The generation, measurement and control of relative humidity of an environment play a significant role in several industries such as aerospace, textile, pharmaceutical, food technology etc. Due to the awareness and need of ISO-9000, it is mandatory for industries to have their hygrometers calibrated periodically. A humidity generator and an accurate calibrated hygrometer are required to calibrate industrial hygrometers. There are several practical methods for producing relative humidity, such as divided flow, two-pressure and two-temperature generator etc. The development of one such generator is essential for the industrial use in our country. We have developed a simple laboratory apparatus for generating different humidity environments. The apparatus function is based on known mixed-flow technique. The working temperature range of the mixed-flow humidity generator is maintained between 18-25 deg C. This paper reports the details of a mixed-flow based relative humidity generator. In this technique there are two streams of nitrogen/air gas. One is a dry stream while the other is a wet stream. These two streams are then combined in another chamber called the test chamber, where device under test is usually inserted. The humidity in the testing chamber is generated by varying the flows of the two streams through needle valves. The relative humidity inside the testing chamber is measured using a capacitance type hygrometer. We have been able to generate different relative humidities in the range (10percent-90percent)RH, with stability of better than +-1 percent RH over a period of 30 minutes. The measurement of different relative humidities is presented in this report. The unit is totally in-house built and has an immense use in several industries for the calibration of their hygrometers.
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