Instrument Uncertainty Effect on Calculation of Absolute Humidity Using Dewpoint, Wet Bulb, and Relative Humidity Sensors

As part of a national program, NREL is currently characterizing the state-of-the-art in desiccant dehumidifiers, the key component of desiccant cooling systems. The experimental data will provide industry and end users with independent performance evaluation and assess the energy savings potential of the technology. Accurate determination of humidity ratio is critical to this work and anunderstanding of the capablilities of the available instrumentation is central to its proper application. This paper compares the minimum theoretical random error in humidity ratio calculation for three common measurement methods to give a sense of the relative maximum accuracy possible for each method assuming systematic errors can be made negligible. A series of experiments are also conductedto illustrate the capabilities of relative humidity sensors as compared to dewpoint sensors in measuring the grain depression of dessicant dehumidifiers. These tests support the results of the uncertainty analysis. At generally available instrument accuracies, uncertainty in calculated humidity ratio for dewpoint sensors is determined to be approximately constant at 2%, wet bulb sensors rangebetween 2% and 6% above 10 g/kg (4%-115% below), and relative humidity sensors vary between 4% above 90% rh and 15% at 20% rh. Below 20% rh, uncertainty for rh sensors increases dramatically. Highest currently attainable accuracies bring dewpoint instruments down to 1% uncertainty, wet bulb to a range of 1%-3% above 10 g/kg (1.5%-8% below), and rh sensors between 1% and 5%.