One of the characteristics of block resistance is that the measured values of blocks of any size are the same, and their block resistances are the same whether the side length is 1m or 0. 1m, so the block resistance is only related to the thickness of the conductive film and other factors, indicating the compactness of the film and the transmittance of thermal infrared spectrum. The larger the measured value of block resistance, the worse the thermal infrared isolation performance, and the smaller the measured value of block resistance, the thermal infrared isolation. For the construction industry, the square resistance measuring instrument must be used to quickly measure the thermal infrared performance of low-emissivity glass. The smaller the measured value, the more energy-saving the building materials, which plays a great role in the building materials industry.
Calculation method
Block resistance: Rs=ρ/t (where ρ is the resistivity of the block and t is the thickness of the block)
Or written as the expression of conductivity: Rs = 1/(σt)
In this way, when calculating the square resistance, we can multiply the square resistance by the aspect ratio, and the calculation process has nothing to do with the dimension:
R=Rs*L/W(L is the length of the block and w is the width of the block)
How to test the square resistance? Can you directly test the material shown in figure 1 with a multimeter resistance file? No, because the multimeter probe can only measure the point-to-point resistance, and this point-to-point resistance doesn't mean anything. To test the square resistance, we first need to press a round copper bar with much smaller resistance than the conductive film on the A surface and the B surface, and the round copper bar should have a high finish so as to make good contact with the conductive film. In this way, we can measure the resistance between two copper bars with a multimeter to measure the square resistance of conductive film materials.
If the square resistance is relatively small, for example, less than a few ohms, because of the contact resistance and the performance of the multimeter itself, the reading will be unstable and inaccurate when testing with the multimeter. At this time, it is necessary to use special four-terminal low-resistance testing instruments, such as milliohmmeter and microohmmeter. The test method is as follows: press four smooth round copper bars on the conductive film, as shown in Figure 2. Four copper bars, denoted by A, B, C and D respectively, are connected by wires and welded on a milliohm meter. We make the distance l between BC equal to the width w of the conductive film. As for the distance between AB and CD, there is no requirement, which is generally10-20 mm. After connecting the milliohmmeter, the resistance displayed by the milliohmmeter is the square resistance of the material. The advantages of this test method are: (1) The square resistance of hundreds of milliohms, tens of milliohms or even smaller can be measured by this method; (2) Because it is a four-terminal test, the contact resistance between the copper rod and the conductive film and the lead resistance from the copper rod to the instrument will not affect the test accuracy even if it is greater than the measured resistance. (3) The test accuracy is high. Because of the high accuracy of instruments such as milliohmmeter, the measurement accuracy of block resistance is mainly determined by the mechanical accuracy of film width W and the distance L between conductive bars BC. Due to its large size, this mechanical precision can be made higher. In practice, in order to improve the test accuracy, in order to test strip materials, W and L are not necessarily equal, and L can be much larger than W. At this time, the square resistance RS = Rx * W/L, and Rx is the reading of milliohmmeter.
Although this method has high accuracy, it is troublesome, especially when the conductive film material is large and irregular in shape, it is necessary to use a special four-probe probe to test the square resistance of the material, as shown in Figure 3.
The probe consists of four probes, and the distance between the heads of the four probes is required to be equal. The four probes are connected to the square resistance tester through four wires. When the probe is pressed against the conductive film material, the square resistance of the material can be displayed immediately. The specific principle is that two probes at the outer end generate a current field, and two probes at the inner end test the potential formed by the current field at these two probe points. Because the larger the square resistance is, the greater the potential is, so the square resistance of the material can be measured. It should be pointed out that although they are all four-terminal tests, they are different from the method of measuring block resistance with copper bars shown in Figure 2 in principle. Because only a small part of the current in the current field produces a voltage (potential) at BC. The sensitivity of the display is much lower, and the ratio is 1: 4.53.