Campbell Scientific Bowen Ratio Instrumentation - Manual (Page 7)

Section 1. System Overview
1-3
FIGURE 1.2-1. Vapor Measurement System
1.2 System Description
1.2.1 Water Vapor Measurement
It is common practice in Bowen ratio measurements to measure wet bulb
depression to develop the water vapor gradient. The position of the two
psychrometers is periodically reversed to cancel systematic sensor errors
(Suomi, 1957; Fuchs and Tanner, 1970).
In the Campbell Scientific system, vapor concentration is measured with a
single cooled mirror dew point hygrometer
1 , using a technique developed for
multiple level gradient studies (Lemon, 1960). Air samples from two heights
are routed to the cooled mirror after passing through mixing volumes (Figure
1.2-1). The problems associated with wick wetting and water supply in
psychrometers are avoided and systematic sensor errors are eliminated.
Air is drawn from both heights continuously through inverted 25 mm filter
holders fitted with Teflon filters with a 1
µm pore size. The filter prevents dust
contamination in the lines and on the cooled mirror. It also prevents liquid
water from entering the system.
A single low power DC pump aspirates the system. Manually adjustable
rotometers are used to adjust and match the flow rates. A flow rate of 0.4
liters/minute with 2 liter mixing chambers gives a 5 minute time constant.
1 Model Dew-10, General Eastern Corp. Watertown, MA