. The pipe is supported by guy wires. The crossarm carries an anemometer to measure wind speed and a vane for indicating wind direction [see illustration at right]. At a point about six feet above the ground the mast supports a louvered wooden box enclosing a pair of dual transistors and a cluster of 13 germanium diodes connected in series. The dual transistors function as a hygrometer of the wet-bulb, dry-bulb type. The 13 diodes and the associated circuit constitute a sensitive thermometer with an expanded scale for observing the size and movement of locally heated domains in the atmosphere.

"The anemometer is essentially a small windmill that generates an electric potential, which varies with the speed of the wind, and a voltmeter that can be calibrated for indicating velocity directly in knots or in miles per hour. Wind direction is sensed by a vane that is supported by the shaft of a self-synchronous motor. The shaft of a companion self-synchronous motor, which is mounted behind the instrument panel, rotates a pointer with respect to a compass card on the front of the panel.

"The operation of self-synchronous motors is analogous to that of meshed gears: when either motor is rotated by an external torque, its companion rotates in lockstep. In my apparatus the wind vane rotates one motor The companion motor simultaneously turns the pointer at the panel to indicate the direction of the vane with respect to the cardinal points of the compass.

"The electric generator of the anemometer consists of a miniature direct-current motor of the type having permanent magnets that provide the required magnetic field. Such motors, which are available at hobby stores, cost about $1. Choose one that turns freely.

"Self-synchronous motors are widely advertised by surplus dealers at prices ranging upward from $5 per pair. The less expensive types are designed for operation at 26 volts and 400 cycles per second. At the reduced loading imposed by the wind-vane system the 400-cycle motors can be operated at 60 cycles per second without overheating if the rated voltage is lowered about 50 percent. The 26-volt motors of my system operate on 12-volt alternating current.

"The generator of the anemometer is mounted vertically inside a block of wood attached to the upper end of the crossarm [see illustration at left]. Both the crossarm and the anemometer block are cut from standard two-by-four-inch pine stock. A hole that makes a snug fit with the motor is drilled approximately three-quarters of the way through the block near the long edge. A drill bit of smaller diameter is then used for extending the hole through the block. The smaller diameter is selected to make a press fit with a rod of nylon.

"A hole is drilled through the rod axially to make a sliding fit with a length of 3/16-inch brazing rod that serves as a shaft to support the impeller of the anemometer. A rectangular transverse opening is cut through the block so that it extends downward about an inch or so from the lower edge of the nylon bearing, The rectangular opening provides access to the shaft of the motor. Insert the motor into the larger hole and couple its shaft to the brazing rod by means of rubber tubing.

"The impeller assembly consists of three cups supported by equally spaced radial spokes fixed to a central hub. The cups were made by cutting off the spouts of aluminum funnels about 1/2 inch from the apex of the cone. The remaining portion of each spout was hammered to a point and sealed with epoxy cement. The hub is a disk of plastic covered with a jar lid for protection from the weather. A snugly fitting washer is fastened with epoxy to the shaft at the upper end of the nylon to act as a thrust bearing.

"The spokes that support the cones were made of brazing rod. The inner ends of the spokes were bent at right angles for insertion into holes of the hub, which are spaced at intervals of 120 degrees. The outer ends of the spokes were threaded for fastening to the cones with nuts. After the assembly has been mounted on the vertical shaft the cups are aligned by bending the spokes as required to make the impeller run true. The leads from the generator are brought out through a small hole in the side of the supporting block. Finally, the supporting block is screwed to one end of the crossarm.

"The wind vane consists of an aluminum tube 3/8 inch in diameter and about nine inches long. It supports a tail of balsa wood at one end and a counterweight at the other. The assembly is mounted to the vertical shaft of a self-synchronous motor by means of a collar that is both wired and cemented with epoxy to the aluminum tube at its balance point [see illustration at right]. I bought the collar from a distributor of bearings and similar machine parts. The tail was cut from a 1/8-inch sheet of balsa wood. The wood was protected with a coat of sealer and two coats of enamel. It was slipped into a saw kerf at one end of the aluminum tube and fastened with epoxy. A carriage bolt that makes a snug fit with the opposite end of the aluminum tube shifts the balance to within about three inches of the weighted end. To protect the upper bearing of the motor from dust and moisture I cemented the screw cap from a bottle to the bottom of the collar. The center of the cap was drilled to admit the shaft of the motor.

"The motor is clamped inside a conduit splice: a short length of tubing fitted with four setscrews. The device is available from dealers in electrical supplies. The bottom screw of the splice clamps the assembly to a cylindrical plug of wood that is fastened to the crossarm with a screw.

"The second setscrew from the bottom of the splice was removed. Leads from the motor were brought out through this hole. One side of the motor shaft was flattened with a file. The set screw of the collar that supports the arm of the vane bears against the flattened side of the shaft. Hence the vane can be removed and replaced without disturbing the orientation of the vane with respect to the pointer on the instrument panel.

"The instrument panel can be improvised according to the tastes of the individual. The self-synchronous motor of the wind-vane system is mounted to the rear of the panel with its shaft extending through the front, where it supports the pointer [see illustration at left]. To the panel must also be mounted at least one meter of the d'Arsonval type capable of measuring current through the range from 0 to 25 microamperes. The panel can include a rotary switch for connecting the meter to the anemometer circuit or to each of the three thermometers to be described. If cost is not crucial, each circuit can be provided with its own meter. The panel is completed by installing a transformer on the rear surface for reducing the power-line potential of 120 volts to 12 volts (alternating current). I also installed an on-off switch and a pilot light for convenience.

"The cable that connects the instrument panel to the anemometer and wind vane must contain seven insulated conductors-two for the anemometer and five for the wind vane. Each of the selfsynchronous motors has five leads: one group of three and one pair. All leads ale identified by either numerals or a color code. Connect all leads of one motor to the corresponding leads of the second motor by means of the cable. All cables are fitted with jacks and plugs for easy connecting and disconnecting.

"To energize the circuit connect the 12-volt output of the transformer to the pair of interconnected leads. When power is applied, each motor will rotate a fraction of a turn and stop. Thereafter when the shaft of either motor is rotated, the other motor will rotate in lockstep.

To align the system, loosen the setscrew that clamps the pointer to the shaft of the motor, turn the pointer to the position on the compass rose that corresponds to the direction in which the wind vane is pointed and tighten the setscrew. Some self-synchronous motors can lock into step at intervals of 90 degrees or 180 degrees. When motors of this kind are used, the alignment between the wind vane and the indicating pointer must be checked each time the power is turned off and on.

"The anemometer can be calibrated most reliably by making a second anemometer of the pendulum type. The pendulum anemometer serves as the calibration standard. The construction is quite simple. It involves suspending a ping-pong ball in the wind by a length of thread and measuring with a protractor the angle the thread makes with respect to the vertical when the ball is forced from its equilibrium position by the wind. Wind speed is calculated by means of a simple formula that takes the angle into account [see "The Amateur Scientist"; SCIENTIFIC AMERICAN, October, 1971].

"To make the calibration, remove the crossarm assembly from the mast and remount it temporarily on a support at eye level. On a breezy day connect the output of the anemometer to the meter with leads of convenient length. Insert a variable resistor of a few thousand ohms in one of the leads. Measure the wind speed with the pendulum anemometer. Increase or decrease the variable resistor to set the pointer of the meter at a desired point on the scale.

"For example, assume that the scale of the meter is graduated from 0 to 50 units and that the pendulum anemometer indicates a wind velocity of 20 miles per hour. The meter can be calibrated to indicate wind speed directly by setting the pointer at 20 units by means of the variable resistor. In general the potential developed by small generators having permanent magnets varies directly with the speed at which they turn. For this reason the anemometer can be calibrated with sufficient accuracy by checking a single point against the standard. A meter that is graduated in inconvenient units can be used with a graph made by plotting wind speed against the units of the scale.

"The optimum size of the variable resistor will vary with the characteristics of the generator and the sensitivity of the meter. Typically it will range from 1,000 to 20,000 ohms and must be determined experimentally. After the calibration has been made measure the exact resistance with a volt-ohm meter and replace the variable resistor with a fixed resistor of the same value. The crossarm assembly can then be permanently mounted on the mast.

"Incidentally, the site of the mast is important. The anemometer and wind vane should be in an open area or on a roof, well removed from buildings, trees and similar obstructions. The instruments should be at least 10 feet above the highest neighboring obstruction.

"The electronic thermometers used for measuring relative humidity and for detecting the movement of local air masses by observing differences in temperature are of two types. The temperature probe of the hygrometer is a pair of dual transistors that can be located several hundred feet from the associated circuitry. The output voltage of each dual transistor circuit varies in direct proportion to the temperature. The circuit [see bottom illustration at right] was developed by T. C. Verster of the National Research Institute for Mathematical Sciences in the Republic of South Africa. Current in the collector circuit of one transistor of the dual pair is made to exceed the current in the collector circuit of the companion transistor by a ratio of more than 10 to one. If the ratio is fixed, the difference between the base-to-emitter voltages of the two transistors is proportional only to the absolute temperature of the transistors. The operational amplifier maintains the current ratio by means of negative feedback.

"The device is remarkably simple to calibrate. To make the calibration connect a voltmeter between the wiper arm of the 1,000-ohm potentiometer (R) and the chassis. Apply power to the circuit. The reference voltage developed by the Zener diode (lN3157) appears across the voltmeter. Adjust the reference potential to-273 millivolts by adjusting the 1,000-ohm potentiometer. (A millivolt is one thousandth of a volt. Insert the transistor and a mercury thermometer in an oil bath at room temperature. Adjust the 750-ohm potentiometer (R) to the point where the meter indication in millivolts matches the temperature in degrees Celsius as indicated by the thermometer. Verster recommends the use of a digital voltmeter with an ungrounded input. A conventional meter of the d'Arsonval type will work, although it is less convenient than the digital voltmeter to read. The calibration need not be checked at more than one reference point.

"Both transistors are cleaned, coated lightly with epoxy and attached to shielded, five-conductor cables. The are suspended vertically side by side about an inch apart and half an inch from the supporting panel of wood. The panel can consist of a piece of quarter inch plywood five inches wide and 10 inches long with a spacer strip half an inch thick, two inches wide and five inches long glued across the upper end. The cables are supported by the spacer strip. A tubular wick of the kind used in hygrometers is slipped over one of the transistors and tied in place with thread The wicks are available from dealers in scientific supplies for $2 per dozen. Wicks should he cleaned about four times a year.

"A glass cistern is clamped to the panel with its opening immediately under the transistor. Glass cisterns are also stocked by dealers in scientific supplies and cost about $2 each. Fill the cistern with distilled water and insert the wick. The companion transistor functions as the dry bulb of the hygrometer.

The temperature probes can be housed in a box that has louvers large enough to ensure the free circulation of air. The box should be painted white. It can be placed at any outdoor site, preferably one that is shaded from the sun. The circuits can be energized by a pair of 12.6-volt batteries such as the Mallory Type TR-289 mercury cells or, preferably, by a regulated 15-volt dual power supply of the type previously described in these columns [see "The Amateur Scientist"; SCIENTIFIC AMERICAN, May 1970]. Relative humidity is determined by measuring the temperature as indicated and referring to a table published by the U. S. Weather Bureau in bulletin No. 1071. The table is reproduced in standard references such as Handbook of Chemistry and Physics (Chemical Rubber Publishing Company). Incidentally, the accuracy of the hygrometer can be improved by installing a small electric fan near the wet bulb to encourage evaporation of water from the wick. Let the fan run for about two minutes before making a reading.

"The sensing probe of the expanded-scale thermometer used for observing micrometeorological effects consists of a string of 13 germanium diodes connected in series. The diodes can be assembled in the form of a compact cluster about 1-1/2 inches square by bending the leads of each unit to form a U and pushing them through a rectangular pattern of holes drilled in a sheet of thin Formica or a similar plastic. The electrical resistance of the diodes varies in inverse proportion to the temperature. Thc potential drop across the string varies by a total of one volt from -24 to 38 degrees C. (-11 to 100 degrees Fahrenheit).

"The diode assembly constitutes one of the four arms of a Wheatstone bridge [see illustration at left]. The meter, which is connected across the bridge, indicates average temperature. The circuit, as developed by Douglas A. Kohl of Minneapolis, includes a rotary switch for observing temperature in four slightly overlapping ranges of approximately 40 degrees each. In effect the insertion of the switch magnifies the three-inch scale of the meter to 12 inches, thus increasing the accuracy with which readings can be made. Rapid fluctuations of temperature appear as a voltage of varying amplitude across the diodes. It can be monitored directly by connecting the input of an operational amplifier, such as the type designated 741C, to the circuit at points V and G. If desired, the output of the amplifier can be used for driving a chart recorder.

"The thermometer circuit is energized by a power supply that includes a stepdown transformer for reducing the line potential to 26 volts. A diode converts the output of the transformer to direct current. The remaining portion of the circuit includes a pair of Zener diodes in tandem that deliver a constant potential of 18 volts to the Whetstone bridge.

"The meter indications vary in direct proportion to the temperature. For this reason a calibration graph that relates temperature to meter indications can be drawn by measuring the temperature of the diodes at two points in each of the four ranges of the instrument and tabulating the corresponding meter indications. For example, set the rotary switch to the 62-to-101-degree range. With a mercury thermometer measure and record the temperature of the room and the meter indication. Transfer both the thermometer and the diodes to a warmer environment, such as the interior of a cardboard box that is heated by a 100 watt incandescent lamp. After the temperature stabilizes make the second set of readings and draw the graph. Lower ranges can be similarly calibrated using the cooling and freezing compartments of a refrigerator. Some of the components of the thermometers, such as the dual transistor and the germanium transistors, are not stocked by distributors that cater to amateurs. They can he obtained from Semiconductor Specialists Inc. (P.O. Box 66125, O'Hare International Airport, Chicago, Ill 60666).

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