Decagon Devices VP-4 Operator's Manual page 12

VP-4
Temperature
The VP-4 has a band gap temperature sensor integrated into the
sensor electronics. The temperature sensor is located with the RH
sensor and accurately measures the sensor temperature. Sensor tem-
perature should remain close to air temperature if you adequately
shield and aspirate the VP-4 radiation shield.
Vapor Pressure
Vapor pressure is calculated from the primary measurements of sen-
sor RH and sensor temperature. The saturation vapor pressure (e )
is calculated from the sensor temperature using the Magnus-Tetens
equation for calculating saturation vapor pressure over liquid water
formulated by Murray (1967)
with coefficients described by Buck (1981): a = 0.611 kPa, b =
17.502, c = 240.97 C, and T is temperature in C. Vapor pressure
is simply the product of saturation vapor pressure and RH, with RH
expressed as a unitless ratio ranging from 0 to 1.
Unlike relative humidity, vapor pressure does not depend on temper-
ature, and is relatively conservative over small spatial scales. This
means that the vapor pressure of the atmosphere near the VP-4 is
the same as the vapor pressure at the VP-4 sensor, even if the VP-4
is not at the same temperature as the atmosphere. Additionally, it
is the vapor pressure of the atmosphere (not RH) that controls the
rate of vapor phase water transport (e.g. evaporation, transpiration,
and distribution of water vapor). As discussed above, RH measure-
ments below a temperature of 0 C introduce errors introduced due
to the use of liquid water as the reference. However, because the
Buck (1981) formulation for liquid water is used to calculate vapor
pressure over the full temperature range, VP-4 vapor pressure output
values are correct over the full temperature range.
e = aexp
Vapor Pressure = e
9
bT
T + c
RH
3 THEORY
(1)
(2)