The necessary and methods to calibrate intelligent pressure transmitters

At present, modern industrial production is developing in the direction of intelligence, digitalization, and networking. With the computer, communication, and microprocessor technology, the control system gradually advances from distributed control (DCS) to field bus (FCS) control mode. Likewise, the in-situ instrumentation equipment also slowly develops from analog to digital instrumentation.

To meet the requirements of Fieldbus control systems, Fieldbus-type intelligent pressure transmitters have been rapidly developed. However, as the field bus type intelligent pressure transmitter and the traditional sense of the analog type, smart pressure transmitter has the essence of the difference, so at this stage of the Fieldbus instrumentation equipment maintenance and calibration traceability method is a wide range of opinions and differences.

 

1. The necessity of Fieldbus-type intelligent pressure transmitter calibration

As an emerging bus protocol, some process users have adopted the Fieldbus system, and there are some typical application cases, such as some petroleum and petrochemical refinery projects.

As a digital communication network, the Fieldbus system has changed the analog signal transmission from point to point into a serial digital message with multiple issues and lines. As a result, the Fieldbus type intelligent pressure transmitter significantly reduces the analog signal in the system, thus virtually eliminating the error in the circuit. Fully digital two-way communication improves the measurement and control accuracy of the system while also making system configuration and commissioning easier. The intelligent pressure transmitter function modules are set up, and the data transmission and exchange can be achieved entirely by digital communication tools or by interaction with the host computer.

At present, some views overly expand the scope of Fieldbus and digital communication tools. We want to eliminate system errors by using Fieldbus systems, and we also can use digital communication tools for the maintenance and calibration of field bus systems. Some people believe that through the powerful function module of the Fieldbus intelligent pressure transmitter, the data interaction function, and its self-test alarm function, through the digital communication tools on the Fieldbus type smart pressure transmitter configuration and debugging can achieve long-term reliability and error elimination of intelligent pressure transmitter.

This is a confusion of the concepts of maintenance and calibration of the system and is entirely contrary to the concepts of calibration and traceability, and does not comply with the relevant provisions of the Law of the People’s Republic of China on the guarantee of unity of the national system of units of measurement and the transmission of values.

Fieldbus-type intelligent pressure transmitters are composed of high-precision sensors, programmable amplifiers, AD conversion circuits, CPU microprocessors, modulation, demodulation chips, and external interfaces. However, the Fieldbus system cannot reduce the errors at the primary sensor level.

The primary sensor is responsible for converting the non-electric/non-linear signal into a linear analog electrical signal. The sensor material and the digital-to-analog conversion circuit in the secondary instrumentation circuit are the main errors. This is because digital communication tools can only configure and adjust the digital part of the system, but not the sensor part and the analog signal part of the calibration and adjustment.

In layman’s terms, no matter what system protocol is used to calibrate the process variable circuit, we need a high precision process variable traceability reference for the transfer of quantities, e.g. to calibrate a pressure smart pressure transmitter, a high precision pressure calibrator, or module is required for calibration comparison.

2. Ways and means of calibrating Fieldbus-type intelligent pressure transmitters.

As the Fieldbus circuit is an entirely digital signal circuit, the output signal in the course is to be monitored and adjusted by a digital communication tool, which the host computer can use to realize the data at the application site. At the same time, as there is no analog signal transmission, the calibration of the Fieldbus-type intelligent pressure transmitter becomes the calibration and traceability of a single process variable value.

The Fieldbus loop is an entirely digital signal loop, and a standard process variable is applied to the Fieldbus Smart Pressure Transmitter by a typical process variable definitive source. The digital output of the Fieldbus Smart Pressure Transmitter is monitored via a digital communication tool or a host computer (which can be set to the same units of measurement as the standard process variable standard source). The Fieldbus Smart Pressure Transmitter is calibrated by a linear comparison of a single process variable range.

3. Intelligent pressure transmitter calibration.

Once communication has been established between the digital communication tool and the Fieldbus-type intelligent pressure transmitter, the calibration mode is accessed from the function menu. Next, the digital communication tool can monitor the signal output of the intelligent pressure transmitter. At this point, the process variable calibration of the smart pressure transmitter can be carried out.

Smart pressure transmitter output indication and common source output is a single unit of process variables, and we have to analyze the procedures and errors of the field bus type innovative pressure transmitter calibration. We can entirely refer to the primary indication table and sensor metrological calibration procedures.

4. Conclusion

The Fieldbus type intelligent pressure transmitter is calibrated in principle by only transferring the values of the process variables. Since the 4-20mA analog comparison is eliminated, the calibration is more straightforward than that of conventional pressure transmitters. However, the introduction of digital quantities places further demands on the signal transmission circuit and the system configuration.

This paper issues the calibration of process variables and does not deal much with the configuration and adjustment of digital signals. This part depends mainly on the Fieldbus type intelligent pressure transmitter,  it will use and what function modules are built-in, and we need to analyze the specific problems.