Errors and efficiency figures during solar survey

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Special report

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Release time:

2020-05-27

　　In the measurement, the amount that can be directly read out by the measuring instrument is called the direct measurement amount; the amount obtained by the direct measurement amount and some constants and function operations is called the indirect measurement amount. The number of significant digits of the direct measurement (the last digit of the significant digit should be the estimated digit) is determined by the accuracy (minimum scale) of the measuring instrument. That is, the maximum absolute error of the measuring instrument is obtained from the accuracy of the instrument (hereinafter referred to as the error), and the error is only reserved for one significant digit. Finally, the position where the effective digit of the error is located determines that the measurement is valid. The cutoff digits. For example, if the length of a wooden stick is measured by a ruler with a minimum scale of i=1mm, the length of a wooden stick is 300mm, because the maximum error of the ruler is $=i/2=0.5mm, the cut-off position of the effective number of the measurement should be 1 At /10mm, it should be expressed as L=300.0mm. Note that the /00 after the decimal point must be written here and cannot be omitted.

　　For indirect measurement, because it is composed of some direct measurement, its maximum error should be related to the size and error of each relevant direct measurement. For general engineering surveys that do not require high accuracy, the maximum error of the indirect measurement quantity f=f(x, y, z, ,) is calculated by the following error transfer formula: the calculation result of $f is a significant number, Finally, the cut-off position of f significant digits is determined by the position of $f significant digits.

　　Taking the thermal performance test of the solar water heater as an example, the method for determining the effective number of the direct measurement quantity and the indirect measurement spectrometer quantity will be explained. In the thermal performance test, the length and width of the lighting surface a and b that belong to the direct measurement quantity; the water capacity of the water tank M; the daily average water temperature of the water tank tm; the average temperature of the pure water flowmeter ta; the irradiance H received by the lighting surface H , Belonging to the indirect measurement of the lighting area A; daily efficiency Gd; instantaneous efficiency G and heat loss coefficient Ul. The measuring instruments are: length ruler with meter ruler (minimum scale is mm); water quality with scale (minimum scale is 015kg); temperature with thermometer (maximum error is $t=0.2e); irradiation with cumulative formula Radiation recorder (maximum error is about $H=2@104Jm-2).

　　Set ts=20e, te=55e, M=80kg, H=1.8@107Jm-2, a=b=1m, Cp=4.18@103Jkg-1e-1. Then as a direct measurement, the correct representations of the lengths a and b of the lighting surface, the water capacity M of the water tank, the temperature and the daily exposure H should be: a=b=1000.0mm, M=80.00kg, ts=20.0e , Te=55e, H=1.800@107Jm-2. As an indirect measurement, the lighting area A=a@b, according to the error transfer formula: $A=a$b+b$a=1000@0.5+1000@0.5=1000mm2=0.001m2(2) Therefore, the lighting area should be written A = 1.000m2 (reserved to the third decimal place).

　　The maximum error is: Substitute each value to calculate $GdU0.01. Considering that the $Gd value is between the hundredth and thousandths, Gd can be retained to the third decimal place, and the daily efficiency should be expressed as Gd=0.650.

　　The error transmission formula is to substitute t1-t4=5e, tm-ta=30e and other quantities into the calculation, and it can get $U1=0.3Wm-1e-1. Therefore, the heat loss coefficient should be expressed as Ul=2.6Wm-1e-1 (significant digits are retained to the first decimal place).

　　To sum up, the expression of measured quantity is regular. Although 118 and 1180 are equal in size, as measurement quantities, the two represent different meanings. Therefore, as a solar science and technology worker, we should develop the good habit of expressing physical quantities correctly with effective numbers.

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