Level Measurement in Closed Tanks with DP Transmitter

💧 Understanding Level Measurement in Closed Tanks
When you’re measuring liquid level in a closed or pressurized tank, it’s not just about the height of the liquid — the vapor pressure above it plays a crucial role.
Here’s the key:
In a differential pressure (DP) transmitter, the Low Pressure (LP) side is connected to the vapor space. This setup ensures that any vapor pressure in the tank is automatically compensated — so the transmitter only “feels” the true liquid head (ρgh).
✅ If the LP side is vented, changes in vapor pressure will directly affect the transmitter’s reading — leading to major level errors.
✅ If the LP side is connected properly, the vapor pressure cancels out, and the transmitter accurately measures only the liquid column.
🎯 In short:
“A properly connected LP line is the difference between an accurate level reading and a costly process error.”
This is a simple yet powerful concept every instrumentation engineer should master — especially during DP transmitter calibration and closed tank design.

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