Lengthening and cutting of ultrasonic flow meter sensor cables

Due to the particularity of industrial sites, the sensor cable needs to be lengthened at some ultrasonic flow meter installation sites. Some ultrasonic flow meter manufacturers, especially foreign products, prohibit lengthening and cutting the sensor cable. If we can understand the structural principle of the sensor, we can lengthen and cut the sensor cable without affecting the accuracy of the measurement.

The resonant frequency of piezoelectric ceramics is around 1MHz, and the resonant frequency of ultrasonic flowmeters (water measurement) from domestic manufacturers is basically 1MHz. The impedance characteristic is capacitive. In other words, strictly speaking, the capacitive reactance of the two sensors in a set of probes should be consistent, so that the static (0 point) measurement will be more accurate. It is not difficult to analyze that if the sensor cable is cut, the two sensor cables in a set of probes should be cut to the same length to keep the impedance characteristics consistent.

Next, let's talk about extending the sensor cable. Before discussing the extension of the sensor cable, we need to analyze the sensor cable first. For a sensor in a set of probes, there are generally two types: 2-wire system and 3-wire system.

1.2-wire sensor cable, generally use coaxial cable. That is, the center line of the coaxial cable is connected to the positive electrode of the piezoelectric ceramic, and the shielding line of the coaxial cable is connected to the negative electrode of the piezoelectric ceramic. The 2-wire sensor cable can be lengthened by using coaxial cables of the same specifications. The point to pay attention to is that the lengthened connection should be handled properly. The center line of the coaxial cable is best connected by welding, and the shielding line should be insulated. For the connection of the shielding line, the entire shielding layer of the connection should be wrapped with copper foil, and finally the external waterproof insulation should be done.

2. 3-wire sensor cable, generally use two-core shielded cable. That is, the two inner core wires of the two-core shielded cable are connected to the positive and negative poles of the piezoelectric ceramic respectively. The shielded wire is not connected at one end of the sensor, and is connected to the terminal position of the GND wire on the transmitter. The general misunderstanding of extending the 3-wire sensor cable is to use a multi-core shielded cable to take 3 of them to connect the original 3-wire sensor cable, and use the selected 3 wires to connect to the corresponding transmitter position. Through the above analysis, such an extended cable has no continuation of the shielding layer, loses the protection of the shielding, and is easily disturbed by the external environment, which manifests as freezing, unstable values, frequent restarts and other phenomena. The correct method is to use a two-core shielded cable to take 2 of the inner cores and connect them to the 2 inner cores of the original 3-wire sensor cable (the 2 non-ground wires), and then connect the shielded wire to the ground wire of the 3-wire sensor cable. The connection method can refer to the above 2-wire sensor cable extension connection method, make a good shielding wire (wrap the entire shielding layer at the connection with copper foil), and finally make a good external waterproof insulation.

Finally, no matter whether the sensor cable is lengthened or shortened, the length of the two sensor cables in a set of probes must be kept consistent.