4–20mA data logger

A 4-20 mA data logger is a data logger equipped with a standardised current interface (inputs and outputs). 4-20 mA data loggers are used in environments where high interference levels prevent precise and safe transmission of measured values. When transmitting measured values by cable over longer distances, 4-20 mA data loggers are also a reliable alternative to data loggers with high-impedance voltage inputs. This article explains what the benefits of the 4-20 mA interface are.

Interference-free signal transmission is key in measurement technology. Analogue data transmission, common in data loggers, transmits measured value signals with the smallest of voltages or currents. Our increasingly "electrified" environment contains ever more active interference factors that can falsify measured values:

• Powerful digital systems with high transmission frequencies
• Electrically operated components, for example drive motors
• Mains-powered contactors
• Electrostatic discharges
• Wireless control and communication equipment

When physical values are measured with sensors, data loggers generally process electrical analog signals. "Analog" here refers to electrical signals that can take on any value within a minimum and maximum. The signals are transmitted from the source - the sensor - via electrical conductors to the target device - the data logger. According to DIN IEC 60381-1, these electrical signals are standardised and referred to as standard signals:

• 0 mA - 20 mA current signal
• 4 mA - 20 mA current signal (offset zero, with offset zero point)

A standard is also defined for voltage signals. DIN IEC 60381-2 lists the following voltage signals:

• <0V - 5V
• <0V - 10V
• 1V - 5V (offset zero point)
• 2V - 10V (offset zero point)
• -10V - 10V (optionally with or without offset zero point)

With a 4 - 20 mA interface on a data logger, measured values can easily be transmitted over greater distances as current signals. Current transmissions make internal resistances and thus voltage drops in the cable irrelevant as long as voltages remain high enough. The current in measured values is greater than the current in interference which therefore ensures precise, interference-free transmission of measured values.

For physical values such as temperatures, humidity or pressures, to be processed by signal conditioning instruments, they must be converted into standardised electrical signals. These signals can be transmitted either as current or voltage signals.

The vast majority of applications use current rather than voltage signals. Current signals have the advantage of being unaffected by electromagnetic interference. When using current signals there is also no problem of voltage loss caused by line resistance.

Signals with an offset zero point, e.g. 4-20 mA, are mainly used in industrial environments. An electrical signal other than zero is assigned to the measurement range start. Using an offset zero point has two advantages:

1) Offset zero point (zero point=4 mA) enables cable break monitoring tasks to be realised. When a signal is present that is below the zero point (e.g. 3 mA), a fault or cable break can be safely assumed.

2) The current unit signal 4-20 mA provides the signal circuit with a continuous power supply. The available energy can then be used by transmitters for their own energy supply. 2-wire connections to the sensor are then sufficient.

Data loggers that process standard signals allow flexibility in using sensors in practical applications. Delphin data loggers can be used with the following external sensors as well as others:

• Temperature sensors
• Thermocouples
• Humidity sensors
• Vibration sensors
• Force sensors
• Water level sensors
• Pressure sensors
• Flow sensors
• Any type of external analog sensor

When using data loggers with analogue inputs, e.g. 4-20 mA data loggers, measurement results may be distorted by earth potential differences. Delphin Technology therefore incorporates galvanic isolation in all its data loggers. Galvanic isolation means that electrical potentials remain permanently isolated from each other and circuits thus remain practically potential-free. Sensitive hardware is thereby protected against overvoltage - and measurement results are always reliable.

Analog signals can be acquired not just by data loggers, but also by controllers and PLCs. For higher resolution signal sampling and significantly improved measurement speeds, data loggers such as the Delphin Expert Logger are ideal additions to existing systems. Sampling rates and speeds are particularly important in advanced measurement tasks, such as vibration measurements. We will be happy to provide you with comprehensive advice on the many options available with Delphin data loggers. Call us now to find out more.

Several factors require consideration when selecting a suitable 4-20 mA data logger. In addition to the key criteria of sampling rate and speed - which need to match the measurement task exactly - the following factors also need to be considered:

• How does the data logger ensure long-term measurement reliability?
• Does the data logger provide galvanic isolation to protect against overvoltage and ensure reliable, high-precision measurement results?
• Is the number of available analog inputs and outputs sufficient for the measurement tasks?

Costs per analog input are often the strongest argument for or against a 4-20 mA data logger. Delphin Technolgy provides an answer here with flexible scalability of available analog outputs and inputs. Delphin 4-20 mA data loggers can be extended as required with add-on devices. Sufficient connection options for sensors are therefore always possible, even for highly complex measuring tasks. The system is complemented by the Delphin Data Center database and DAQ measurement data software. The Delphin Data Center can be used to record large numbers of analog signals from multiple data loggers throughout a measurement network, as well as from third-party hardware such as a PLC control system. Large amounts of measurement data can therefore also be acquired decentrally without having to route data over long cables to a single data logger.

Do you want to find out more about Delphin data loggers or do you have a specific measurement task that you would like to discuss with our experts? Then call us. We look forward to the opportunity to impress you with our Delphin products.