Believe it or not, even in present day where everything from our personal lives to industrial processes is run by digital signals, there is still a place for analog pressure transmitters.
How a pressure transducer communicates with the rest of your facility’s systems is just as much of an efficiency and safety measure as it is an accuracy specification. In fact, there is a wide array of industries in which an analog 4 to 20 mA or 0 to 10 V output is preferred over their digital counterparts. Several factors go in to deciding whether an analog or digital precision transducer is best for your system.
Analog vs Digital Pressure Transducers
System compatibility
Digital transducers have many design choices in different communication protocols such as RS-232, RS-485, Modbus, Profibus, and Canbus, to name a few. With so many options, it's extremely easy for a digital transducer to be incompatible with your current system. On the other hand, analog transducers or digital transducers with analog output, either in current (4 to 20 mA) or voltage (0 to 5 or 10V), can start transmitting readings as soon as power is applied. Additionally, there is no need for further engineering to code, as it will communicate seamlessly with other instruments in a system.
Cost of installation
Digital transducer outputs such as RS-232 or RS-485, must be able to read command sets. This additional requirement will often call for programming to properly communicate with other devices before it can go into a production environment. While this can add a few different features to your system, it will also require additional infrastructure cost to ensure compatibility. Since analog devices use an electrical signal rather than a digital interface, they can be interpreted and translated by almost any device as soon as it is introduced into a system.
Remote and complex communication
Remote communication is one of the biggest strengths for digital output instruments wherever data collection is required. Modern data acquisition systems are based off widely spread, local transducers transmitting data to a central hub or cloud, like an industrial Internet of things network. As all this communication is digital, digital transducers are the preferred choice for this type of an industrial system.
With some of the above mentioned cases, both digital and analog pressure transducers have different applications in the real world. Unlike their digital counterparts, analog pressure transducers primarily come in two formats, a current output type and a voltage output type.
Current vs Voltage Output
Long distance communication
When it comes to transmitting information over a distance, current (4 to 20 mA) is the preferred analog interface. This is due to the fact that voltage outputs can be more susceptible to noise interference and the signal itself can degrade due to cable resistance. However, a current output can withstand long distances and deliver a complete and accurate pressure reading from transmitter to the data acquisition system.
Robustness to radio frequency interference
Cable runs are susceptible to electromagnetic (EMI) / radio frequency (RFI) / electrostatic (ESD) interference from adjacent cabling and wiring. This unwanted electrical noise can wreak havoc to a high impedance signal like the voltage signal. This is easily overcome by using a low impedance high current signal like the 4-20 mA.
Troubleshooting
The 4-20 mA signal has a 4 mA output for zero pressure value. This essentially means the signal has a "live zero," so it draws 4 mA of current even at zero pressure reading. This feature provides the user with a clear indication of an erroneous reading or signal loss if the signal drops to 0 mA. This practice can't be followed in the case of voltage signals, which typically range from 0-5 V or 0-10 V, where 0 V output represents zero pressure. Mensor's analog pressure transducer draws a 4-20 mA output across its entire pressure range.
Signal isolation
A 4-20 mA output signal is a low impedance current signal that can cause ground loops when connected to ground on both ends (transmission and receiving), which can then lead to inaccurate signal. Each 4-20 mA transducer line should be properly isolated to avoid this scenario. However, this does prevent the transducers from being daisy chained with a single cable infrastructure compared to a 0-10 V output.
Receiver accuracy
When transmitted from a pressure transducer, a 0-10 V signal can easily be interpreted on the receiving end by a voltmeter. With a 4-20 mA output, the signal can only be read with conversion to voltage at the receiver. In order to convert this signal to a voltage drop, a resistor is added in series to the output. This resistor accuracy is critical to the measurement accuracy of the received signal.
Analog pressure transducers form a critical part in a variety of applications. The choice between a 4-20 mA version or a 0-10 V version is determined by the need of the application. As a general rule of thumb, 4-20 mA output is ideal for long distances and reliable communication with just two cables, whereas a 0-10 V output is great for local environment and provides cost effective, fast and easy communication.
Related Reading:
- Analog Output from a Digital Pressure Transducer
- Serial Communication Interface: Differences Between RS-232, RS-485 and RS-422
- How to Zero Absolute Pressure Transducers and Sensors
- Long-term Stability in Silicon Pressure Transducers
- Calibration Terms: Differences in Sensor, Transducer and Transmitter
