Why is it critical to select a pressure-based level sensor with a range that closely matches the maximum liquid depth, especially when measuring low levels?
Selecting a sensor with a measurement range that closely matches the application’s required depth is crucial for maximizing measurement resolution and accuracy. This is especially true for low-level applications. The principle is to use as much of the sensor’s analog output signal (e.g., 4-20 mA) as possible to represent the process variable.
Consider an application to measure a maximum level of 20 cm of diesel, which exerts about 17 mbar of pressure.
- Case 1: Poorly Matched Sensor. If a sensor with a range of 0-100 mbar (the lowest range for a typical 18.605G series) is used, the 17 mbar of pressure only utilizes 17% of the sensor’s full-scale capability. The corresponding change in the 4-20 mA output signal would be small, making it difficult for the control system to resolve small fluctuations in level.
- Case 2: Correctly Matched Sensor. If a sensor with a range of 0-40 mbar (such as the LMK382 series) is used, the 17 mbar of pressure utilizes about 42% of the sensor’s range. This provides a significantly larger change in the output signal for the same change in level, resulting in higher resolution and more precise process control.
Furthermore, sensor accuracy is typically specified as a percentage of full-scale output (% FSO). A higher-range sensor will have a larger potential error in absolute terms. For example, a 0.5% FSO accuracy on a 100 mbar sensor is ±0.5 mbar, which is a significant error relative to a 17 mbar measurement. In contrast, a 0.35% FSO accuracy on a 40 mbar sensor is ±0.14 mbar, providing a much more accurate reading at that specific low level.
By properly matching the sensor’s range to the application, engineers can ensure higher resolution, better accuracy, and more reliable system performance.
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Related Technical Terms
- Accuracy
- BSL – Best Straight Line
- Compensated Temperature Range
- Digital Compensation
- g Effect
- Hysteresis
- LHR – Linearity, Hysteresis and Repeatability
- Long Term Stability/Drift
- NL – Non-Linearity
- PPM – Parts Per Million
- Precision
- Pressure Hysteresis
- Repeatability
- RTE – Referred Temperature Error
- Secondary Pressure Standard
- TEB – Temperature Error Band
- TEB – Total Error Band
- Temperature Compensation
- Temperature Error
- Thermal Hysteresis
- Threshold
- TSL – Terminal Straight Line
- TSS – Thermal Span or Sensitivity Shift
- TZS – Thermal Zero Shift
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