Process instrumentation is an integral part of industrial automation and process control systems. This guide explains key terminology in process instrumentation, complete with detailed explanations and practical examples. The goal is to help engineers, technicians, and professionals understand and apply these concepts effectively.
1. Range
The algebraic difference between the upper and lower limits of a specified measurement range.
Example: A temperature sensor with a measurement range of -50°C to 150°C has a range of 200°C.
2. Output Signal
The form in which an instrument transmits the measured value to other systems. Common types include current (e.g., 4-20 mA), voltage (e.g., 0-10 V), or digital protocols (e.g., Modbus).
Example: A pressure transmitter outputs a 4-20 mA signal to indicate the measured pressure.
3. Response Time
The time taken for an instrument to react to a change in input and stabilize at a new output value.
Example: A temperature sensor may take 1 second to reflect a new temperature change in its output.
4. Step Response Time
The time required for an instrument to stabilize its output after a sudden, large change in input value.
Example: When pressure is abruptly increased from 10 bar to 20 bar, the transmitter stabilizes its output in 0.5 seconds.
5. Time Constant
Represents how quickly a system responds to input changes, typically defined as the time to reach 63.2% of the final value after a step input.
Example: A temperature sensor with a time constant of 2 seconds will reach 63.2°C when the input changes from 0°C to 100°C within 2 seconds.
6. Reverse Polarity Protection
A feature that prevents damage to an instrument when the power supply’s positive and negative terminals are accidentally reversed.
Example: A transmitter remains functional even if the power supply connections are reversed.
7. Turndown Ratio
The ratio of the maximum measurement range to the minimum measurable range.
Example: A sensor with a range of 0-100 bar and a turndown ratio of 10:1 can measure as low as 0-10 bar.
8. Long-term Stability
The ability of an instrument to maintain its accuracy over an extended period.
Example: A pressure transmitter retains an accuracy of ±0.1% even after 5 years of operation.
9. Process Temperature
The temperature range of the medium (liquid or gas) in direct contact with the instrument.
Example: A flow meter measuring hot water may have a process temperature of up to 90°C.
10. Ambient Conditions
Environmental factors such as temperature, humidity, and vibration that affect instrument operation.
Example: The ambient conditions for a device might range from -20°C to 50°C with up to 80% relative humidity.
11. Storage Temperature
The temperature range in which an instrument can be safely stored without operation.
Example: A device can withstand storage conditions as low as -40°C in a warehouse.
12. Climatic Class
Defines an instrument’s ability to operate under specific climatic conditions, often dictated by standards.
Example: A sensor with an industrial-grade climatic class is suitable for humid environments.
13. IP Rating (Ingress Protection)
Indicates the level of protection against dust and water, represented as “IPxx.”
Example: An IP68-rated sensor is fully dustproof and can be submerged in water indefinitely.
14. Electromagnetic Compatibility (EMC)
The ability of an instrument to operate without being affected by electromagnetic interference (EMI) and without emitting interference itself.
Example: A factory-installed transmitter resists EMI from nearby motors.
15. Process Connection
The interface type that connects the instrument to the process equipment, such as threads or flanges.
Example: A level transmitter may use a flange connection to mount onto a storage tank.
16. Electrical Connection
Specifies how the instrument connects to electrical systems, such as via terminal blocks or connectors.
Example: An M12 connector is commonly used for electrical connections.
17. Ripple
Small, periodic variations or noise in the output signal.
Example: A 4-20 mA output signal may have a ripple of ±0.1 mA.
18. Noise
Unwanted signal disturbances that can obscure accurate measurement.
Example: Electromagnetic interference might cause fluctuation in a sensor’s output signal.
19. NAMUR
An international organization that sets standards for instrumentation interfaces and usage.
Example: NAMUR-compliant sensors integrate seamlessly with PLCs.
20. Low Flow Cut-off
Filters out signals below a set threshold to avoid interpreting insignificant measurements.
Example: A flow meter disregards flow rates below 1 L/h.
21. Start-up Time
The time required for an instrument to become fully operational after being powered on.
Example: A sensor may take 3 seconds to begin transmitting accurate data after startup.
22. Excitation Frequency
The electrical frequency used to drive a sensor.
Example: An electromagnetic flow meter might operate at an excitation frequency of 50 Hz.
23. Mechanical Load
The maximum external force an instrument can withstand, such as pressure or vibration.
Example: A sensor installed on a vibrating platform can tolerate up to 20 N of vibrational force.
24. Power Consumption
The amount of power an instrument consumes during operation.
Example: A pressure transmitter may consume 1.5 W of power.
25. Explosion-proof Certification
Certification indicating an instrument’s safe operation in explosive atmospheres.
Example: ATEX-certified transmitters can be used on offshore oil platforms.
26. Pollution Degree
Specifies the suitability of equipment for different pollution environments.
Example: Devices with Pollution Degree 3 are suitable for industrial environments.
27. Repeatability
The consistency of an instrument’s output under identical conditions.
Example: A temperature sensor repeatedly measures 99°C for the same input.
28. Non-repeatability
The inconsistency in an instrument’s output for the same input, expressed as error.
Example: Measurements of the same temperature vary by ±0.5°C.
29. Write Protection
Prevents accidental changes to configuration settings.
Example: A pressure transmitter with write protection enabled prevents unauthorized parameter adjustments.
30. Signal-to-Noise Ratio (SNR)
The ratio of signal strength to noise level, indicating signal quality.
Example: High SNR indicates precise measurements with minimal noise interference.
31. Signal Dynamic Range
The range between the smallest and largest signals an instrument can process effectively.
Example: A sensor with a wide dynamic range can detect both weak and strong signals accurately.
32. Load Stability
The stability of the output signal when the load conditions change.
Example: A pressure transmitter’s output remains stable despite variations in connected loads.
33. Unipolar and Bipolar Signals
Unipolar: Signals with only positive or negative values.
Example: 0-10 V signals.
Bipolar: Signals with both positive and negative values.
Example: -10 V to 10 V signals.
This glossary aims to serve as a comprehensive reference for process instrumentation terminology. By understanding these concepts, professionals can better design, operate, and troubleshoot industrial systems.