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> Wm weather measure bt-901 balloon theodolite vintage
Wm weather measure bt-901 balloon theodolite vintage
WM Weather Measure Corporation BT901 Balloon Theodolite
Used in nice condition, unable to test
This instrument is ordinarily used to track a pilot balloon in free flight thus determining the winds in the upper atmosphere. The observer follows the rising target through the telescope and records the horizontal and vertical angular readings, usually at equally spaced intervals of one minute. By reference to tables (available from the weather bureau) for the appropriate balloon size (commonly 30 or 100 grams) balloon altitude as a function of time may be obtained. Altitude in conjunction with azimuth and elevation is sufficient to locate the balloon in space and hence to derive the winds aloft by use of tables and a plotting board. A winds aloft observation is usually called a pibal, a contraction of the expression pilot balloon.
A more accurate procedure for obtaining the winds aloft is to use two theodolites spaced at known distances apart along a baseline. Observers at each location track the same target and record elevation and azimuth angles at the same intervals of time. This technique is usually described as a double theodolite observation of the winds aloft. The procedure is more accurate since balloon altitude is obtained by actual measurement (triangulation) thus avoiding the necessary but sometimes inaccurate assumption of a constant balloon assention rate in order to obtain balloon altitude in the single theodolite procedure. However, reduction of the data of a double theodolite run is more tedious if done graphically. It is, however, quite easy to do if a computer is available.
Double theodolite observations are also very useful for tracking a tetroon (a tetrahedron shaped balloon of myler ballasted to fly at approximately constant altitude). Double theodolite tetroon data. Like double theodolite balloon data, are most conveniently reduced with a computer. Weather Measure has the appropriate computer programs for reducing single and/or double theodolite data. These are available at a very nominal rate.
