Immediately before the balloon release, when the clock is wound and the pens lowered against the drum, the pressure and temperature pens should be tapped lightly so as to make short marks and the time noted.
When the barograph is recovered the smoked foil should be treated to preserve the record. A solution of clear shellac diluted with about ten times its volume of alcohol may be used. The drum is immersed in the shellac and allowed to dry thoroughly before further handling.
(f) Evaluation of the record. In evaluating, the record heights of significant points are measured vertically from the reference line. The pressure calibration steps are measured first and plotted on graph paper, vertical distance versus pressure or altitude. Each significant point on the flight trace is then measured and the corresponding altitude determined from the calibration curve.
The same procedure is followed in evaluating the temperature record, measuring from the reference line.
The curvature of the record due to the motion of the pens must be corrected for. Since the temperature record covers a short vertical range, the time correction may be neglected. Corrections for curvature of the pressure record may be read directly from Figure 37, which gives the correction in inches as a function of the distance of the point in question from the center of the record.
The final time correction is made to correlate the temperature and pressure records. This may be done by measuring the horizontal distance between the temperature and pressure marks as made before release and correcting this amount for vertical position. The rotation of the drum is once in 12 hours and, therefore, the time-distance relation may be computed by noting the total length of record obtained in one revolution.
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