4 20mA Pressure Transmitter Level Measurement Pressure Transmitter

NCS Series of Intelligent Transmitters are digital, intelligent, network-based, merged with advanced, reliable, and stable sensor technology, to be designed as a new generation of Fieldbus Intelligent Transmitters. 

NCS Series of Intelligent Transmitters have been designed with high precision and high reliability, high stability. The pressure, temperature, currents, and other physical quantities are displayed on LCD module. Zero drift, range settings, and many other functions, are realized by the magnetism bar operation.

The interoperability between different systems and equipment is realized by the Equipment Description Document, which is coMpatible with a variety of device management software, and adapts to harsh electromagnetic environments. NCS Series of Intelligent Transmitters have been used in inflammable and explosive places according to Intrinsically Safety Design.

NCS-PT105 series of Intelligent Pressure Transmitters are well-designed with advanced, mature and reliable capacitance sensors, combined with advanced microprocessor technology and digital capacitance measurement technology. Internal microprocessor make it intelligent, high-precise, high reliable and zero-point stable. It is easy to debug in the field.

NCS-PT105ⅡSG

The measurement accuracy of the smart transmitter depends largely on the correct installation of the smart pressure transmitter and the pressure inlet tube. In particular, the measurement of the flow mainly relates to the correct installation of a measurement device.

Transmitter Installation

For transmitter installation, three types of brackets are provided (pipe mounting flat bracket, pipe mounting angle bracket, and plate mounting angle bracket). Accordingly, there are three installation methods as the following. 

The installation of pipe mounting flat bracket: the typical installation as Figure 2-1 shows. Fix the transmitter in a flat bracket using the four bolts provided, and then fix the flat bracket on the vertical pipe of Φ50mm around with the U-shape bolt provided.

The installation of pipe mounting angle bracket: the typical installation as Figure 2-2 shows. Fix the transmitter in the angle bracket using the four bolts provided, and then fix the angle bracket on the horizontal pipe of Φ50mm around with the U-shape bolt provided.

The installation of plat mounting angle bracket: the typical installation as Figure 2-3 shows. Fix the transmitter in the angle bracket using the four bolts provided, and then fix the angle bracket on the plate with the M10 bolt.

The correct installation of pipeline depends on the measurment medium. Smart transmitter can measure liquid, steam or other gases. The pressure port, smart transmitter and related position of flow pipe are different according to different measurment medium.

Liquid: The pressure port must be selected on side of the flow pipe to avoid slag deposition. Smart transmitter should be installed under the pressure port, so that the gas can be drained into flow pipe. As shown in Figure 2-4.

Stream: The Inlet pressure pipe must be installed on the side of the flow pipe, and smart transmitter must be installed under the pressure port. The inlet pressure pipe should be filled with water to prevent the direct contaction between smart transmitter and stream. Smart transmitter should not exceed the working temperature limit when stream or other high-temperature medium is measured. As shown in Figure 2-5.

Gas: The pressure port should be installed on the top or on the side of the flow pipe. Smart transmitter should be installed on the top of the pressure port, so that the liquid can be drained into flow pipe. As shown in Figure 2-6.

1) The inlet pressure pipe should be as short as possible in the case of meeting the needs.

2) Corrosive or overheated medium should not be in contact with the transmitter directly.

3) The inlet pressure pipe should be installed in a place where the temperature gradient is low and fluctuation is small.

4) Working temperature limit must be paid attention to when a high temperature medium is to be measured.

5) For the differential pressure transmitter, the two inlet pressure pipes should be kept at the same temperature, and the hydraulic height should be kept balanced (For differential type). 

6) The inlet pressure pipe should use a large-diameter pipe as far as possible in case of the friction effect.

7) For the differential pressure transmitter, the liquid level of the two inlet pressure pipes should be kept at the same height when isolating gas is used.

8) When an injection system is used, the system should be as close as possible to the pressure port of the flow pipe. The purifying liquid should get through from the pipes of the same size, and length of pipes to the transmitter. Also, the injection of liquid through the transmitter should be avoided (For differential type).

The power and signal of Smart Transmitter are sharing one pair of cables (Bus Cable).

The ordinary cables can be selected by HART smart transmitter, but FF, PA Smart Transmitter is suggested to use specific Fieldbus cable the IEC61158-2 recommended.

The terminal is at the rear cover, the terminal wiring board could be seen after screwing the rear cover (indicated “FIELD TERMINAL”).

The left terminal of the terminal wiring board is the “+” signal terminal, the right terminal is the testing“+” terminal, and the middle terminal is the “-” signal and testing “-” terminal (sharing). A power supply is provided to a smart transmitter with a signal wire. The testing terminal is only used in the HART smart transmitter to connect the testing instrument.

Signal wires should be holed to the wire terminal through the wiring hole. And the “+” signal wire should be connected to the “+” signal terminal (left), and the “-” signal wire should be connected to the “-” signal terminal (middle). Signal wires shielded layer can be floating ground, and is also available for single-point grounding at any point in the signal circuit. When the testing instrument is connected to test the analog signal of the HART smart transmitter, the thread method is also in the same way of the signal wire, but the “+” test wire should be connected to the right terminal of the terminal wiring board in HART smart transmitter, and the “-” test wire should be connected to the middle terminal.

Caution: When connecting the HART smart transmitter, do not connect the signal wire to the testing terminal. It will damage the diodes. But when the diodes are burnt, the emergency response is to short the circuit of the two testing terminals.

The Bus Cable of the smart transmitter should not share the line pipe or trunkings with other devices, and should be away from the high-power device.

The measurement principle of smart transmitter based on the 3151 capacitive pressure sensor, using advanced micro-controller technology, sensor digital measurement technology and high accuracy algorithm to ensure the high-precision pressure measurement. The advanced HART, FF H1 and PROFIBUS PA Fieldbus technologies are used respectively by the communication interface of smart transmitter.

The logical structure of smart transmitter, which is the same, can be divided into five parts: sensor, instrument card, communication card, LCD card and port card, as picture shows. The FF, PA Smart Transmitters are identical in terms of hardware due to the same physical layer specification, the communication card and port card of the FF, PA Smart Transmitters are different from those of Hart, but other parts are the same.

The widely used 3151 capacitive pressure sensors, which were developed by Rosemount initially in USA, have been produced on a large scale in China. The core of its sensor is differential capacitive membrane, as picture shows.

There are two measurement capacitances CH and CL distributed in differential capacitive membrane. 

The two measurement capacitances are almost equal to two plate capacitances because of their mechanical structure. The two measurement capacitances share one polar plate, which is a measure membrane in the center. 

And the other polar plate is fixed on the two sides. 

When the pressures of two sides are equal, the measure membrane is in the center, the capacitances of two sides are equal too. 

But when the pressure of high pressure side is higher than that of low pressure side, the guide pressure liquid filled in the membrane guides the differential pressure, so that the measure membrane moves to low pressure side. 

As a result, the capacitance of high pressure side is lower than that of the low pressure side.