Pressure Transducers are critical components of various applications. For example, your automotive vehicle has pressure changes for oil, liquid, and fuel. Similarly, hydraulic machines use oil to push and create a force.
Pressure changes should be accurate to ensure the proper pressure transmission and force. One of the most critical pressure sensors is the pressure transducer 4-20mA. Today, we will discuss various aspects of this pressure transducer and understand how it works.
What is a pressure transducer 4-20mA?
A pressure transducer, which operates on a 4-20mA signal, tests the pressure of liquid or fuel and converts it into electrical signals. Instead of voltage as an output, this system offers current output.
You might wonder why you should consider the pressure transducer 4-20mA; the logic is straightforward.
- 4 mA represents the lowest measurable value (e.g., 0 bar or minimum range).
- 20 mA represents the highest measurable value (e.g., full scale, such as 100 bar).
- The measurements and indications are easier. The four mA baseline ensures that a “live zero” is present, meaning that if the loop reads zero mA, it indicates a failure—such as a broken wire or power loss—rather than just zero pressure.
Typical applications of a pressure transducer include:
- Industrial automation
- HVAC systems
- Hydraulic and pneumatic control
- Process monitoring in chemical plants
- Water treatment facilities
Working Principle of pressure transducer 4-20mA
The working principle of a flush mount pressure transducer 4-20mA involves three main stages. These are:
Sensing elements
Sensing elements are the game-changing tools that shape the pressure sensors. In the pressure transducer 4-20mA, there are three crucial types of sensors.
- Piezoresistive. In this type, there is a change in the resistance that incites the pressure changes in a strain gauge. Based on the change, there is an electrical signal that provides the data.
- Capacitive sensors are good at detecting pressure changes. They alter the distance between two plates, changing capacitance and interpreting the data.
- Resonant Wire or Crystal. If there is a pressure change, it affects the vibration frequency.
Signal conversion
Sensing elements aim at pressure sensing. The next step is the signal conversion. Here is how this occurs.
- Sensors detect the pressure changes through resistance or capacitance.
- This change is transmitted into the electrical signal.
- This electrical signal causes the voltage output in the system, providing the readings.
Current Loop Output
Since you are using the current output system, the voltage conversion is converted to current values. Current translation occurs between the 4mA and 20mA, ensuring a secure output of the system.
This change is proportional to the pressure changes. Suppose a transducer has a range of 0–100 psi.
- At 0 psi → 4 mA output
- At 50 psi → 12 mA output
- At 100 psi → 20 mA output
Instead of showing the values in voltage, why should you rely on the current? The logic is straightforward.
- The impact of electrical noise is minimal on the current signals.
- There are no more voltage drop errors due to increasing cable length.
- Many industrial devices interpret 4–20 mA loops.
Step-by-step Guide to Wiring a pressure transducer 4-20mA
If you want to get the ultimate pressure sensing job from your pressure sensors, wiring and power supply can help you with that.
Two-wire, three-wire, and four-wire connections are available. Let’s evaluate each connection.
2-wire connection
The 2-wire connection is the most common and standardized because of its ease and quick installation process. The chances of errors are less as two wires have two terminals.
These are:
- Positive wire (+) connects to the positive power supply terminal. It links with multiple devices and the signal input of the receiver (PLC, controller, display).
- Negative wire (–) connects back to the negative terminal of the power supply.
The voltage varies between 12 and 16V DC.
Once you have it, go ahead with the connection. Here are a few simple wiring steps.
- Step 1: Turn off the power supply
Safety is the first choice. To avoid accidents, you should determine if the power supply is connected. If it is, let’s turn it off immediately to ensure no current or voltage flows.
- Step 2: Connect Terminal
As we have already determined, there are two terminals. One is the positive terminal that should be connected to the positive terminal. You should connect the positive terminal to the loop power positive terminal.
The negative terminal connection is different. You should connect the negative terminals to the negative electrode of the measurement device and then to the negative terminal of the loop power.
- Step 3: Power up the system
Check the connections and polarity before you power up the system. Ensure the system is working.
3-wire connection
3-wire connection offers a separate route for the signal output from the power return. This configuration is ideal for the industrial sectors and provides better signal durability.
There are three wiring terminals.
- V+ (Power Positive). It connects the positive terminal of the DC power supply (typically 12–36 V DC).
- Signal Output. This terminal sends the 4–20 mA current to the PLC, display, or controller.
- V– (Power Negative) is a common terminal with a shared return path. It provides both power and signal transmission.
The wiring steps are easy. You can follow:
- Positive terminal connection. Connect V+ to the power supply positive terminal.
- Signal output terminal Connection. The signal output terminal to the signal input of your receiving device.
- Negative terminal connection. Connect V– to the power supply negative terminal and the signal input’s common/ground.
4-wire connection
A 4-wire connection has a separate route for the power and signal. It provides better signals than the 3-wire, but is used in specific industrial sensors.
Let’s take a look at the terminals.
- V+ (Power Positive): Positive DC power supply input.
- V– (Power Negative): Negative DC power supply input.
- Signal + (Output Positive): Positive side of the 4–20 mA loop signal.
- Signal – (Output Negative): Negative side of the 4–20 mA loop signal.
Testing a pressure transducer 4-20mA
Testing a pressure transducer 4-20mA is essential to confirm if it works in the specified pressure range and provides the optimal functionality. For that, you have to go through various steps and bring some tools.
We have listed the necessary tools and steps here.
Tools required
Here is the list of tools required to test a pressure transducer 4-20mA.
- Digital multimeter (DMM) to measure DC
- Loop calibrator. It is an optional tool but recommended for precise testing.
- Power supply (24 V DC is standard)
- Known pressure source (hand pump, deadweight tester, or process pressure)
Basic test procedure
Once you gather all the tools, it is time to test the pressure transducer 4-20mA with the following steps.
- Safety first.
Turn off any power source and depressurize the system before connecting or disconnecting to prevent any mishaps or accidents during the operation.
- Connection
You should bring your power supply and connect the transducer to the power supply from one terminal. On the second terminal, you can connect the measuring equipment that helps test the pressure transducer 4-20mA.
- Apply known pressures.
Testing at known pressure can help evaluate the range. Measurement tools grab the relevant values. Here are a few ranges of pressure. You have to alter the pressure between 0%, 25%, 50%, 75%, and 100% of the range.
- Measure current output at each step.
All you have to do is measure the current output at each step. Test each level separately and calculate it.
- Compare readings to expected mA values.
Once you have all the values, it is time to compare them and determine the range. Here is how you should go.
- 0 psi → 4.00 mA
- 25 psi → 8.00 mA
- 50 psi → 12.00 mA
- 75 psi → 16.00 mA
- 100 psi → 20.00 mA
Troubleshooting guide for the pressure transducer 4-20mA
| Symptom | Possible Cause | Solution |
| No Signal (0 mA) |
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| Output Stuck at 4 mA |
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| Output Stuck at 20 mA |
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| Fluctuating or Noisy Reading |
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| Incorrect Reading vs. Actual Pressure |
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Conclusion
Choosing a pressure sensor is challenging. It is because you have to look at the quality, grades, output, and several accuracy factors. Not all the pressure transducers are suitable for your choice. Even many vary based on the system.
Do you want to explore the top-class pressure sensors? Sendo-sensor offers a small pressure sensor SS311 that features a 4-20mA output and provides an accurate measurement of the pressure in the system. Check out the quality options now!