INSTART Instrumentation Course Chapter 5 focuses comprehensively on Level Measurement
- Tank stratification (if two level sensors show offset).
- Fouling of sensor (signal noise increases over time).
- Link latency (time delta between sensor read and cloud arrival).
5.2 Types of Level Instruments and Their Output Links
Every level instrument creates a physical or electronic link between the process fluid and the control system.
Capacitive Level Meters: Using a probe and the vessel wall as a capacitor; the material's dielectric constant determines the capacitance change as the level shifts. 3. Non-Contact Level Transmitters
Lab 5.1: Calibrating a DP Cell for Level
- Goal: Calculate lower range value (LRV) and upper range value (URV) given tank dimensions and fluid density.
- Task: Simulate wet leg compensation for a closed tank with condensable vapor.
Capacitive & Nuclear Meters: Methods for measuring level when materials are difficult to touch or see.
Review Questions
5.8 Best Practices for a Secure and Reliable Level Link
- Separate power and signal cables – High voltage motors induce noise in the level link.
- Use shielded twisted pair for 4-20 mA loops longer than 30 meters. Ground shield only at the InStart gateway side.
- Implement watchdog timers – InStart should set a "Level Link Health" boolean. If false, override the control loop to a safe state.
- Document the link path – Create a signal flow diagram: Sensor → Terminal strip → I/O module → Tag → PID → Actuator.
- Regular link validation – Every 3 months, perform a loop test by physically changing the level and verifying InStart and cloud values match.
- Raw value → e.g., 12,000 counts from a 16-bit ADC.
- Scaling: Lower Range Value (LRV) = 0 inches → 4 mA (3,276 counts); Upper Range Value (URV) = 120 inches → 20 mA (26,212 counts).
- Linearization formula:
( Level_in = \frac(Raw - 3276)(26212 - 3276) \times 120 ) - Filter link: Add a moving average (2 seconds) to ignore waves in a tank.
- Output link: Forward to: