Because a range of duct air velocities and two different temperature step changes were considered in this study, a more general analysis is provided by considering the time constants of each sensor ([[tau].sub.RH] and [[tau].sub.T]) as opposed to 90% response times.
Figure 12 indicates that the temperature time constant of the Candidate X sensor was similarly affected by the direction of the RH step change. Like the RH time constant, the temperature time constant for positive RH step changes at zero velocity was, on average, about 170 s greater than for negative RH step changes.
To characterize the transient response of thin-film capacitive humidity sensors under a range of duct air velocities, an experimental apparatus was designed and fabricated that was capable of exposing test sensors to step changes in RH and temperature.
Marchgraber and Grote (1963) studied the response of a carbon humidity element exposed to a step change in RH.
A test section was designed and built that was capable of exposing duct-mount humidity sensors to a step change in RH and temperature at different duct air velocities.
To expose the test sensor to a step change in RH, the sensor was pushed (or pulled) by hand from one duct to another (Figure 2b).
The test sensor was quickly moved into the adjoining duct, subjecting it to a step change in conditions.
To investigate the differences between exposing the test sensors to a positive (absorption) or negative (desorption) step change in humidity, two RH step change tests were conducted for each temperature step change and duct air velocity condition.
Testing Hierarchy for a Single Test Sensor RH Step Duct Air Change Test Number Velocity Direction Temperature Step Change (+/-) 0[degrees]F 9[degrees]F 18[degrees]F (0[degrees]C) (5[degrees]C) (10[degrees]C) 0 ft/min + 1 11 21 (0 m/s) - 2 12 22 200 + 3 13 23 ft/min (1.0 m/s) - 4 14 24 500 + 5 15 25 ft/min (2.5 m/s) - 6 16 26 800 + 7 17 27 ft/min (4.1 m/s) - 8 18 28 1100 + 9 19 29 ft/min (5.6 m/s) - 10 20 30 A consequence of the test section configuration was that whatever the direction of a given RH step change, the temperature step change was opposite.