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Radar Level Transmitter Output Signals: Choosing the Right One
Choose a radar level output by working backwards from the receiving system, required response and diagnostic functions. Confirm the actual interface offered for the selected model. A protocol comparison does not mean that every radar family is available with every interface listed below.
Which interfaces are being compared?
| Interface | Information path | Integration questions |
|---|---|---|
| 4-20 mA | An analogue process value on a current loop | Compatible input, active or passive wiring, supply and fault recognition |
| Wired HART | Digital communication superimposed on a compatible analogue loop | Host support, loop impedance and available device descriptions |
| Modbus RTU over RS-485 | Serial register-based communication | Register map, separate power requirements, topology and polling time |
| PROFIBUS PA | Digital process fieldbus | Segment design, power, host support and device integration files |
| FOUNDATION Fieldbus H1 | Digital process fieldbus | Segment power, scheduling, device capabilities and host engineering |
| Ethernet-APL | Two-wire Ethernet physical layer | APL ports, power profiles, application protocol and network design |
| Wireless arrangement | Radio link with a defined power and network system | Coverage, update rate, battery plan and required local protection |
A PLC does not necessarily accept a current loop without a suitable analogue card. Likewise, an RS-485 pair is not normally the power supply for a conventional Modbus transmitter. Check the terminal diagram rather than inferring the number of conductors from the protocol name.
Is the transmitter loop-powered or separately powered?
In a two-wire 4-20 mA transmitter, the same pair carries power and the measurement current. Other designs have a separate supply and an active or passive output. Match the receiving input and do not connect two incompatible active circuits together.
Check the loop at the minimum supply voltage and highest current required by the instrument, including its configured fault condition. Account for input resistance, cable resistance and the voltage drop of barriers, isolators and other series equipment.
As an assumed calculation, take a 24 V supply, 300 ohms total resistive load, 22 mA current and another 2 V of series-equipment drop. The transmitter receives 24 - 0.022 x 300 - 2 = 15.4 V. If that model requires 12 V at its terminals under those conditions, the remaining margin is 3.4 V. These are example inputs, not universal equipment ratings.
How should analogue fault currents be interpreted?
An upscale or downscale fault output is diagnostic information, not a command that is inherently safe for the process. Configure the PLC or logic solver to distinguish valid measurement, overrange, underrange and fault according to the device and system requirements.
A broken loop cannot carry an upscale fault current: an open circuit normally drives the received current towards zero. Test this separately from an internal sensor fault that produces a configured current while the loop is intact.
Do not automatically start a wet-well pump on an upscale fault, or close a valve simply because a generic table calls a current 'fail-safe'. The required action belongs to the cause-and-effect and risk assessment, including dry-run, flooding and independent protection where needed. A held last value must be recognised as stale rather than silently accepted as a fresh reading.
What does HART require?
Wired HART allows configuration and supported diagnostic data over the current-loop wiring. The available variables and access to an echo curve depend on the transmitter, host software and device integration. A plain analogue input may read the current without making digital information available.
A common nominal load is 250 ohms, but check the actual permitted communication impedance, capacitance and loop-voltage budget. An input card may already provide the required load; adding another resistor without checking can leave too little voltage at the transmitter.
Multidrop capability depends on the HART revision, devices and host. The electrical budget and intended control use can limit a network before its address capacity is reached. In fixed-current multidrop operation, each device's live value is read digitally rather than from a separate varying analogue current.
What changes with Modbus and process fieldbus?
Modbus RTU needs agreed addresses, baud rate, parity, register types, units and data order. RS-485 node capacity depends on transceiver loading, topology, termination and the installed design; it is not always exactly 32 instruments. Power wiring must be designed separately where required.
Calculate polling time from actual requests, responses, turnaround, retries and network size. An assumed one-second cycle for twenty tanks is not a guarantee. Test timeout and stale-value handling when a device or cable fails.
PROFIBUS PA and FOUNDATION Fieldbus H1 require a compatible host and engineered segments. Device count is constrained by power, communication and installation requirements, not just a table of maximum addresses. Their standard physical data rate does not establish the application's effective update time.
What is different about Ethernet-APL?
Ethernet-APL provides a 10 Mbit/s two-wire physical layer for process field networks. Its links are point-to-point: a field-device spur port is not a bus connection for 25-50 instruments. An engineered network can carry power and data, and suitable port profiles can support intrinsically safe connections.
APL does not automatically make every connection intrinsically safe or specify the application protocol. Check the switch, port and device profiles, power budget, cable, hazardous-area requirements and DCS support. It can be useful in a new plant or a retrofit, but the project design must justify the change.
What belongs in the integration acceptance test?
| Test | Required evidence |
|---|---|
| Measurement and scaling | Correct units and agreement between transmitter and receiving system |
| Diagnostics | Fault and stale-data states recognised by the host |
| Electrical budget | Terminal voltage and load within the permitted envelope |
| Communication loss | Agreed timeout, alarm and protective response |
| Recovery | Controlled behaviour after power or network restoration |
| Documentation | Wiring, addresses, files, settings and approved change record |
Follow the manufacturer's and site's EMC and protection design for shielding, grounding and cable separation. There is no universal 50 mm clearance or one-end-only shield rule for every interface and installation.
Provide METRAVON with the receiving card or controller model, required data, update time, supply, cable and classification so the offered interface can be checked as part of the whole system.
FAQ
Can a Modbus PLC read a 4-20 mA sensor?
With a suitable analogue input or remote I/O conversion, it can. Confirm scaling, diagnostics and fault handling; a serial port alone cannot directly measure loop current.
Must I add a 250-ohm resistor for HART?
Not always. Check existing loop impedance and the model's communication and power requirements. The receiving card may already provide the necessary load.
Does a cable break produce a high fault current?
No. An open loop normally produces near-zero received current. Test that condition separately from a device's configured upscale fault output.
How many devices fit on an RS-485 network?
It depends on loading, topology, cabling and the permitted installation. Address capacity alone does not determine the number of working devices or the polling time.
Does one Ethernet-APL spur connect many transmitters?
No. The physical link is point-to-point. A field switch provides separate appropriately designed device ports.
Does a digital output remove every measurement error?
No. It can avoid parts of an analogue conversion chain, but sensor performance, datum, echo selection and process effects still remain.
Technical references
Related applications
How to Specify a Radar Level Transmitter: The 14 Fields That Decide the Quote | Explosion Proof Radar Level Transmitter: Reading the Certificate, Not the Brochure
