Design And Realization Of Multi-channel Temperature And Humidity Inspecting Instrument

In order to make electric and electronic product be reliable, people pay incresing attention to the reliability test. The temperature and humidity climate environment test cabinet is one of the important equipment for reliability test. The cabinet provides test environment, which is an important factor determining the test result. Therefore, it is especially important to calibrate the temperature and humidity climate environment test cabinet and the calibration mainly include temperature tests and humidity tests.According to actual demand, a kind of portable temperature and humidity inspecting instrument based on ADI MicroConverter is designed. This inspecting instrument mainly includes multi-channel temperature and relative humidity measurement, RS-232 serial communication interface and man-machine interactive interface. The theoretical basis and realization method of inspecting instrument are introduced in the front five parts of this paper, and the function and performance index of the inspecting instrument are showed in the conclusion part, which also includes what needs to do to improve and upgrade this instrument.In chapter one, the present state of calibration of temperature and humidity climate environment test cabinet is summarized. According to the relevant national standard, the design task and target of temperature and humidity inspecting instrument are put forward. The instrument realizes temperature measurement with 15-channel and humidity measurement with 4-channel. The measurement range of temperature is -70℃~ +200℃, and the measurement range of ??tive humidity is 20%~100%; the measurement accuracy of temperature is less then±0.1℃, and the measurement accuracy of relative humidity is less then±3%RH.In chapter two, the conceptions of temperature and relevant humidity are described in detail, and measurement methods are introduced. The theory of using platinum resistance wine four pins to measure temperature and the way of wet and dry bulb to measure humidity are introduced. Because the wind speed and atmospheric pressure influences the accuracy of humidity measurement, the method of decreasing the influence is described in this chapter too.In Chapter 3, hardware circuits of temperature and humidity inspecting instrument are analyzed. Platinum resistances are used to measure humiture.ADuC834 is used as the central processor.CD4097 and CD4067 are used to design multi-channel selector and realize multi-channel temperature and humidity measurement.This method takes full advantage of analog signal acquisition integrated by microprocessor ADuC834, which makes the hardware circuit simplest. Through ADuC834 internal software calculation, the temperature and relative humidity values can be directly read, or the data are transmitted to a computer, facilitating the work of chamber calibration. MS5534B is used as a pressure sensor to measure the internal air pressure of temperature and humidity climate environment test cabinet, and the measurement results are parts of data to calculate relative humidity.In Chapter 4, software parts of temperature and humidity inspecting instrument are introduced, the calculation ways of temperature and humidity are analyzed in detail, and the appropriate way to program efficient code is chose. Humidity measurement has always been a difficulty in measurement field, its key is to calculate the saturation vapor pr??re, and some measuring mechanism had given some different ways to calculate it, among them, seven formulas are famous. Before programming the software, the writer has done many studies about these seven formulas, finding that the Michell Instruments Ltd's formula is suitable to program for microprocessor. Because the relative humidity is related to the wind, but the inspecting instrument can't measurement wind speed, the wind speed value is a constant in the software,When needing to change wind speed value and platinum resistance'value at 0℃, values can be input from key board. In addition the platinum resistances value at 0℃maybe not are correct 100?, the user also can amend it by keyboard. This chapter also introduces atmospheric pressure measurement program, ADC's calibration program and communication interface program.In Chapter 5, the tests of temperature and humidity inspecting instrument are introduced, the factors which cause measurement error are analyzed, and the ways to reduce error are introduced. Measurement results are analyzed, the performance index of the temperature and humidity inspecting instrument is confimed. The test method which the writer used is that a KEITHLEY2700 was used to measure platinum resistances value and calculate them to temperature and humidity value, and the calculation results was used to compare with the data which come from the Multi-channel Temperature and Humidity Inspecting Instrument. the comparison results was that the temperature measurement error was less than±0.1℃, and humidity measurement error was less than±3% RH, which achieves the actual needs.