R.M. Young 61402L Barometric Pressure Sensor

Barometric pressure is the least visible measurement on a weather station and one of the most consequential. It is the basis of the altimeter setting an aircraft flies on, so an error in the reported pressure becomes an error in the aircraft's indicated height - which on an offshore approach in poor visibility is the measurement least tolerant of being wrong. It is also the primary short-term forecasting signal: the rate at which pressure is falling tells you more about the next twelve hours than any single reading.

The 61402L covers 500 to 1100 hPa. That range is wider than a coastal site needs, and deliberately so - it spans from well below any sea-level low pressure system to above the highest recorded anticyclone, and the lower end accommodates installation at altitude. YOUNG pair that with high accuracy and low power consumption over a wide temperature range, and the low power figure is the one that matters on a remote or solar-powered station where the barometer runs continuously.

The output options are unusually generous for one instrument, and they are the reason this particular model gets specified: 4-20 mA, RS-232, RS-485 and SDI-12 are all available. That covers essentially every way a met sensor gets connected. The current loop suits a long run into a PLC or an analogue input. RS-232 is the direct connection to a single logger or display. RS-485 is the multi-drop bus where several instruments share one cable back to the data acquisition system, which on a platform saves a great deal of cable. SDI-12 is the low-power serial standard used across environmental monitoring, which lets the sensor share a bus with other SDI-12 instruments and be interrogated by address.

Installation is simpler than for the other met sensors, since pressure does not need exposure - the sensor can live inside an enclosure, which is why it usually outlasts everything on the mast. The one requirement is a static pressure port that is not affected by wind: a pressure tapping in a windy position reads dynamic pressure as well as static, and the error appears exactly when the weather is worth measuring. Where the reading feeds an altimeter setting, the sensor's height above the reference datum has to be recorded and corrected for, which is a commissioning matter rather than a hardware one.