Diaphragm Seal Fill Fluid Selection Guide
The fill fluid inside a diaphragm seal assembly plays a critical role in transferring process pressure from the diaphragm to the measuring instrument. Selecting the correct fill fluid is essential for maintaining accuracy, response time and long-term reliability, particularly where high temperature, low temperature, vacuum or hygienic service conditions are involved.
The guide below provides a comparison of PCI’s available fill fluids, including operating temperature, viscosity, specific gravity, thermal expansion and typical applications, to help identify a suitable option for each process condition.
Scroll across to view the full table.
| Fill Fluid | Operating Temperature (°C) | Viscosity | Specific Gravity | Thermal Expansion Coefficient | Description |
|---|---|---|---|---|---|
| DC200 | -45 to +205 | 20 cSt @ 25°C | 0.95 @ 25°C | 0.00107 /°C | General-purpose silicone fill fluid for standard industrial applications. Vacuum applications up to +100°C. |
| DC550 | -40 to +300 | 125 cSt @ 25°C | 1.07 @ 25°C | 0.00076 /°C | Silicone fill fluid for wider temperature capability. |
| DC704 | 0 to +315 | 39 cSt @ 25°C | 1.07 @ 25°C | 0.00095 /°C | High-temperature silicone fill fluid for vacuum-related service. Vacuum applications up to +200°C. |
| DC705 | +20 to +370 | 175 cSt @ 25°C | 1.09 @ 25°C | 0.00077 /°C | High-temperature silicone fill fluid for demanding vacuum-related service. Vacuum applications up to +250°C. |
| Fomblin Y06 | -40 to +230 | 71 cSt @ 25°C | 1.88 @ 25°C | 0.00086 /°C | Highly inert fill fluid for specialist inert-fluid applications. |
| Halocarbon 1.8 | -79 to +100 | 1.8 cSt @ 25°C | 1.82 @ 25°C | 0.00084 /°C | Low-viscosity inert fill fluid for low-temperature service and applications where silicone-based fluids are not preferred. |
| Halocarbon 4.2 | -45 to +160 | 6.5 cSt @ 25°C | 1.85 @ 25°C | 0.000864 /°C | Inert fill fluid suitable for oxygen service and applications where silicone-based fluids are not preferred. |
| Halocarbon 6.3 | -40 to +200 | 6.3 cSt @ 25°C | 1.87 @ 25°C | 0.00084 /°C | Standard inert fill fluid suitable for oxygen service and applications where silicone-based fluids are not preferred. |
| Neobee M20 | -15 to +225 | 9.6 cSt @ 25°C | 0.94 @ 25°C | 0.001008 /°C | FDA food and beverage fill fluid. Suitable for hygienic applications; response time should be checked for colder applications due to increased viscosity. |
| Syltherm 800 | -40 to +400 | 9.1 cSt @ 25°C | 0.93 @ 25°C | 0.00136 /°C | Very high-temperature silicone fill fluid for non-vacuum service. Not recommended for vacuum service or at high temperatures under low static pressure. |
| Syltherm® XLT | -100 to +150 | 1.4 cSt @ 25°C | 0.85 @ 25°C | 0.001198 /°C | Low-temperature silicone fill fluid with very low viscosity. Vacuum or low absolute pressure applications must be checked against the vapour-pressure curve. |
Important note: Operating temperature should not be used as the only selection criterion. Final fill fluid suitability depends on process pressure, vacuum conditions, instrument configuration, diaphragm seal design, capillary length and response-time requirements. For low absolute pressure or vacuum service, vapour pressure should be checked separately.
Trademark notice: DOWSIL, DC200, DC550, DC704 and DC705 are trademarks of The Dow Chemical Company or its affiliates. SYLTHERM is a trademark of The Dow Chemical Company or its affiliates. Halocarbon is a trademark of Halocarbon Products Corporation. Fomblin is a registered trademark of Solvay Specialty Polymers Italy S.p.A. NEOBEE is a registered trademark of Stepan Company. All other trademarks are the property of their respective owners.
