Basic Measurement Concepts
Measuring and operating conditions for transmitter
We use the term transmitter or instrument for equipment that converts a measurement variable into a measurement signal. Transmitters can be divided into two concepts:
- Concepts related to signal and measurement range
- Concepts related to accuracy
1. Concepts related to signal and measurement range
Here's a description of several basic concepts you should know. A diagram illustrating these concepts follows below.- Process Value (PV): This is the actual measurement being read by the instrument in real-time. It's the live reading that shows what's happening in the process right now. This value will be moving accordingly to the process.
- Lower Range Limit (LRL): This is the absolute lowest value that the instrument can physically measure. It's a hardware limitation built into the device that cannot be changed. This is the lowest measurement variable the instrument can be adjusted to measure.
- Upper Range Limit (URL): This is the absolute highest value that the instrument can physically measure. Like the LRL, it's a hardware limitation that's built into the device and cannot be changed. This is the highest measurement variable the instrument can be adjusted to measure.
- Lower Range Value (LRV): This is the lowest value of the configured measurement range. Users can set this value anywhere between the LRL and URL. It typically corresponds to 0% of the output signal (often 4mA in a 4-20mA system).
- Upper Range Value (URV): This is the highest value of the configured measurement range. Users can set this value anywhere between the LRL and URL. It typically corresponds to 100% of the output signal (often 20mA in a 4-20mA system).
- Range: This is the span of values between the LRV and URV. It represents the configured working range of the instrument where it provides its specified accuracy.
- Span: This is the mathematical difference between URV and LRV. For example, if LRV is 20 PSI and URV is 100 PSI, the span is 80 PSI. It's same as the calibrated measurement range.
- ZERO: This is a reference point that can be set anywhere within the configured range. It doesn't necessarily have to be at the LRV or at actual zero. It's used as a calibration reference point.
- Overrange: This is the condition when the Process Value exceeds the URV but remains below the URL. The instrument continues to measure, but may have reduced accuracy.
- Overrange Limit: This is the maximum safe value beyond the URV that the instrument can measure without damage. Exceeding this limit risks damaging the instrument.
Practical Example 1:
- Consider a temperature transmitter:
- Physical limits: LRL = -50°C, URL = +250°C
- Configured range: LRV = -20°C, URV = +200°C
- Span: 220°C
- ZERO might be set at +20°C
- Overrange occurs between 200°C and 250°C
- Overrange Limit might be set at 230°C for safety