A Smart Mixed-Air Temperature Sensor

Accurate mixed-air temperature (MAT) measurements are notoriously difficult to obtain in packaged air-conditioning equipment for small commercial applications because of space constraints and the use of small mixing chambers. However, MAT is important for control of economizers and in diagnostic algorithms for both economizers and vapor compression equipment. This paper demonstrates that a single-point measurement of MAT can provide accurate results when combined with a correlation for bias error that depends on damper control signal and difference between outdoor and return-air temperature. The correlation could be determined using a self-calibration procedure with measurements of mixed, outdoor, return, and supply air temperatures. A system was tested in a laboratory over a wide range of outdoor conditions and damper positions, and the single-point measurement errors were reduced by about a factor of five when using the bias correction. Furthermore, errors in outdoor air fraction (OAF) determined from temperature measurements were reduced by about a factor of four.