A customer needed to add alarm and control capabilities to an existing airflow monitoring system that used a sensor with a 4-20mA output. The primary goal was to integrate relay switching functionality for high or low flow conditions directly into the signal conditioning chain, thereby eliminating the cost and complexity of a separate relay module. The 4002ALM-6 trip amplifier was selected as the ideal solution to meet these requirements.
- Application: Monitoring and alarming on volumetric airflow in a ducting system.
- Input Signal: 4-20mA from an air velocity sensor.
- Requirement: Add relay outputs for process control/alarming without a separate module.
- Solution: A universal trip amplifier with dual setpoint relays and a re-transmitted analogue output.
A customer in the hazardous location services industry required a solution for monitoring and controlling volumetric airflow within a ducting system. The application involved taking the standard 4-20mA analogue signal from an existing air velocity sensor and using it to trigger alarms. A primary requirement was to integrate this alarm functionality directly into the signal conditioning instrument to avoid the cost, wiring complexity, and additional panel space required by a separate, standalone relay module. The ability to consolidate components was a key consideration for simplifying the installation and enhancing system reliability.
Initially, the customer inquired about incorporating a square root extraction function. This is a common requirement in airflow applications where the primary sensor, such as a differential pressure transmitter, produces a signal proportional to the square of the flow rate. Applying a square root function to this signal linearises the output to be directly proportional to the volumetric flow. However, to meet specific budgetary targets for the project, the decision was made to proceed without the square root function and focus on the core requirement of providing reliable relay switching from the sensor’s native 4-20mA velocity signal.
The proposed solution was the 4002ALM-6 series universal trip amplifier. This DIN rail mountable device was perfectly suited to the application’s needs. It accepts the 4-20mA signal directly from the air velocity sensor and provides two key outputs. Firstly, it features an isolated 4-20mA re-transmission output, allowing the original velocity signal to be passed on to a control room or data logger without the risk of ground loops or signal interference.
More importantly for this application, the 4002ALM-6 includes two independent, single pole change-over (SPDT) relay outputs. These relays serve as the core of the solution, enabling the customer to set precise high and low trip points based on the incoming air velocity signal. The configuration of these setpoints is handled directly via the front panel interface. This provides the flexibility to create multiple alarm conditions, such as a low-flow warning, a high-flow alarm, or a high-high level for initiating a shutdown procedure. The ability to configure the relays as energised or de-energised in the alarm state also allows for the implementation of fail-safe or non-fail-safe logic, enhancing the safety and integrity of the process control system. This integrated approach successfully provided the required alarm functionality, simplifying the system architecture and meeting the project’s financial and technical goals.
4002ALM-6 24V ac/dc Powered 4-20mA to 4-20mA Signal Converter with Dual Relay Outputs
- Part No: II-4002ALM-JJ-6-00-00
- Input Type: 4-20mA
- Output Type: 4-20mA, 2 x Relays
- Supply: 12-36 Vdc or 12-32Vac
- Isolation: Full 3-Port
- Installation: DIN Rail TS35
- Electrical Connections: Screw clamp with pressure plate