An SF6-insulated switchgear system is to be refilled after maintenance. The system documentation specifies, for example, a defined filling pressure referenced to 20 °C. During filling, however, the ambient temperature is only 8 °C and the connected pressure gauge shows a lower pressure than the specified target value. Should more SF6 now be added until the pressure gauge shows exactly the specified value?
This is where an important risk of confusion arises in practice. The directly measured gas pressure is temperature-dependent. If SF6 is heated while the gas quantity remains constant, the pressure increases. If the gas cools down, the pressure decreases. The actual gas quantity or gas density present in the closed gas compartment can remain unchanged.
For this reason, the filling condition of SF6 switchgear is often referenced to a defined reference temperature – typically 20 °C. A gas density monitor compensates for the influence of temperature and displays a temperature-compensated condition. A conventional pressure gauge, by contrast, generally shows the current pressure under the present conditions.
When filling an SF6 system, three parameters must therefore be clearly distinguished: the actual gas pressure currently present, the current gas temperature and the resulting or temperature-compensated filling condition. Anyone who directly compares a target value referenced to 20 °C with a conventional pressure gauge without temperature correction can fill the system incorrectly.
Why does SF6 gas pressure change with temperature?
The pressure of an enclosed gas depends, among other things, on temperature, gas quantity and volume. If a closed SF6 gas compartment is heated, the pressure increases even though no additional gas has been added. If the temperature falls, the pressure decreases accordingly.
In switchgear, this means that a different raw pressure may be measured in the morning at a low outdoor temperature than in the afternoon, even though the system is completely tight. A simple pressure measurement cannot distinguish this temperature effect from an actual loss of gas.
This relationship is particularly important during filling. If a pressure measured at a low temperature is compared directly with a target value specified for 20 °C without considering temperature, an apparent deficit can arise even though the correct quantity of gas is already present.
What is the difference between gas pressure and gas density?
Gas pressure describes the mechanical pressure effect of the gas under the conditions currently present. Gas density, by contrast, describes in simplified terms the gas mass present in relation to the volume of the gas compartment.
| Parameter | Dependence on temperature | Meaning for SF6 systems |
|---|---|---|
| Current gas pressure | Changes significantly with gas temperature | Current physical pressure in the gas compartment |
| Gas temperature | Changes with ambient conditions and operating state | Required for evaluating the gas condition |
| Gas density | Much less temperature-dependent in a closed, constant-volume gas compartment | Measure of the actual gas filling condition |
| Pressure referenced to 20 °C | Temperature-compensated reference value | Facilitates standardized assessment of the filling condition |
For the operational safety of gas-insulated switchgear, the key factor is that the required gas quantity or gas density is present. This is why gas density monitors and gas density sensors are used for continuous monitoring.
What does “filling pressure referenced to 20 °C” mean?
An SF6 filling pressure stated in the technical documentation is often referenced to a temperature of 20 °C. This value describes the gas condition that the system would have at 20 °C.
It does not automatically mean that a conventional pressure gauge must display exactly the same pressure at every actual gas temperature.
If the gas temperature is significantly below 20 °C, the pressure currently measured will be lower for the same quantity of gas. At a temperature above 20 °C, it will be correspondingly higher.
Before filling, it must therefore be clearly established what the value in the system documentation refers to:
- current pressure at a defined temperature,
- pressure compensated to 20 °C,
- gas density,
- absolute or gauge pressure,
- or a manufacturer-specific filling specification.
What actually needs to be set during filling?
The filling condition specified by the equipment manufacturer and the prescribed procedure are always decisive. If a filling specification referenced to 20 °C is provided, the actual gas temperature must be taken into account during filling.
For this purpose, a pressure-temperature table provided by the manufacturer, an appropriate calculation method or a suitable temperature-compensated measuring system can be used.
A general procedure such as “fill gas until the conventional pressure gauge reaches the pressure stated on the nameplate” is not sufficient, particularly when the actual temperature differs significantly from 20 °C.
The specific gas type must also be taken into account. Pressure-temperature relationships for pure SF6 must not be applied to alternative insulating gases or gas mixtures without verification.
What does a gas density monitor display?
A gas density monitor is not a conventional process pressure gauge. Its measuring system is designed to compensate for pressure changes caused by temperature.
The WIKA GDM-100, for example, displays pressure referenced to a temperature of 20 °C. If the ambient temperature falls, the actual physical pressure in the gas compartment decreases, but the display of the gas density monitor should remain largely stable if the gas quantity remains unchanged.
If the quantity of gas actually decreases, however, the temperature-compensated value also decreases. Integrated switching contacts can then be used to generate warning and alarm signals, for example.
This makes a gas density monitor much better suited to long-term filling-condition or leakage monitoring than a conventional pressure gauge.
Why can a conventional pressure gauge show a different value?
During service and filling work, an additional pressure gauge is often used in the filling line or on the service equipment. This measuring instrument records the current pressure.
At the same time, a gas density monitor may display a value compensated to 20 °C. The two instruments can therefore show different numerical values while both are operating correctly.
| Measuring instrument | Typical measured value | Temperature compensation |
|---|---|---|
| Conventional pressure gauge | Current gas pressure | No |
| Reference pressure sensor | Current gas pressure | Normally no |
| Gas density monitor | Temperature-compensated gas condition or pressure referenced to a reference temperature | Yes |
| Electronic gas density sensor | Gas density or compensated values derived from it | Yes |
Before troubleshooting, it must therefore always be clarified which physical parameter the respective display actually represents.
Which temperature is relevant for assessment?
The actual temperature of the SF6 is relevant for assessing the gas condition. Particularly during a rapid filling process, this does not necessarily correspond exactly to the measured ambient temperature.
As the gas flows in and pressure changes occur, the gas can undergo thermal changes. At the same time, gas vessels, pipelines and switchgear have considerable thermal mass. After filling, some time may therefore be required before a largely stable temperature condition is reached.
For precise filling, the procedure specified by the equipment or device manufacturer for temperature measurement and stabilization should therefore be followed.
Why should the gas be allowed to stabilize after filling?
During a rapid filling process, the measured pressure may continue to change even after no additional gas mass is being introduced. This can be caused by temperature equalization and pressure distribution within the gas compartment.
If a final value is read immediately after the filling valve is closed and assessed without allowing for stabilization, the later equilibrium condition may differ from this value.
Particularly with larger gas compartments, sufficient time should therefore be allowed for pressure and temperature equalization, after which the final filling condition should be checked again.
How long this stabilization takes depends on the system volume, temperature difference, filling rate and design.
How can overfilling occur?
A typical misinterpretation can occur at low ambient temperature. The current pressure is below the nominal value referenced to 20 °C because the gas filling is cold.
If SF6 is now added until a conventional pressure gauge shows exactly the numerical value specified for 20 °C, too much gas may be present in the gas compartment.
If the switchgear later warms up, the actual pressure rises accordingly. In addition to the intended filling condition, the permissible operating and design pressures must therefore also be taken into account.
The filling process should therefore not be based on a single uncompensated pressure value unless that value is explicitly specified for the current temperature.
How can underfilling occur?
The opposite misinterpretation can occur at a high gas temperature. A warm gas compartment has a higher pressure for the same gas quantity.
If only the current pressure is checked to see whether the desired value appears to have been reached, the actual gas filling condition may still be too low. When the gas later cools down, the pressure drops significantly and a gas density monitor may reach a warning level.
This again demonstrates why a temperature-compensated reference condition is much more meaningful for filling than a single pressure value without temperature reference.
What role do moisture and gas quality play?
The correct pressure or gas density alone does not confirm that the gas filling is of suitable quality. During SF6 service work, moisture, purity and possible decomposition products are also relevant parameters.
Excessive moisture can impair the electrical properties of the insulation system and, under unfavorable conditions, contribute to the formation of unwanted reaction products.
Depending on the requirements of the system, additional SF6 analysis instruments are therefore used during maintenance to check parameters such as moisture, purity and decomposition products.
Gas density and gas quality therefore answer different questions:
- Gas density: Is there sufficient gas mass in the intended gas compartment?
- Moisture and purity: Does the gas present have the required quality?
Practical example: filling at low ambient temperature
An SF6 switchgear system is recommissioned after maintenance at a low outdoor temperature. The target filling condition specified on the nameplate or in the system documentation is referenced to 20 °C.
During filling, a conventional reference pressure gauge shows a value lower than this nominal value. At the same time, the gas density monitor is already in its intended operating position.
If the technician were to look only at the pressure gauge and continue adding gas until the numerical value of the 20 °C nominal pressure was reached, the system could be overfilled.
Instead, it is first determined which current pressure at the existing gas temperature corresponds to the required reference filling condition. The relevant manufacturer data or a suitable temperature-compensated measuring method are used for this purpose.
After the filling process has been completed, sufficient time is allowed for thermal equalization and the condition is then checked again.
The decisive factor is therefore not that two different displays show the same numerical value. What matters is that both measured values describe the same physical gas condition when their measurement variable and temperature reference are taken into account.
Systematic filling and inspection procedure
- Check the switchgear manufacturer’s specifications: Clearly determine the target filling condition, reference temperature and permissible pressure limits.
- Check which gas type or gas mixture must be used.
- Evacuate or prepare the gas compartment according to the prescribed procedure.
- Use suitable SF6 service equipment, hoses, couplings and measuring instruments.
- Before filling, clarify whether the display being used represents current pressure or a temperature-compensated gas condition.
- Take the current gas temperature or the temperature specified by the manufacturer into account.
- Introduce the gas in a controlled manner until the filling condition specified for these conditions is reached.
- After filling, allow pressure and temperature to equalize.
- Check the temperature-compensated display or gas density again.
- Check service connections and couplings for leaks.
- If required, check the moisture, purity and decomposition products of the SF6.
- Document the final condition and the quantity of gas used or other relevant service values.
Common mistakes
- Setting the filling pressure referenced to 20 °C directly on a conventional pressure gauge: At a different gas temperature, this can lead to overfilling or underfilling.
- Treating a gas density monitor like a conventional pressure gauge: Its display is temperature-compensated and therefore has a different meaning.
- Automatically assuming ambient temperature equals gas temperature: Differences may exist, particularly during and immediately after filling.
- Assessing the final value immediately after filling: Thermal and pneumatic equalization can change the measured value afterwards.
- Assessing gas mixtures using an SF6 characteristic curve: Temperature compensation must match the specific gas composition.
- Equating gas density with gas quality: Correct gas density provides no information about moisture or purity.
- Looking at only one display: If values conflict, it must first be clarified which measurement variable and temperature reference are being used.
- Refilling without investigating the cause of a low density value: If there is an actual gas loss, the leak tightness of the system should first be assessed.
WIKA SF6 measurement technology and service equipment
For continuous monitoring of the gas filling condition of an SF6-insulated system, the WIKA GDM-100 gas density monitor is one suitable example. It combines a local temperature-compensated display with electrical switching contacts for defined warning and alarm levels.
The local display of the GDM-100 represents the gas condition as pressure referenced to 20 °C. This allows normal temperature-related pressure changes to be distinguished to a large extent from an actual loss of gas quantity.
Special SF6 service equipment is available for filling, evacuation, recovery and gas processing. One example is the WIKA GPU-B-2000. The unit is designed for filling, recovering, evacuating and processing SF6 gas compartments and enables a controlled closed-loop service process.
Suitable devices and components can be found under SF6 gas solutions at ICS Schneider. Further information is available for the WIKA GDM-100 gas density monitor and the WIKA GPU-B-2000 SF6 service unit.
Conclusion
When filling an SF6-insulated switchgear system, the directly measured gas pressure must not automatically be equated with the specified gas filling condition. The current pressure changes with temperature even if the quantity of gas present in the gas compartment remains unchanged.
For this reason, filling conditions are often referenced to a defined temperature of 20 °C. A gas density monitor compensates for the influence of temperature and therefore enables much more reliable assessment of the quantity of gas present.
A conventional pressure gauge and a gas density monitor can show different numerical values on the same system without either instrument being faulty. The important point is to understand which measurement variable each instrument represents.
During filling in particular, gas temperature, thermal equalization and the equipment manufacturer’s specifications must also be taken into account. A pressure that appears uncritical at low temperature can rise significantly after warming. Conversely, a warm gas compartment can initially conceal an insufficient actual filling condition.
For reliable SF6 filling, the following therefore applies: consistently distinguish between current gas pressure, gas temperature and temperature-compensated filling condition, always interpret the target value together with its temperature reference and assess the final gas condition only after suitable pressure and temperature equalization.
FAQ: SF6 gas pressure and gas density during filling
What is the difference between SF6 gas pressure and gas density?
Gas pressure describes the pressure currently present and changes with temperature. Gas density describes the gas mass present in relation to the volume and is therefore more suitable for assessing the actual filling condition.
Why is SF6 filling pressure often referenced to 20 °C?
Using a reference temperature allows gas filling conditions to be compared independently of different ambient temperatures. A value referenced to 20 °C describes the condition that the gas filling would have at this reference temperature.
Does a conventional pressure gauge always have to show the filling pressure specified for 20 °C during filling?
No. If the actual gas temperature is not 20 °C, the currently measured pressure can differ. The required value must be assessed with reference to temperature according to the equipment manufacturer’s specifications.
What does an SF6 gas density monitor display?
A gas density monitor compensates for temperature-related pressure changes. Depending on the design, it may display pressure referenced to 20 °C or a density value derived directly from the gas condition.
Why does the gas density monitor show a different value from the service pressure gauge?
The service pressure gauge typically measures the current gas pressure, while the gas density monitor displays a temperature-compensated value. The two values can therefore differ while still correctly describing the same gas condition.
Can an SF6 system be overfilled in cold weather?
Yes. If a pressure specified for 20 °C is set directly on a conventional pressure gauge at a low gas temperature without temperature correction, too much gas can be introduced. The pressure then rises accordingly when the system later warms up.
Can a warm SF6 system be underfilled even though the pressure appears correct?
Yes. A higher gas temperature increases the current pressure. As a result, an insufficient gas filling condition may initially appear unremarkable when viewed only on a conventional pressure gauge.
Why should you wait before performing the final check after filling?
During filling, both the gas and the system can undergo thermal changes. After filling, pressure and temperature need time to equalize. Only then can the final filling condition be assessed reliably.
Does correct gas density also provide information about SF6 moisture?
No. Gas density and gas quality are different parameters. Gas density describes the filling condition, while moisture, purity and decomposition products must be assessed separately using suitable SF6 analysis instruments.
Which specific device is suitable for SF6 gas density monitoring?
A typical example is the WIKA GDM-100 gas density monitor. It provides a temperature-compensated local display and electrical switching contacts for monitoring defined gas density limits.
Which device is suitable for filling an SF6 switchgear system?
For professional service work, dedicated SF6 service units such as the WIKA GPU-B-2000 can be used. They are designed for controlled filling, evacuation, recovery and processing of SF6 gas.