3416 -- An Equation and Computer Program to Determine the Partial Pressure of Water Vapor in the Air Between the Dry-Bulb Temperatures of -60 deg F and +2 deg F PDF

Existing thermodynamic equations, Carrier's equation and Ferrel's equation, which are used to determine the partial pressure of water vapour in the air, Ps, have proved to be valid if used in the air conditioning range, 30 deg F to 100 deg F. A new equation was derived to determine Ps for use in other equations dealing with the properties of moist air between -60 deg F and +2 deg F, and an algorithm was developed using the new equation and the steam tables (Keenan 1969). A mathematical model was prepared to predict the vapour pressure of the steam in the air using the new equation. Thirty-one tests were then conducted in a psychrometric chamber isolated from ambient temperature and humidity. The chamber temperature was lowered from -5 deg F to -60 deg F while dry-bulb temperature and relative humidity were measured by NBS traceable instruments. The results were evaluated and the new equation was found to correlate with test results. Twenty additional tests were conducted by varying humidity between the dry-bulb temperatures of 32 deg F and -4 deg F, and again, the new equation was found to correlate with test results. The conclusion made from comparing the predicted values of Ps with the experimental tests is that the equation is valid over its designated range of -60 deg F to +2 deg F. The algorithm (with the new equation) was programmed for the digital computer, in GW Basic, and the source code is included.

KEYWORDS: computer programs, calculating, pressure, water vapour, temperature, air conditioning, moisture, testing, algorithms, psychrometry, climate chambers, relative humidity, measuring, comparing, experiment.