Campbell Scientific HMP60 HMP60 Temperature and Relative Humidity Probe - Page 20
Campbell Scientific HMP60 Manual
![]() |
View all Campbell Scientific HMP60 manuals
Add to My Manuals
Save this manual to your list of manuals |
Page 20 highlights
HMP60 Temperature and Relative Humidity Probe reference and power ground are connected to ground at the datalogger. The signal reference/power ground lead serves as the return path for 12 V. There will be a voltage drop along this lead because the wire itself has resistance. The HMP60 draws approximately 2 mA when it is powered. The wire used in the HMP60 (P/N 18159) has resistance of 26.2 Ω/1000 feet. Using Ohm's law, the voltage drop (Vd), along the signal reference/power ground lead, is given by Eq. (1). Vd = I ∗R = 2 mA ∗ 26.2 Ω 1000 ft = 52.4 mV 1000 ft (1) This voltage drop will raise the apparent temperature and relative humidity because the difference between the signal and signal reference, at the datalogger, has increased by Vd. 7.6 Absolute Humidity The HMP60 measures the relative humidity. Relative humidity is defined by the equation below: RH = e ∗ 100 es (2) where RH is the relative humidity, e is the vapor pressure in kPa , and es is the saturation vapor pressure in kPa. The vapor pressure, e, is an absolute measure of the amount of water vapor in the air and is related to the dew point temperature. The saturation vapor pressure is the maximum amount of water vapor that air can hold at a given air temperature. The relationship between dew point and vapor pressure, and air temperature and saturation vapor pressure are given by Goff and Gratch (1946), Lowe (1977), and Weiss (1977). When the air temperature increases, so does the saturation vapor pressure. Conversely, a decrease in air temperature causes a corresponding decrease in saturation vapor pressure. It follows then from Eq. (2) that a change in air temperature will change the relative humidity, without causing a change in absolute humidity. For example, for an air temperature of 20°C and a vapor pressure of 1.17 kPa, the saturation vapor pressure is 2.34 kPa and the relative humidity is 50%. If the air temperature is increased by 5°C and no moisture is added or removed from the air, the saturation vapor pressure increases to 3.17 kPa and the relative humidity decreases to 36.9%. After the increase in air temperature, there is more energy available to vaporize the water. However, the actual amount of water vapor in the air has not changed. Thus, the amount of water vapor in the air, relative to saturation, has decreased. Because of the inverse relationship between relative humidity and air temperature, finding the mean relative humidity is meaningless. A more useful quantity is the mean vapor pressure. The mean vapor pressure can be computed on-line by the datalogger. CRBasic dataloggers use the 14
![](/manual_guide/products/campbell-scientific-hmp60-hmp60-temperature-relative-humidity-probe-f832e3e/20.png)