Campbell Scientific Bowen Ratio Instrumentation - Manual (Page 8)

Section 1. System Overview
1-4
A datalogger is used to measure all sensors and control the valve that switches
the air stream through the cooled mirror.
The resolution of the dewpoint temperature measurement is ±0.003°C over a
±35°C range. The limitation is the stability of the Dew-10, approximately
0.05°C, yielding better than ±0.01 kPa vapor pressure resolution over most of
the environmental range.
Every 2 minutes the air drawn through the cooled mirror is switched from one
height to the other with the valve. Forty seconds is allowed for the mirror to
stabilize on the new dewpoint temperature and 1 minute and 20 seconds worth
of measurements for an individual level are obtained for each 2 minutes cycle.
The dewpoint temperature is measured every second and the vapor pressure is
calculated by the datalogger using the equation described by Lowe (1976).
The average vapor pressure at each height is calculated every 20 minutes.
CR23X
FIGURE 1.2-2. Thermocouple Configuration
1.2.2 Air Temperature Measurement
Air temperature is measured at two heights with chromel–constantan
thermocouples wired as in Figure 1.2-2. The differential voltage is due to the
difference in temperature between T
1
and T
2
and has no inherent sensor offset
error. The datalogger resolution is 0.006°C with 0.1
µV rms noise.
The thermocouples are not aspirated. Attempts to aspirate the TCs with the air
from the vapor measurement system were not successful. Testing under 1000
W m
-2 solar radiation, with several radiation shield designs and aspiration rates
of up to 80 cm s -1 (1 l min -1 ), showed a significant increase in temperature due
to radiation from the shield/ducting.
Calculations indicate that a 25
µm (0.001 in) diameter TC experiences less
than 0.2°C and 0.1°C heating at 0.1 m s -1 and 1 m s -1 wind speeds, respectively,
under 1000 W m -2 solar radiation (Tanner, 1979). More importantly, error in