A digital display, pressure sensor and digital interface module package for use on portable data acquisition trolleys to measure pressure of air over a range of -20 to 20 kPa diff from two 6.6mm dia x 11mm barbed hose connections, and sending the corresponding reading via a RS485 Modbus RTU interface through the terminal block connector electrical connections.
- Customer’s Application: Measuring bidirectional air pressure in a test chamber and across an orifice plate.
- System Constraint: Existing data acquisition system only accepts Modbus RTU inputs.
- Key Requirements: Bidirectional ±17 kPa range, integral sensor display, and IP54 environmental protection.
- Product Solution: A DPS300 series differential pressure transmitter paired with an SIAi-8 series analogue input to RS-485 Modbus module.
A project engineer from a multi-disciplinary engineering and scientific consultancy required a pressure measurement solution for a portable data acquisition trolley. The trolley was designed for versatile use in both semi-covered indoor and outdoor environments, necessitating a system with a degree of weather protection.
The primary measurement task involved monitoring low differential air pressures, specifically the pressure difference between a test chamber and the surrounding atmosphere. A secondary, but equally important, application was measuring airflow by determining the pressure drop across an orifice plate. A critical constraint of the project was that the company’s unique data logging system, installed on the trolley, was limited to accepting inputs via Modbus RTU communication protocol only. This meant any proposed sensor would either need a native Modbus output or be paired with a reliable analogue-to-digital converter.
The engineer specified a set of precise requirements for the instrumentation. A sensor was needed to accurately measure a bidirectional pressure range of at least ±20 kPa to handle both positive and negative gauge pressures within their test setups. For immediate, on-site verification of readings without referencing the main data logger, the sensor had to include an integral digital display. To withstand the varied working environments of the portable trolley, an ingress protection rating of at least IP54 was necessary. For connectivity, the sensor needed to have barbed pressure ports suitable for 4mm or 6mm internal diameter flexible tubing. The most significant challenge was the required Modbus output to interface with their existing equipment.
The proposed solution consisted of two components that worked together to meet all the customer’s technical requirements: a DPS300 differential pressure transmitter and an SIAi-8 analogue to Modbus RTU interface converter.
The DPS300 series transmitter was selected as the primary sensing instrument. Its robust ABS housing is rated to IP54, making it perfectly suited for the semi-protected, portable nature of the data acquisition trolley. The transmitter was configured for the required ±17 kPa differential pressure range and equipped with the optional two-line LCD display, allowing engineers to view live pressure readings directly at the point of measurement. This model offers exceptional flexibility with user-selectable analogue outputs, which for this application was set to a 4-20mA two-wire signal.
A key advantage of the DPS300 for this application is its range of features tailored to airflow measurement. The response time is adjustable via a potentiometer, allowing the user to dampen the output signal to smooth out fluctuations from turbulent air conditions. Furthermore, the transmitter can be supplied with a square root extraction function. This feature is particularly useful for the customer’s secondary application of measuring flow with an orifice plate, as it linearizes the output signal relative to flow rate, simplifying subsequent calculations in the data acquisition system.
To bridge the gap between the transmitter’s analogue output and the data logger’s Modbus input, the SIAi-8 serial transmitter module was specified. This powerful DIN-rail mounted module converts the 4-20mA signal from the DPS300 into the Modbus RTU protocol for seamless integration. While the immediate application only required one input, the SIAi-8 provides a total of eight independent channels, offering significant capacity for future expansion of the portable trolley with additional sensors.
Two crucial features of the SIAi-8 are galvanic isolation and advanced signal processing. All inputs are galvanically isolated from the power supply and the RS-485 interface, which is vital for eliminating ground loops and protecting the sensitive data acquisition hardware from electrical noise, an essential consideration for a portable system used in various locations. Beyond simple conversion, the module allows for sophisticated signal characterization. Using the Modbus communication link, the engineer can set scaling parameters (“Lo CAL” and “Hi CAL”) to convert the 4-20mA signal into precise engineering units (e.g., -20.00 to +20.00 kPa). The module also features options for linear, square, or even user-defined 20-point linearization curves, providing an additional layer of signal processing capability to refine the measurement data before it ever reaches the central logger.
DPS 300 DP 4-20mA Output Sensor and Display
- SKU ID: s1-dps300-91-24318
- Pressure Range: -20 to +20
- Units: kPa
- Range Type: Differential (0 to +P)
- Output Signal: 4-20mA 2 wire
- Output Options: Linear
- Switch Contacts: None
- Accuracy (linearity only): 1% full scale
- Electrical Connection: M16 x 1.5 cable gland
- Process Connection: 6.6mm dia x 11mm barbed connections
- Media Compatibility: Air
- Optional Extras: integral LCD readout
SIAi-8 4-20mA to RS-485 Modbus RTU Converter
- SKU ID: s1-siai8-0002
- Part No: SIAi-8P-001
- Input: current 0-20 mA, 4-20 mA
- Number of Inputs: 8
- Communication Interface: RS-485
- Transmission Protocol: Modbus RTU
- Operating Temperature: 0 to +50degC
- Power Supply: 24Vdc