Process Instrumentation
Pressure Transmitter Temperature Effect in a Remote Seal System and Compensation
A the system isolates a pressure transmitter from the process fluid using a flexible diaphragm and a capillary tube filled with hydraulic fill fluid.
This protects the transmitter from corrosive, viscous, or high-temperature process fluids. However, the capillary fill fluid expands with ambient temperature, causing a significant measurement error.
This guide covers how a this seal system works, why the temperature effect occurs, how to calculate it, and what engineering choices reduce it.
Capillary Fill Fluid
Zero Shift and Span Error
Silicone vs Glycerine Fill
Equal Capillary Lengths
In a remote seal system, the capillary fill fluid changes volume with temperature. This volume change applies a pressure to the transmitter sensing element that does not come from the process. The result is a false reading that tracks ambient temperature, not process pressure.
How the Remote Seal System Works
Hello! Today we are going through the pressure transmitter with a remote seal system: how the diaphragm seal and capillary work, why temperature causes a measurement error, and how engineers design around it. This is one of the most common sources of unexplained zero drift and reading error in process plants, and understanding it well separates a good instrument engineer from one who simply replaces parts.
A pressure transmitter normally connects directly to the process via an impulse line. When the process fluid is corrosive, viscous, or prone to plugging, a direct impulse line is not practical.
A remote seal system solves this with a diaphragm seal at the process connection and a capillary tube filled with hydraulic fill fluid linking it to the transmitter. Click any term to expand.
Diaphragm Seal: A thin, flexible metal diaphragm (typically 316 SS, Hastelloy C276, or tantalum, depending on process fluid compatibility) is welded or clamped to the process connection. Process pressure deflects the diaphragm, and this deflection is transmitted hydraulically through the fill fluid to the transmitter sensing element. The diaphragm seal is the wetted part in contact with the process; the transmitter itself is fully isolated from the process chemistry.
Capillary and Fill Fluid: The capillary is a small-bore (typically 1.5 to 2.5 mm inner diameter), stainless-steel-armoured tube that connects the diaphragm seal to the transmitter. The capillary and the cavity behind the diaphragm are completely filled with a hydraulic fluid (silicone oil, glycerine/water mixture, or other fill fluids depending on temperature range). This fluid transmits the pressure from the diaphragm to the transmitter's sensing element with no air gaps. The fill fluid is chosen based on the process temperature, the ambient temperature range, and the required viscosity.
0.4%/10°C
Typical volume expansion of silicone DC200 fill fluid per 10°C temperature rise
±5 mbar
Typical temperature-induced zero error for a 5-metre capillary on silicone fill per 10°C change
Equal lengths
On DP systems with remote seals, equal capillary lengths on high and low sides cancel the temperature error
Glycerine
Lowest thermal expansion coefficient of common fill fluids: preferred for minimising remote seal temperature effect
Why Temperature Causes a Remote Seal Measurement Error
Fill fluid expands when temperature rises. In a sealed system, this expansion pushes against the transmitter sensing diaphragm, producing a pressure signal indistinguishable from a real process change.
Error magnitude depends on three factors: the thermal expansion coefficient of the fill fluid, the total fill fluid volume (capillary length and diameter), and the stiffness of the sensing element.
A stiffer element resists deflection, so more expansion appears as pressure error. A more compliant element absorbs some expansion, reducing the error.
Remote Seal Temperature Error Formula
ΔP_error = (β × V_cap × ΔT) / C_element
ΔP_error: temperature-induced pressure error (Pa or mbar)
β: volumetric thermal expansion coefficient of fill fluid (per °C)
V_cap: total fill fluid volume in the capillary and seal cavity (cm³)
ΔT: temperature change (°C)
C_element: compliance of the transmitter sensing element (cm³/Pa)
Practical approximation used in engineering:
ΔP_error ≈ β × ΔT × (capillary length × capillary cross-section area) × (1 / element stiffness factor)
For silicone DC200 fill, β ≈ 0.00096 per °C.
A 5 m capillary at 2 mm ID: V_cap ≈ 5 × π/4 × (0.002)² × 100 = 1.57 cm³
For ΔT = 10°C: volume change = 0.00096 × 1.57 × 10 = 0.0151 cm³
This volume change against a typical stiff element produces roughly 3 to 8 mbar of zero error.
β: volumetric thermal expansion coefficient of fill fluid (per °C)
V_cap: total fill fluid volume in the capillary and seal cavity (cm³)
ΔT: temperature change (°C)
C_element: compliance of the transmitter sensing element (cm³/Pa)
Practical approximation used in engineering:
ΔP_error ≈ β × ΔT × (capillary length × capillary cross-section area) × (1 / element stiffness factor)
For silicone DC200 fill, β ≈ 0.00096 per °C.
A 5 m capillary at 2 mm ID: V_cap ≈ 5 × π/4 × (0.002)² × 100 = 1.57 cm³
For ΔT = 10°C: volume change = 0.00096 × 1.57 × 10 = 0.0151 cm³
This volume change against a typical stiff element produces roughly 3 to 8 mbar of zero error.
The temperature error acts primarily as a zero shift: it offsets the reading up or down depending on whether temperature rises or falls. It can also produce a small span error if the high and low capillary sides of a DP transmitter are at different temperatures. See the pressure transmitter zero shift guide and the span drift guide for how these errors are characterised in transmitter specifications.
Fill Fluid Selection for Capillary Systems
| Fill Fluid | Thermal Expansion Coefficient (β) | Temperature Range | Best For |
|---|---|---|---|
| Silicone DC200 (standard) | 0.00096 per °C | minus 40°C to 200°C | General service, most process temperatures. Most widely used remote seal fill fluid. |
| Glycerine/water (80/20) | 0.00045 per °C | minus 10°C to 120°C | Low temperature error applications. Preferred when ambient temperature variation is large. Not suitable below minus 10°C (freezing point). |
| Silicone DC704 (high temp) | 0.00078 per °C | minus 40°C to 315°C | High-temperature process connections above 200°C. Steam service, reactor hot-side seals. |
| Halocarbon (Fluorolube) | 0.00080 per °C | minus 18°C to 175°C | Oxygen service where silicone fill is prohibited due to combustion risk. |
| Propylene glycol/water | 0.00050 per °C | minus 45°C to 150°C | Cryogenic and low-temperature service. Lower expansion than silicone. |
| Food-grade silicone (H1) | 0.00095 per °C | minus 40°C to 200°C | Food, pharmaceutical, and potable water applications where fluid contact with product is possible. |
Glycerine/water fill has the lowest thermal expansion of common fill fluids. For applications with wide ambient temperature swings (outdoor installations, uninsulated piping, seasonal temperature variation of 30°C or more), specifying glycerine/water fill instead of silicone can reduce the capillary temperature error by a factor of two or more. The trade-off is a higher minimum temperature limit and slightly higher viscosity at low temperatures (which increases response time).
How to Reduce the Temperature Error
Four practical approaches reduce the temperature-induced error in a remote seal system. In practice, multiple approaches are combined for the most demanding applications.
| Approach | How It Reduces the Error | Limitation |
|---|---|---|
| Use equal capillary lengths (DP systems) | If both high-side and low-side capillaries are identical in length, diameter, and fill fluid, and at the same ambient temperature, their thermal expansions cancel at the DP transmitter. Zero error is eliminated. | Only works for differential pressure systems. Does not help gauge or absolute remote seal transmitters. Requires the two capillaries to experience the same ambient temperature. |
| Select low-expansion fill fluid | Glycerine/water or propylene glycol fill has roughly half the thermal expansion of silicone. Directly reduces ΔP_error by the ratio of β values. | Narrower operating temperature range. Higher viscosity at low temperatures slows system response. |
| Minimise capillary length and volume | Shorter capillary = less fill fluid volume = less volume change per degree. Reducing capillary from 10 m to 3 m cuts the error by 70%. | Not always possible due to installation geometry. Requires transmitter to be mounted close to the process. |
| Mount transmitter in a temperature-stable location | If the transmitter and capillary are in a climate-controlled room or shaded instrument enclosure, ΔT is small. Error is proportional to ΔT, so minimising temperature variation directly minimises error. | Requires longer capillary to reach a stable-temperature location, which conflicts with minimising capillary length. |
Remote Seal Temperature Error Calculator
Capillary Temperature Error Estimator
Estimate zero shift error from fill fluid thermal expansion
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Watch: Remote Seal DP Level Transmitter with Capillary: Full Guide
Remote Seal System Questions
What is a remote seal system on a pressure transmitter?
A remote seal is a flexible diaphragm at the process connection, connected to the transmitter by a capillary filled with hydraulic fluid. It isolates the transmitter from the process while transmitting process pressure hydraulically.
Why does a remote seal cause temperature measurement error?
The capillary fill fluid expands with ambient temperature. In a sealed system, this volume change pushes against the sensing element, producing a false pressure reading that tracks temperature rather than process pressure.
How do you minimise the temperature effect on a remote seal system?
Use equal capillary lengths on DP systems so errors cancel, choose low-expansion fill fluid (glycerine/water), minimise capillary length, and mount the transmitter in a temperature-stable location.
Which fill fluid has the lowest thermal expansion for remote seals?
Glycerine/water (80/20) has β ≈ 0.00045 per °C, roughly half that of silicone DC200. It is preferred for outdoor installations and applications with large ambient temperature swings.
Why must the capillary lengths be equal on a DP remote seal level system?
Equal capillary lengths ensure identical volume changes on both sides of a DP transmitter, so the errors cancel at the differential element. Unequal lengths leave a residual zero error proportional to the length difference. See the DP transmitter guide.
External References
- Remote Seals: Beyond the Basics | Emerson Rosemount (Technical Article)
- Diaphragm Seal Systems: Fill Fluids and Temperature Effects | WIKA
What We Learn Today
- A remote seal system isolates a pressure transmitter from the process using a flexible diaphragm and a capillary filled with hydraulic fill fluid. The transmitter never contacts the process fluid.
- Temperature-induced error occurs because fill fluid expands with temperature and pushes against the transmitter sensing element, creating a false pressure signal (zero shift).
- Error magnitude depends on fill fluid thermal expansion coefficient (β), capillary volume, temperature change (ΔT), and element stiffness.
- For DP remote seal systems: use equal capillary lengths on both sides so temperature errors cancel at the differential element. This is the most effective single compensation technique.
- Lowest thermal expansion fill fluids: glycerine/water (β ≈ 0.00045 per °C) and propylene glycol/water (β ≈ 0.00050 per °C). Standard silicone DC200: β ≈ 0.00096 per °C.
- Additional error reduction: minimise capillary length, mount the transmitter in a temperature-stable location, and combine fill fluid selection with equal capillary lengths for demanding applications.
“A sealing system that saves the transmitter from a corrosive process can create a new problem: a reading that drifts with the weather. Equal capillary lengths and the right fill fluid are the two answers most engineers reach for first.”