A 4-20 mA process meter is used to troubleshoot, test, and calibrate analog devices. It may be configured to simulate a two-wire sensor or it can provide a controlled loop current to an actuator.
This brief presents two experiments to get you started with your process meter. It is focuses on the two modes associated with the 4-20 mA current loop including:
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Simulation mode: The process meter simulates a 4-20 mA sensor.
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User-selected current source mode: The meter provides a current as manually set by the user. Alternatively, the meter provides a current that automatically varies with time. The user can configure the meter to provide a smooth current sweep or a stairstep change.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Measure It
Difficulty: 
Technician — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 29 Apr 2026
When Does a Process Meter Become Essential?
The process meter moves from luxury to essential when system downtime exceeds the cost of the meter. Depending on the size and complexity of your facility, that’s somewhere between 5 minutes and one hour of lost production. However, maintenance crews need to be trained to quickly configure the instrument and interpret the results. Hence this document. There is also an argument for saving time by testing and calibrating field devices either on the bench or locally in the installed equipment.
If you are looking for a rule of thumb, may I suggest the process meter becomes increasingly important in an industrial setting with between 10 and 20 analog sensors or actuators.
Required Components
- Process meter such as the Fluke 789 shown in Figure 1
- Conventional multimeter
- 24 VDC power supply
- 1.5 kΩ 1/2 W (minimum) resistor
Figure 1: Image of the process meter configured for transmitter mode simulating a sensor operating at 75% of span which corresponds to 16 mA.
Experiment in Sensor Simulation Mode
Configure the process meter experiment using familiar test equipment. Figure 1 shows a current loop including:
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24 VDC power supply
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Process meter wired to simulate a 4-20 mA sensor
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Conventional meter set to measure current
The conventional meter stands in for the device that normally receives and processes the sensor’s output such as a PLC or an industrial panel mount display. Without the current meter, we are missing a critical component that is present in every operational current loop. It also provides a measure of truth indicating that the current loop is active. For additional information, refer to this article which provides a brief introduction to analog scaling and I/O card configuration for a Siemens S7 PLC.
Adjusting the Current
The user may adjust the loop current using the meter’s arrow keys as shown in Figure 2. Alternatively, the meter may be configured to automatically change the loop current. There are settings for a smooth ramp up and down through the entire 4-20 mA range. The user can also command a stairstep with steps at 4, 8, 16, 20, and 25 mA. Consult the datasheet for additional settings such as configuring the meter for a 0-20 mA range.
Figure 2: Manual adjustment to the loop current as shown in the Fluke 789 datasheet.
Tech Tip: The process meter is not supplying energy while operating in simulation mode. Instead, it is acting as a two-wire transmitter allowing more or less current to pass through the loop. This is the same operation performed by a 4-20 mA sensor as it responds to its environment.
As a mental model, the meter behaves like a variable resistor adjustable from 6.0 kΩ to 1.2 kΩ corresponding to 4 to 20 mA. As a student, make sure you understand the Ohm’s law relationship between the 24 VDC (nominal) supply and stated resistance.
In practice, the “variable resistor” is a pass transistor such as a BD139 (or SMD equivalent) tightly controlled by a dedicated 4-20 mA transmitter IC such as the Texas Instruments XTR116.
Next Step for Sensor Simulation Mode
In this experiment, the process meter simulates a sensor. The next step is to connect our simulated sensor to a real 4-20 mA receiver such as a PLC analog input module or an industrial display like the Red Lion PM-50.
Tech Tip: A connection to a real-world device will reveal a minor technical point that is not addressed in this article. Specifically, the concept of a burden resistor. Up until now, the current is passing through the near zero impedance of the Fluke 87 meter’s current measurement loop. By contrast, a PLC or an analog process meter will have a fixed input resistance. This is OEM- and equipment-specific, but is generally in the 100 Ω to 1.5 kΩ range.
The voltage developed across this burden resistor may be used during analog troubleshooting. For example, by measuring the voltage drop across a PLC’s analog 4-20 mA input resistor, we can identify gross loop malfunctions such as short or open circuits.
Experiment in Fixed Source Mode
The process meter is configured to source current.
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A 1.5 kΩ resistor serves as a load. This resistance was chosen to pull approximately 16 mA from a 24 VDC source.
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The Fluke 87 in Figure 3 is used to measure the voltage drop across the resistor.
Figure 3 reveals the cause and effect nature of the setup. The process meter is configured to supply 16.000 mA. In an ideal setting, we expect the voltmeter to display 24 VDC. However, the resistor is not precisely 1.5 kΩ. In fact, given the 5% tolerance, we can realistically expect a voltage reading between 22.8 and 25.2 VDC. I invite you to verify my calculations.
Continue the experiment by manually adjusting the loop current (Figure 2). Also explore the sweep functionality of the process meter.
Next Step for Source Mode
In this experiment, the meter is configured to simulate a PLC’s 4-20 mA analog output. You are encouraged to use the process meter to drive an actuator. A representative example of a 4-20 mA field device is a proportional pressure regulator such as the Festo VPPE-3-1-1/8-6-420-E1.
Figure 3: Process meter configured for source mode driving a 1.5 kΩ load resistor.
Tech Tip: Note that the resistor gets noticeably warm. This is a clear indication that power is being supplied by the meter. It also explains this note from the Fluke 789 datasheet, “Source mode depletes the battery faster than simulate mode, so use simulate mode whenever possible.”
Parting Thoughts
Let’s recap the functionality of a process meter:
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It can stand in for a sensor where it modulates a known user-adjustable current within the externally power loop. This is useful for troubleshooting or commissioning a system. For example, alarm and trip points can be quickly verified using the meter as opposed to swinging the real-world plant to those extremes.
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It can command an actuator via its 4-20 mA current output. The current-to-pressure regulator is a good example. With the meter connected directly to the regulator, you can quickly set the minimum and maximum pressure to match the desired 4-20 mA control signal.
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As a student, the process meter is a natural companion to PLC programming. It allows you to simulate field devices (sensors and actuators). The meter configuration and physical connection to the PLC analog module become part of the lesson plan.
As a final thought, be sure you can explain the difference between setting the current in simulate vs setting the current source mode. Remember that the terms “setting” has two very different meanings.
Related Information
Please follow these links to related and useful information:
About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.