The Kelvin Bridge, also known as the "armed bridge", is an instrument that uses the potential comparison method to measure. It has high sensitivity and accuracy, and is widely used in electrical measurement technology and automatic control measurement. The Kelvin bridge is also a variant of the Wheatstone bridge. It has a fairly high accuracy when measuring small resistance resistors (usually "1 ohm") and is suitable for measuring 10-5 to 10 ohm low resistance resistors.
In 1862, when W. Thomson of the United Kingdom encountered difficulties in measuring small resistance using a single-bridge, it was found that the cause of the large error in the measurement was the lead resistance and the contact resistance at the connection point. These resistance values ​​may be much larger than the measured resistance value. Therefore, he proposed the bridge as shown in Figure 1, known as the Thomson Bridge. Later, because he was promoted to Lord Kelvin, it is also known as the Kelvin Bridge. In the figure, R3 and R4 are standard resistors and small resistors to be tested, and R1 and R2 are two bridge arms forming a desired ratio. r is the line resistance (including the lead resistance, contact resistance and internal wiring resistance between the two resistors R3 and R4).
In order to obtain accurate measurement results and eliminate the influence of r, r must be assigned to R3 and R4 in the same proportion of R1 and R2, and R姈 and R娦 are set for this purpose. When the bridge is balanced, the ratio of R姈 and R娦 of the DC bridge should be kept equal to the ratio of R1R2. Due to this feature, this bridge is also called a double bridge. The measured resistance value can be as low as milliohms or less. According to the double-bridge principle, the Smith bridge, the three-balanced bridge and the four-span bridge are developed, which makes the theory and practice of measuring the small resistance of the bridge road perfect.
The double-arm bridge is a kind of bridge that combines the four-terminal lead method and the bridge balance comparison method to accurately measure low resistance.
Connect the low-resistance of the four-terminal method (such as the low resistance to be tested and the low resistance of the comparator arm) to the original single-arm bridge, as shown in Figure 9.
This is an extra arm, and finally evolved into the electrical schematic of the dual-arm bridge of Figure 10. It is easy to see from the schematic: In order to further consider the effects of lead resistance and contact resistance, and access the resistors R3 and R4, And their value must be greater than 10Ω. In order to consider the influence of the difference between R4/R2 and R3/R1 on the measurement result when the bridge balance is made, the value of the shunt current I3 is small, and we use a thick wire R of less than 0.001 Ω to connect the resistors Rn and Rx. In order to increase the sensitivity, an amplifying circuit is added to make the unbalanced current I0, which is amplified and then indicated by the galvanometer.
When the bridge reaches equilibrium, the current in the galvanometer, I0 = 0 indicates that the potentials of C and D are equal. R1, R2, R3, and R4 are far greater than the additional leads and contact resistance, according to Kirchhoff. The second law, the following equations can be derived:
Why can the use of a double-arm bridge to measure low resistance eliminate or reduce the effects of wiring resistance and contact resistance on the measurement results?
1 is shown in Figure 10: the thick wire connection with the resistance value less than 0.001 Ω between the measured resistance Rx and the standard resistance Rn makes the shunt current I3 smaller. It can be seen from the above equation that even if there is a small difference, The measured resistance is always calculated as above. This reduces the effect of this part of the resistance and contact resistance on the measurement results.
2Rx and Rn connection resistance to the power supply, Cn1 and Cx2 contact resistance, only affect the total operating current I, but has no effect on the balance of the bridge, so this part of the wire resistance and contact resistance to the measurement results It also has no effect.
3 The contact resistance of the potential joints Pn1, Pn2, Px1, Px2 and the wire resistance are included in the corresponding bridge arm branch; since the resistors R1, R2, R3, and R4 are all selected to be 10Ω or more, the contact resistance and the wire resistance are compared with this value. It is trivial, so the impact on the measurement results is extremely small. This reduces the impact of this part of the contact resistance and the wire on the measurement results.
Kelvin Bridge Technical Specifications1, the measurement range is divided into three files a. × 0.1 0 ~ 2.17 × 10 Ω
b, ×1.0 0~2.17×10 Ω
c, ×10 0~2.17×10 Ω
2. Working current: 4.0 amps;
3, scale division: 100 grid even scale;
4. The uniform and uniform brass bar resistance is evenly distributed, the sliding contact is in good contact, and the adjustment is convenient;
5. The proportional arm resistance is arranged in a certain order by 8 standard resistors with an error of ±0.1%;
6. Metal rod to be tested: one each of brass rod, copper rod, aluminum rod and iron rod;
7, instrument dimensions: 500 × 250 × 60mm
The cigarette vape pod is usually called the electronic cigarette holder, a necessary part which installed upon electronic cigarette atomizer. The pod is made of oil cartridge or tank which stores the e-juice, and drip tip. Powered by battery, the e-liquid is heated by atomizer, finally forms into vapor which is look like the smoke. Generally speaking, the larger pod tank, the more liquid stored in the flammable, and the longer vaping times.
The pod can be various in flavors, fruits, nuts, tobacco....the strength can be 2-5% salt nic, or no nic.
The pod is consumable! It`s time to replace when the e-liquid in cartridge is ready to run out of, It's like replacing the ink cartridge in an inkjet printer when run out of it. Since 2018, the Electronic Cigarettes on the market are directly pod vape system, it is very simple and convenient, perfect alternative to box mode which is not only very heavy to carry but sldoand need refill oil frequently.
For some smokers who need for long-term use are more likely to buy liquid alone, fill themselves. Although this method looks more economical, it is not sanitary and have healthy threatens. It is easy to breed germs when using the same cigarette holder for a long time, finally enter human body through the mouth, causing bacterial infection. Therefore, for the sake of your health, please try to buy pod vape for replacement. Nothing is more precious than health!
Vape Pen,Disposable Vape Pod,Pod Mod,Vape Pod Pen,Vape Pod Kit
Shenzhen Axiswell Technology Co., Ltd , https://www.medhealthycare.com