Sonic post-project temperature chamber tests

Began January 9, 2013

Following the SCP project, we tested each sonic in a zero-wind enclosure within the EOL temperature chamber, measuring wind component offsets and sonic temperature errors over the nominal range -30 C to 55 C.  The procedure is to warm the chamber up to 55 C and then slowly decrease the temperature linearly to -30 C, followed by slowly increasing the temperature back to 55 C.  The zero-wind enclosure holds two sonic heads on their sides (with the v-axis vertical) and electronics, one above the other and separated with a horizontal layer of rigid blue foam.  On the cool-down cycle, the atmosphere around the top sonic is unstably stratified (cold enclosure lid above warmer air) and that around the bottom sonic is stably stratified (cold enclosure bottom below warmer air).  The opposite is true during the warm-up cycle.  Both the wind component data and the sonic temperature data have significantly less variance when the air is stably stratified, and thus we use data from the bottom sonic during the cool-down cycle and data from the upper sonic during the warm-up cycle.

The following table shows wind component offsets as okay if the u and v offsets are less than +/- 4 cm/s and the w offsets less than +/- 2 cm/s (between - 20 C and 30 C).  If the zero wind offset exceed these thresholds, the table lists the temperature range of the over-limit offset and the largest amplitude of the offset.  The plots of offsets versus temperature can then be examined to determine the exact nature of the offset.

The sonic temperature corrections are listed as the slope and offset of a linear fit (between -20 C and 30 C), Tc = offset + slope*tc.sonic, where Tc is calculated from temperature, relative humidity, and pressure measured in the zero-wind enclosure.

Note that we have repeated the laboratory tests on SN0923 and SN0833 to determine the reproducibility of the results.   The velocity component offsets agree within less than 2 cm/s and the temperature offsets agree within 0.06-0.10 C and the slopes agree within 0.2-0.3%, for an overall agreement of no worse than 0.1 C.