With the popularization of sensor technology and computer data acquisition technology, more and more air compressor manufacturers have upgraded the air compressor exhaust volume measuring devices. The electric measuring sensor replaces the traditional glass mercury thermometer, u-tube and pointer type pressure gauge, and the computer communication technology automatically collects data to replace the traditional manual reading mode, which can not only greatly improve the automation level of the test equipment, but also improve the exhaust volume. The accuracy of the measurement is convenient for the company to carry out quality tracking management and enhance the competitiveness of the industry. Since the release of the energy efficiency labeling management method for air compressors, the accuracy of the exhaust gas measurement results is more important for the production enterprises. This paper starts from the general aerodynamic air compressor exhaust gas test system, and integrates the errors affecting the measurement results to determine the total measurement uncertainty, find out the main factors of the impact and propose solutions.
2 Evaluation method of accuracy measurement According to the error transfer theory, the absolute error of indirect measurement F and each direct measurement parameter are not (the relationship between the absolute error of i is 2+...+()2(1) and the relative error is f =V (the most contending W is the most influential factor of the 3 exhaust volume measurement according to the national standard GB/T3853-1998, when using the throttle nozzle to measure the air compressor displacement, when the suction pressure is equal to the local atmosphere When pressure is applied, the volumetric flow rate of the air compressor that does not take into account the condensed water can be expressed as the use of maintenance s - gas expansion coefficient C - nozzle coefficient d - nozzle throat diameter T0 - suction temperature H nozzle before and after pressure difference ph experiment The atmospheric pressure T, the gas temperature upstream of the nozzle is based on the error synthesis formula, and the uncertainty of the exhaust gas volume, ie, the mean square relative error, is the relative error of the exhaust gas volume depending on the relative error values ​​of the items on the right side of the equation: Error in nozzle coefficient, error in nozzle diameter, error in absolute temperature of inhaled gas, error in differential pressure, error in local atmospheric pressure, and absolute temperature before nozzle. The following will use electrical sensor and computer technology.
3.1 Relative error of the nozzle coefficient The nozzle coefficient C=ay. According to the error synthesis formula, the relative error of the root mean square of the nozzle coefficient is f=爨 according to the standard requirement, and the 04 heart is as the minimum requirement of the standard, then the maximum relative error of the flow coefficient is In fact, for a certain type of throttling device and pressure mode, the flow coefficient is related to the Reynolds number, the diameter ratio and the roughness of the pipe; a is obtained by experimental method and is given by the relationship curve between the two. The relative error of the root is ±0.5%; the uncertainty of the expansion coefficient Pi——the gas pressure upstream of the nozzle 3.2 The diameter of the nozzle d The error of the nozzle diameter depends on the machining accuracy of the nozzle and the measurement accuracy of the inner diameter after machining. With the improvement of machining accuracy and measurement accuracy, the requirement of “measurement error not to be greater than ±0.01 mm†according to the standard is easy to implement. g small ASME nozzle throat diameter is 3.18mm (measurable volume flow range is 瞧, the largest ASME nozzle throat diameter is 152.40mm, the maximum relative error is only visible here, the smaller the air volume The error of the machine is affected by the nozzle diameter measurement. In this paper, the maximum relative error of the ASME nozzle with the measurable volume flow rate of 1.756~3.597m3/min is calculated as ±±=±0.12%. d 3.3 The pressure difference is ugly Measurement error Traditional measurement generally uses U-tube. It is generally considered that when the scale is 1mm, the maximum possible reading error is Ai/=±1mm. When the graduation value is 0.5mm, it is Afl=±0.5mm. The relative error of measuring the pressure difference ugly with U-tube differential pressure gauge mainly depends on the manufacturing precision of the scale, the reading error 4 ugly and the height of the liquid column. For the measurement of the displacement of the general air compressor, the pressure difference is The minimum relative error is 0.5~1.0%. If the pressure measuring tube is installed improperly, the relative error of the pressure difference ugly measurement will be super. If the electric measuring sensor is used, the accuracy of the sensor directly determines the relative error of the measurement. Electrical measurement sensor, the pressure difference The relative error is only M=±0.2%. 3.4 Atmospheric pressure /> J!iJ amount error test The atmospheric pressure can be measured by a precision atmospheric pressure gauge. For example, the absolute pressure transmission of 0.15 grade is adopted according to GB/T3853-1998. The maximum relative error of the differential pressure is 3.5. The measurement error of the inhalation temperature r. The conventional measurement method is measured by a glass mercury thermometer.
I 维修 maintenance generally considers that the reading error is equal to half of the thermometer's minimum division value. If the division value is 0.1, the reading error is ±. 5 feet. It can be seen that when the thermometer is placed correctly, the thermometer for the laboratory (with a graduation value of 0.1), taking into account the reading error and the basic error of the thermometer itself, the absolute error of the temperature measurement is /=±0.2; When the degree is 0.5, z 0=± If the general industrial grade A precision platinum resistance is used, the absolute error is / = ± (0.15 + 0.2% llf), so its relative error is: M = ± + if inhaled gas The temperature is calibrated in the range of -2080 °C, the maximum relative error is 3.6 nozzle gas temperature 7; the measurement error In order to improve the accuracy of the measurement, the national standard specifies the temperature of the gas before the nozzle with 2 thermometers without temperature measuring sleeve For the measurement, the insertion depth of the thermometer is such that the low-pressure box diameter D of the low-pressure box has a heat-insulating layer outside the temperature-measuring area.
Using the Class A precision platinum resistance sensor, the absolute error of the absolute temperature measurement of the gas before the nozzle is 4. The maximum relative error determined by the above parameters is brought into Equation 4, and the air compressor displacement measurement result is obtained. 45 pages) Su Changzhen, Tan Liancheng, Liu Guiyu. Higher engineering thermodynamics. Beijing: How to be a senior director, Xiao Bihua, Fang Yongshui. Theoretical analysis methods and applications of orthogonal experimental design. Journal of Anhui Institute of Architecture and Industry (Natural Science Edition), 2004, 12: 103-106. HC Alrannikov, BH Maltsev, Zhang Bingzhen, et al. Aerodynamics (revision). Beijing: Higher Education Press, 1959, 1:255-279. Xia Yajun. Engineering mass transfer. Beijing: Mechanical Industry Press, 1985, 07:49-50. Uncertainty As can be seen from the above analysis and calculation, under the condition that the ASME nozzle coefficient is constant, the error of the nozzle before and after pressure difference and temperature measurement The air compressor displacement measurement results have a greater impact, the more accurate the differential pressure and temperature measurement, the higher the accuracy of the displacement measurement results. Therefore, the exhaust gas test equipment should use electrical measurement sensors that meet the requirements of national standards and even higher precision, using computer data acquisition technology combined with filtering technology. By performing centralized background processing on a large amount of measurement data, it is possible to effectively prevent reading errors, coarse errors, and calculation errors caused by manual operations, thereby improving the stability and accuracy of the exhaust gas measurement results. In fact, due to the processing, installation deviation of the measuring hardware system C low-pressure box, nozzles, etc., and the additional uncertainty of the valve, as well as the correction calculation of the exhaust volume caused by the ambient temperature and humidity, the actual air compressor displacement The uncertainty of the measurement results may be greater, but it is still feasible to control the accuracy of the overall test equipment within 1% by strictly controlling the design and processing of various processes and the reasonable application of electrical measurement sensors and computer technology.
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