Process industries require monitoring of pressure, temperature, pH level, etc. for smooth operation.

These transmitters send information to PLCs or controllers on a 4 mA to 20mA loop.

Often engineers troubleshoot these transmitters by checking the 4 mA to 20 mA loop signal.

How the 4-20 mA loop works

The loop’s operation is straightforward: a sensor’s output voltage is first converted to a proportional current, with 4 mA normally representing the sensor’s zero-level output and 20 mA representing the sensor’s full-scale output. A reading of 20 mA means that a direct-acting valve, for example, is 100% open, and a reading of 4 mA means that it is closed. (The opposite is true for a reverse-acting valve). Readings between the maximum and minimum values indicate that the circuit is controlling the valve.

Verifying a 4-20 mA loop is a crucial step in both troubleshooting and calibrating process systems.

Full verification includes testing the output of the transmitter, the wiring, input to the control system and control system input card, and the return wiring back to the transmitter.

How to Measure a 4-20 mA loop Signal With a Multimeter

Here, I am going to show you two different methods on how to measure a 4-20 mA loop signal using a multimeter.

Method #1: Measure the 4-20 mA loop signal with the help of a multimeter

Identify the Loop Wiring

Find the two wires carrying the 4–20 mA signal from the transmitter or sensor to the controller/PLC.

Tip: Consult the device manual or wiring diagram.

Break the Circuit (Series Measurement)

Unlike measuring voltage (parallel), current measurement requires the multimeter to be inserted in series with the loop.

Choose a point where the loop can be safely opened (often at a terminal block or test point).

Power must remain ON in the loop for accurate measurement.

Configure the Multimeter

• Turn the dial to DC mA (usually a 20 mA or 50 mA range is best).
• Plug the red lead into the mA input jack (NOT the voltage or A high-current jack).
• Plug the black lead into the COM jack.

Connect the Multimeter in Series

• Disconnect one wire at the test point.
• Connect the red lead to the wire coming from the transmitter.
• Connect the black lead to the terminal going to the PLC/controller.
• This way, the loop current flows through the meter.

Read the Measurement

A normal signal should read between 4.00 mA and 20.00 mA.

If you see:

• <4 mA → Possible wiring issue, sensor fault, or calibration error.
• >20 mA → Overrange condition or device malfunction.

Remove the Multimeter

Power down or hold the loop signal as needed.

Reconnect the wire directly to restore normal operation.

Method #2: Measure the 4-20 mA loop signal with the help of a process clamp meter

The functions of an advanced loop calibrator allow technicians to troubleshoot on the spot without disconnecting wires or “breaking the loop.”

Multifunction process calibrators can also be used to test 4-20 mA loops as well as digital controls.

Step 1: Access the wire connected to the ‘+’ terminal of the transmitter (no need to remove the wire).

Step 2: Put the process clamp meter in ‘Measure’ mode.

Step 3: Clamp the wire connected to the positive terminal of the transmitter and measure the mA signal as shown below.

You can use Fluke’s 771 or 772 or 773 process clamp meters to measure mA without breaking the loop.

Advantage of this method

You need not break the loop to measure the mA signal. This is the biggest advantage.

Disadvantage of this method

Process clamp meters are expensive.

Safety Tips when using a multimeter to measure 4-20 mA loop

Check Multimeter Rating

Ensure the multimeter’s current rating is suitable for the expected measurement.

Proper Probe Insertion

Confirm that the multimeter probes are correctly inserted into the appropriate ports (usually labeled COM and mA/µA).

Avoid Overloading

Do not exceed the multimeter’s current measurement limit to prevent damage to the multimeter and ensure personal safety.

Conclusion

Measuring a 4-20 mA signal with a multimeter involves setting the multimeter to the appropriate mode, connecting it in series with the current loop, and interpreting the readings.

This process helps ensure that sensors and transmitters in industrial control systems are operating correctly.

Always prioritize safety by powering down the circuit before making connections and ensuring the multimeter is properly rated for the measurements being taken.

Selecting the right gas detection solution is critical to maintaining safety and compliance across various environments.

The process requires evaluating potential hazards, understanding site-specific conditions, and ensuring compliance with regional standards. Here’s a comprehensive guide to making an informed choice.

Why do you need a gas detection system?

Depending on the industry, many gases can be produced as a by-product or part of the process.

Gases such as benzene can cause cancer, and flammable gases such as propane and methane need to be monitored, and you need a system that will let people in those areas evacuate in case of any danger. 

What are gas detectors?

Gas detectors can detect explosive, flammable, and toxic gases and oxygen depletion.

This device is used widely in industry and can be found in locations such as oil rigs to monitor manufacturing processes and emerging technologies such as photovoltaics.

How to select the right gas detection solution for your plant

Many gas detection products on the market might appear to be the same. Still, a closer inspection of specifications, functionality, and features reveals major differences in what products can do and their potential value.

Similarly, individual applications’ designs, needs, and processes are unique.

Understand Site-Specific Risks and Hazards

Before selecting a gas detector, conduct a thorough risk assessment of the site:

Identify Potential Risks

Knowing which gases are present is essential. As you mentioned, a reliable approach is to use a gas analyzer or consult a third party for a detailed site evaluation. This will provide clarity on detectable gases and help mitigate risks effectively.

Consider Industry-Specific Requirements

Each industry faces unique risks. For instance, environments that involve chemical processing may require benzene detection.

In confined spaces, monitoring for gases like oxygen, hydrogen sulfide, carbon monoxide, and volatile organic compounds (VOCs) is essential.

To select the right gas detection system, it’s essential to understand the risks on-site. Using a gas analyzer or a third-party service can quickly reveal the gases you need to monitor.

For example, confined spaces often need detectors for oxygen, hydrogen sulfide, carbon monoxide, and VOCs.”

Choose the Right Type of Gas Detector

Different environments and applications may require either fixed or portable gas detectors:

Fixed vs. Portable Detectors

Fixed gas detection systems are ideal for continuous monitoring in high-risk or permanent areas, such as warehouses or industrial plants. Portable detectors, on the other hand, offer flexibility and are suited for dynamic settings.

Single vs. Multi-Gas Detectors

Multi-gas detectors are efficient and cost-effective for industries where several gases are a concern. Single-gas detectors, meanwhile, are ideal for settings where one primary gas is the main risk.

In my experience, portable gas detectors are ideal for confined spaces and are versatile in various environments.

For stationary areas, however, fixed solutions provide continuous coverage, which can be essential in high-risk zones.

Consider Environmental Conditions

Environmental factors significantly influence gas detector performance. For example:

Adapt to Temperature and Climate

In high- or low-temperature environments, standard gas detectors may underperform. Select a detector with appropriate specifications for extreme conditions—like mining or offshore rigs—where durability is essential.

Specialized Detectors for Specific Conditions

Some environments, such as humid or dusty locations, may require detectors with moisture-proofing or particulate filters.

Each environment requires specific detectors; extreme cold or high heat can impact performance. In mining, for instance, rugged, temperature-resistant models are necessary.”

Check for Necessary Certifications

Compliance with regional standards is vital for safety and legal reasons:

Regional Certification Standards

Different countries have distinct certification requirements. For instance, Brazil uses the InMetro certification, while the U.S., Canada, and Mexico rely on UL or CSA certifications. Europe adheres to ATEX or IECEx standards.

Importance of Compliance

Meeting local standards is essential to avoid potential penalties and to ensure that the equipment is reliable. Uncertified equipment may not perform optimally, especially under challenging conditions.

In my experience, certifications vary widely. For example, projects in the U.S. need UL or CSA certification, while European projects often require ATEX or IECEx compliance.”

Evaluate Integration Capabilities

Integration is crucial, particularly if the detector will connect to a broader safety system:

Compatibility with Existing Systems

For fixed systems, check if the detector is compatible with protocols like Modbus, HART, or BACnet, especially if connecting to building automation systems. This makes monitoring seamless and efficient.

Software and Data Management for Portable Detectors

Many portable detectors come with software for data collection and analysis. Look for models that offer free software, which can reduce costs and simplify monitoring.

Integration options can make a huge difference. If you’re adding a fixed system to a building automation network, compatibility with protocols like Modbus or BACnet is key. Portable detectors also benefit from having free software for easier data access.”

Assess the Support and Maintenance Requirements

Regular maintenance ensures that gas detectors remain accurate and reliable over time:

Routine Calibration and Testing

Regular calibration is necessary to maintain detector accuracy. Ensure that the manufacturer provides calibration support or has local service options.

Ease of Use and Training Needs

Some gas detection systems may require specialized training. Choose equipment that matches your team’s skills, and provide additional training if necessary to ensure proper operation.

Additional Considerations for Choosing a Gas Detection Solution

Finally, a few extra factors can help maximize value and usability:

Budget vs. Long-Term Value

While cost is always a factor, investing in high-quality detectors often saves money over time by reducing maintenance needs and increasing reliability.

Availability of Spare Parts and Replacement Options

Ensure that you can easily obtain replacement parts or additional units if needed. Quick access to parts minimizes downtime and keeps safety measures in place.

Key Takeaways: How to Select the Right Gas Detection Solution

Selecting the right gas detection solution requires a blend of industry-specific insight, regional compliance knowledge, and practical considerations based on site requirements.

With careful attention to these factors, you’ll be well-prepared to choose a system that enhances safety, supports regulatory compliance, and meets the unique demands of your environment.