Pressure and Temperature (MET PT)

See also MET Overview

See also MET File Naming

In situ measurements of near-surface temperature, pressure, humidity and wind are essential to improve our understanding of Mars weather and current climate. Phoenix, as the first mission to the high northern latitudes, will provide in situ meteorological information from an environment with a seasonal maximum of atmospheric water vapour, and an expectation of access to sub-surface ice. Basic meteorology measurements will support all aspects of the surface science mission and have been included as measurements critical to mission success.

Temperature Sensor

Air temperatures and temperature differences between levels will be monitored, almost continuously, by three temperature sensors based on fine-wire, butt-welded thermocouples (75 micron diameter, Constantan-Chromel) mounted in C-frames on a 1m mast, coupled with a reference platinum resistance thermometer in an isothermal block containing the 'cold' junctions of the thermocouples. Levels on the mast are 0.25, 0.5 and 1.0 m above the Lander deck, which itself is approximately 1m above the ground. There will be three thermocouple junctions in parallel in each of the air temperature sensors, providing a degree of redundancy.

The micro-voltages generated by the thermocouples are measures at 2-second intervals and converted to digital signals with a 16-bit analogue to digital converter. Drifts in the read-out electronics are calibrated and corrected for at the same interval.

Pressure Sensor

An FMI (Finnish Meteorological Institute) sensor, based on a silicon diaphragm sensor head manufactured by Vaisala Inc., combined with MDA data processing electronics will measure pressure. The FMI unit has 3 pressure sensor heads. One of these is the primary sensor head and the other two  will be used for monitoring the condition of the primary sensor head during the mission. During the mission the primary sensor will be read with a sampling interval of 2 s and the other two will be read much less frequently as a check of instrument health.

The pressure sensor system has a sophisticated real-time data-processing and calibration algorithm that allows the removal of temperature dependent calibration errors.

The time constant of the pressure sensor had not been specified but it was originally hoped that it would be short compared to the sampling interval of 2 s. In fact, it is a little longer than that, approximately 3s due to locational constraints and dust filtering requirements, but should still be short enough to detect pressure drops associated with the passage of a nearby dust devil.

Source: MET PT Instrument Catalog File