1. The Physics of Thermal Drift

Remote seal systems use a closed volume of oil to transmit pressure. Liquids are generally incompressible, but they expand significantly with heat. In a capillary tube, this expansion has two effects:

• Density Change (Head Effect): As the fluid warms up, it becomes less dense ($\rho$ decreases). The weight of the vertical column of oil in the capillary decreases, reducing the hydrostatic pressure exerted on the transmitter. This causes a Zero Shift. This calculator focuses on this effect.
• Volume Expansion (Stiffness Effect): The expanding oil volume pushes against the diaphragm stiffness. This increases internal pressure. This is minimal in modern low-volume systems but significant in older designs.

The Density Effect is often dominant in tall towers or distillation columns where capillaries are long (e.g., 10m).

2. The Myth of "Balanced" Systems

A common design rule is to make both capillary legs the same length to "cancel out" errors. While this works for the Volume/Stiffness effect, it DOES NOT cancel out Head Effect errors unless the vertical elevation is also identical.

Example:
HP Leg: 1m drop. LP Leg: 5m drop.
Even if both capillaries are physically 5m long (coiled up), the vertical head is different (1m vs 5m).
If temp rises, the 5m leg loses 5x more weight than the 1m leg. The DP ($P_H - P_L$) will shift because the LP side gets lighter faster than the HP side.
Conclusion: Symmetrical capillaries do NOT eliminate density-based zero shift in level applications.

3. Choosing the Right Fill Fluid

The choice of fill fluid is a trade-off between temperature range, response time (viscosity), and expansion.

• Silicone DC200 (Standard): Good general purpose. Low viscosity, moderate expansion. Not for oxidizers (Chlorine/Oxygen).
• Syltherm 800 (High Temp): Designed for stability at 300°C+. Higher viscosity at ambient temps = slower response time.
• Halocarbon/Fluorolube: Inert. Required for Oxygen/Chlorine service to prevent explosion. extremely high density (SG ~1.9) and high cost.
• Neobee M-20: Food grade (vegetable based). Used in sanitary/hygienic applications. Limited temp range.

4. The Vacuum Hazard

In vacuum vessels (distillation columns), the weight of the fill fluid in the capillary pulls outward on the seal diaphragm. If the transmitter is mounted above the bottom tap, this "pull" combined with the process vacuum can drop the absolute pressure inside the seal below the fluid's vapor pressure.

Result: The fill fluid boils (cavitates), forming a bubble. The measurement becomes noisy and eventually fails.
Rule: In vacuum service, always mount the transmitter below the bottom tap so the hydrostatic head adds positive pressure to the seal.

5. Reducing Zero Shift

If the calculated error is too high (>2-3%):

• Minimize Vertical Run: Keep capillaries as short and horizontal as possible.
• Insulate/Trace: Heat trace the capillaries to maintain a constant temperature (e.g., 40°C) regardless of weather.
• Electronic DP: Use two independent pressure transmitters connected digitally (Electronic Remote Seal). This eliminates the capillaries entirely, removing the head error, though it introduces two sensor errors.