Features & Configuration of a Smart Pressure Transmitter

Smart/Intelligent pressure transmitters are microprocessor-based, and they provide additional functional capabilities beyond their basic role of just sensing and representing a process variable; for example they provide computing, diagnostics, and networking capabilities .

We shall consider a capacitive sensing pressure transmitter with a typical functional diagram illustrated below:

The process pressure is transmitted through the isolating diaphragm and the fill fluid to the sensing diaphragm in the center of the capacitance cell. Electrodes on both sides of the sensing diaphragm detect its position and the differential capacitance between the sensing diaphragm and the electrodes is directly proportional to differential pressure.

The capacitance cell is laser welded and isolated mechanically, electrically and thermally from the process medium and the external environment. The mechanical and thermal isolation is achieved by moving it from the process flange to a position in the neck of the electronics housing. Glass-sealed pressure transport tubes and insulated cell mountings provide electrical isolation and improve the performance and transient protection.

The signal from a temperature sensor incorporated in the cell is used to compensate for thermal effects.

During the characterization at the factory, the sensors are run through pressure and temperature cycles over the entire operating range. The data from these cycles is used to generate correction coefficients which are then stored in the sensor module memory to ensure precise signal correction during operation. The sensor module also facilitates repair, since all the module characteristics are stored with the module so that the electronics can be replaced without having to recalibrate or substitute different correction PROMs (Programmable read-only memories). Also, located in the sensor are the electronics that convert the differential and temperature input signals directly into digital formats for their processing by the electrons module.

The electronics module consists of a single board incorporating ASIC and other surface mounted components. This module accepts the digital input signals from the sensor module, along with the correction coefficients and then corrects and linearizes the signal. The output section of the electronics module converts the digital signal into a 4-20 mA output signal and handles the communication with the handheld terminal which may be connected at any access point in the 4-20 mA loop, as illustrated below. 

Configuration of a Smart Pressure Transmitter

The configuration data are stored in non-volatile EEPROM (Electrically Erasable Programmable Read Only Memory) in the electronics module. This data is retained when the transmitter power is interrupted so that the transmitter is functional immediately it is powered again.

The process variable is stored as digital data, enabling precise corrections and engineering unit conversions to be made. The corrected data is then converted to a standard 4 – 20 mA current in the measurement circuit.

The sensor signal can be assessed directly as a digital signal by the handheld terminal bypassing the digital to analog conversion and thereby providing a higher accuracy. For example a smart pressure transmitter that can communicate via HART protocol which employs the industry standard Bell 202 frequency shift keying (FSK) technique whereby remote communication is achieved by superimposing a high frequency digital signal on top of the analog 4 – 20 mA output signal. The HART protocol, allows the user easy access to the configuration, test and format capabilities of the smart pressure transmitter.

Note, different manufactures have different configuration procedures and type of data that is entered into the transmitter may differ from one manufacture to another. What is discussed here, is just an illustration to show you which kind of information maybe required to enter into the transmitter.

The configuration is done in two parts; the transmitter operation parameters are first set and consist of the following:

• 4 and 20 mA points
• Damping
• Selection of engineering units

Secondly, the information data is entered into the transmitter to allow for identification and physical description of the transmitter. This data include:

• Tag
• Descriptor
• Message
• Date
• Integral meter installation
• Flange type
• Flange material
• Drain/vent material
• O-ring material
• Remote seal information

Besides the configurable parameters, smart pressure transmitters contains several kinds of information which are not user-changeable, such as transmitter type, sensor limits, minimum span, fill fluid, isolator material, module serial number and software revision number.

Continuous diagnostic tests are run by the transmitter. In the event of a fault, a user-selected analog output warning is activated. The handheld terminal can be used to interrogate the transmitter to establish the fault. The transmitter outputs specific information to the handheld terminal or control system to facilitate fast and easy correction of the fault. If the operator believes there is a fault in the loop the transmitter can be directed to give specific outputs for loop testing.

Related: How to Connect a DP (Differential Pressure) Flow Sensor to a DP Transmitter

In smart pressure transmitter you can find the format function which is employed during the initial set-up of the transmitter and for maintenance of the digital electronics. It allows the sensor and the 4 -20 mA output to be trimmed to meet the plant pressure standards. Also, you find the characterize function that allows the user to prevent accidental or deliberate adjustments of the 4-20 mA set points.

Related articles:

• What is a Smart Sensor?
• Types of Sensors used in Measurement and Process Control
• 4-20 mA Transmitters (2-wire, 3-wire and 4-wire Types)