In SF6-insulated medium- and high-voltage switchgear, gas density is a decisive operating parameter. If the quantity of gas inside the gas compartment decreases due to a leak, the insulation and switching performance of the installation deteriorates. Reliable monitoring must detect this condition before a safety-critical limit is reached.
Different instrument concepts are available for this task. A conventional gas density monitor displays the temperature-compensated gas condition locally and switches electrical contacts at defined alarm limits. An electronic gas density sensor, by contrast, continuously transmits measured values via 4–20 mA or Modbus to a PLC, protection system or control system.
The more suitable solution depends on whether only fixed warning and shutdown limits are required or whether trends, gradual gas losses and centralised condition data are also to be evaluated. In many installations, a combination of local indication, switching contacts and a continuous output signal is also beneficial.
Table of Contents
- Why monitor gas density rather than gas pressure alone?
- Direct comparison of gas density monitors and gas density sensors
- How does a gas density monitor work?
- How does an electronic gas density sensor work?
- Planning warning, alarm and shutdown contacts
- Continuous monitoring with 4–20 mA
- Transmitting gas-condition data via Modbus
- Detecting gradual leaks at an early stage
- When is a hybrid solution appropriate?
- Retrofitting existing switchgear
- Process connections, valves and gas tightness
- Electrical connection and evaluation
- Commissioning and functional testing
- Testing gas density monitors and sensors
- Typical selection and installation errors
- Practical example: Monitoring an SF6 switchgear installation
- Which system is suitable for the application?
- Which products are suitable?
- Conclusion
- Frequently asked questions
Why monitor gas density rather than gas pressure alone?
The gas pressure in a sealed SF6 compartment changes with temperature. When the ambient temperature falls, the pressure decreases even if no gas has escaped from the installation. When the temperature rises, the pressure increases accordingly.
A conventional pressure gauge therefore cannot clearly distinguish whether a pressure change has been caused by an actual gas loss or merely by a temperature change.
Gas density measuring instruments compensate for this temperature influence. Depending on the measuring principle, the gas condition is converted to a defined reference temperature, frequently 20 °C, or output directly as a density value.
This makes it possible to distinguish between two different situations:
- Temperature-related pressure change: The quantity of gas remains constant and the temperature-compensated value remains largely stable.
- Actual gas loss: The quantity of gas decreases and the temperature-compensated pressure or gas density value falls.
Further information on temperature compensation is provided in the article Temperature-compensated SF6 gas density measurement: Why pressure alone is not sufficient.
Direct comparison of gas density monitors and gas density sensors
| Criterion | Gas density monitor | Gas density sensor |
|---|---|---|
| Main function | Local indication and switching of defined limits | Continuous electronic measurement and transmission |
| Output | Volt-free switching contacts | 4–20 mA, Modbus or another digital output |
| Measured-value trend | Readable locally, but no trend without external recording | Can be recorded continuously in the control system |
| Power supply | Mechanical indication normally requires no power supply | Electrical power supply required |
| Alarm generation | Directly through defined contact points | Limits are often generated in the PLC or control system |
| Early leak detection | Alarm only when the configured contact point is reached | Trend monitoring can detect gradual decreases earlier |
| Communication | Binary signals | Analogue measured value or several digital process values |
| Typical application | Independent protection and alarm function directly at the switchgear | Centralised condition monitoring and condition-based maintenance |
A gas density sensor is not inherently safer than a gas density monitor. The required function and the way in which the signals are processed are decisive.
A mechanical gas density monitor can continue to provide a local indication and warning through independently wired contacts even if the PLC or data communication fails. A sensor enables considerably more detailed analysis but depends on the power supply, signal transmission and evaluation system.
How does a gas density monitor work?
A gas density monitor combines a temperature-compensated mechanical measuring system with a local indication and one or more electrical switching contacts.
Depending on the version, the scale indicates:
- the gas pressure compensated to 20 °C,
- a gas density range,
- colour-coded operating, warning and alarm ranges,
- where applicable, a vacuum or filling range.
If the gas density falls below a defined value, the pointer passes the configured contact point. The contact can then, for example:
- trigger a warning message,
- transmit an alarm to the station control system,
- withdraw an enable signal,
- inhibit switching operations,
- initiate a controlled shutdown.
The switching points are generally configured at the factory according to the switchgear data. In many versions, they are fixed and are not intended to be freely adjusted by the plant operator.
Gas density monitors are particularly suitable when a simple, robust and independently operating limit-monitoring system is required.
How does an electronic gas density sensor work?
An electronic gas density sensor measures the pressure and temperature of the insulating gas. A microprocessor uses these values to calculate the temperature-compensated gas condition.
Depending on the version, the following values can be provided:
- SF6 gas density in g/l,
- compensated pressure referenced to 20 °C,
- current gas pressure,
- gas temperature,
- instrument and diagnostic information.
With an analogue 4–20 mA output, one selected measured variable is generally transmitted. The PLC then scales the current value, for example, into g/l or bar at 20 °C.
With a digital interface such as Modbus, several measured variables and diagnostic values can instead be read from the same instrument.
A gas density sensor is therefore particularly suitable for:
- centralised monitoring of several gas compartments,
- recording long-term trends,
- remote monitoring of unmanned substations,
- condition-based maintenance,
- early detection of gradual gas losses,
- integration into SCADA or condition-monitoring systems.
Planning warning, alarm and shutdown contacts
Depending on the version, gas density monitors can have several independent switching contacts. The precise number and function are defined for the relevant switchgear installation.
| Stage | Typical function | Possible response |
|---|---|---|
| Warning | First detectable loss of density | Inform maintenance personnel and observe the trend |
| Alarm | Gas density approaches the minimum permissible operating range | Plan a service visit and assess switching readiness |
| Lockout or shutdown | Impermissibly low gas density | Inhibit switching operations or terminate operation in a controlled manner |
The specific switching values are defined by the switchgear manufacturer or on the basis of the permissible gas density of the relevant gas compartment.
The limits must not simply be adopted from another installation. Even switchgear panels with the same rated voltage may have different filling pressures, gas volumes, gas mixtures and permissible operating limits.
The following must also be considered during electrical planning:
- contact type and switching function,
- maximum switching voltage,
- maximum switching current,
- AC or DC operation,
- inductive loads,
- closed-circuit or open-circuit principle,
- cable-break monitoring,
- separation between warning and shutdown.
Continuous monitoring with 4–20 mA
An analogue gas density sensor transmits the measured or calculated gas condition as a 4–20 mA signal. In this arrangement, 4 mA corresponds to the lower and 20 mA to the upper end of the configured measuring range.
Example:
- 4 mA corresponds to 0 g/l,
- 20 mA corresponds to 60 g/l,
- 12 mA corresponds to 30 g/l.
The actual scaling must be taken from the data sheet, ordering code or sensor configuration.
The advantages of the 4–20 mA signal include:
- simple integration into existing analogue inputs,
- robust transmission over longer distances,
- detection of certain cable faults through currents outside the operating range,
- manufacturer-independent processing in PLC and control systems,
- simple testing with a current-loop calibrator.
As only one current value is transmitted, a single current loop normally provides only one main measured variable. If gas density, pressure and temperature are all to be transmitted simultaneously, Modbus or another multivariable solution is more suitable.
The UPS4E current-loop calibrator can be used to test the current loop. It can measure the loop current, simulate defined mA values and provide a 24 V loop power supply where required.
Transmitting gas-condition data via Modbus
With Modbus RTU, digital measurement and diagnostic data are transmitted via an RS-485 interface. Depending on the bus design, addressing and cable length, several sensors can be operated on a common bus.
Typical values that may be available include:
- gas density,
- compensated pressure,
- current pressure,
- gas temperature,
- instrument status,
- fault or diagnostic information.
Modbus offers advantages when several items of information from each gas compartment are to be recorded centrally. However, the requirements for electrical and software integration are correspondingly higher.
The following must be planned:
- unique instrument addresses,
- baud rate and communication parameters,
- register assignment and data format,
- bus termination at the ends of the cable,
- line topology rather than uncontrolled star wiring,
- shielded and twisted data cable,
- galvanic isolation and equipotential bonding,
- communication monitoring in the PLC.
The presence of a measured value in the control system does not automatically confirm fault-free communication. The PLC should also detect timeouts, implausible values and communication failures.
Detecting gradual leaks at an early stage
A gas density monitor responds only when a defined limit is reached. An electronic sensor, by contrast, can reveal smaller changes at an earlier stage, provided that its measured values are stored and evaluated regularly.
Early leak detection is typically based on:
- long-term recording of the compensated gas density value,
- calculation of the change per week, month or year,
- comparison of similar gas compartments,
- detection of accelerating density losses,
- correlation with maintenance and refilling data.
A single slightly deviating measured value is not yet reliable proof of a leak. Possible causes also include:
- measurement tolerance,
- temperature gradients between the gas compartment and sensor,
- incomplete thermal equalisation,
- valve position or trapped dead volume,
- electrical interference,
- incorrect scaling in the control system.
The reproducible trend of the temperature-compensated value is therefore decisive.
A gas density sensor does not locate the leaking point. If a density loss is detected, the actual leak must subsequently be located using suitable SF6 leak-detection instruments and a defined test procedure.
When is a hybrid solution appropriate?
Hybrid gas density measuring instruments combine the advantages of a conventional gas density monitor with those of an electronic transmitter.
Such a solution can simultaneously provide:
- local mechanical indication,
- independent warning and alarm contacts,
- a continuous analogue measured value,
- digital communication,
- centralised trend monitoring.
Hybrid instruments are particularly useful when the local protection function must remain available even if the control system fails, while continuous condition monitoring is also required.
The two signal paths should be considered separately from a functional perspective:
- switching contacts for direct warning, lockout or protection functions,
- transmitter output for indication, diagnostics and trend analysis.
Continuous measurement may replace a safety-related contact function only if the complete protection concept, including the sensor, power supply, PLC, software and output stage, has been designed and assessed for this purpose.
Retrofitting existing switchgear
Older SF6 switchgear installations frequently have only mechanical gas density monitors with switching contacts. An electronic sensor or hybrid solution can be retrofitted for centralised monitoring.
The following points must be clarified before retrofitting:
- Is a free, gas-tight measuring connection available?
- Can a combination valve or manifold be retrofitted?
- Will the existing gas density monitor remain fully functional?
- What additional dead volumes will be created?
- Are the pressure range and gas mixture compatible with the sensor?
- Are a power supply, analogue input or RS-485 communication available?
- Is the mechanical load on the connection permissible?
- Which approvals from the switchgear manufacturer are required?
A sensor must not be separated from the actual gas compartment by a permanently closed or incorrectly positioned service connection. Otherwise, it measures only the trapped gas volume between the valve and sensor.
The retrofit must be designed so that as little insulating gas as possible is released during installation, testing or replacement.
Process connections, valves and gas tightness
The measuring instrument forms part of the gas-tight pressure system. The process connection, valves, seals and couplings must therefore be selected just as carefully as the sensor itself.
The following must be checked:
- connection geometry and thread,
- permissible operating pressure,
- materials and seals,
- gas compatibility,
- leak rate of the connection,
- mounting position,
- mechanical support,
- access for functional testing and calibration.
A gas density monitor must not be installed under mechanical stress through the process connection or used as a support for heavy pipework.
Suitable calibration or combination valves can be provided for servicing and recalibration. They allow the measuring instrument to be isolated from the gas compartment and connected to a test system without completely removing the monitor.
A suitable leak-tightness test must be performed after any work on the connection.
Electrical connection and evaluation
Switching contacts
The contacts of a gas density monitor are normally connected to digital inputs, protection relays or alarm circuits.
The following must be checked:
- contact assignment,
- normally open, normally closed or changeover function,
- switching state at normal gas density,
- closed-circuit principle,
- permissible contact load,
- protective circuitry for inductive loads.
Larger loads must not be switched directly through sensitive instrument contacts. Solenoid valves, contactors or audible alarms should be controlled through suitable interposing relays or protective modules.
4–20 mA signal
With a two-wire sensor, the power supply and measuring signal are transmitted through the same current loop. For reliable operation, the supply voltage, cable resistance and input resistance of the PLC must be compatible.
At least the following must be documented during configuration:
- measured variable assigned to the analogue output,
- lower and upper measuring range,
- unit,
- behaviour in the event of a sensor fault,
- PLC alarm limits,
- filtering and averaging.
Modbus
With digital sensors, the registers, byte order and data type must also be interpreted correctly. A floating-point value read using the wrong byte order can produce completely implausible measured values.
Commissioning and functional testing
The complete measuring point should be checked before commissioning:
- Check the instrument data: Compare the measuring range, gas type, gas mixture, switching points and output signal with the ordering data.
- Check the process connection: Verify the valve position, seals and gas-tight installation.
- Allow thermal equalisation: Before evaluating the values, allow the measuring instrument and gas compartment to reach as uniform a temperature as possible.
- Compare the local indication: Compare the value with the installation and filling data.
- Test the switching contacts: Verify the contact assignment and alarms through to the control room.
- Test the analogue signal: Measure the loop current and check the PLC scaling.
- Test Modbus communication: Check the address, registers and measured values.
- Test the alarm limits: Trigger warning, alarm and lockout functions using a suitable simulation.
- Document the results: Record the measured values, contact states, configuration and serial number.
A comparison with a conventional pressure gauge is of limited significance because it indicates the current pressure rather than necessarily the temperature-compensated gas condition.
Testing gas density monitors and sensors
Testing a gas density measuring instrument must cover both the pneumatic or gas-density-related function and the electrical output.
Testing a gas density monitor
The following are typically checked on a gas density monitor:
- local indication,
- warning contact,
- alarm contact,
- reset points,
- repeatability,
- leak tightness of the measuring connection.
Testing is performed using a suitable gas density calibration system or in accordance with the procedure specified by the manufacturer. Pressure and temperature must be considered together.
Testing the 4–20 mA output
With an electronic sensor, the actual gas condition is compared with a suitable reference. The electrical current loop can also be tested independently.
A current-loop calibrator can be used to:
- measure the actual output current,
- inject defined mA values into the PLC input,
- check the scaling in the control system,
- simulate warning and alarm limits,
- check cable and supply voltages.
However, a simple mA simulation tests only the electrical signal path. It does not calibrate the pressure sensor, temperature sensor or gas density calculation.
Typical selection and installation errors
A conventional pressure gauge is used as a gas density monitor
Temperature changes produce apparent gas losses or conceal an actual leak.
Switching contacts are interpreted as a continuous measured value
Between two contact points, no statement can be made about the exact gas density or the rate of change.
A gas density sensor is installed, but no trend is stored
The advantage of continuous measurement is not used. A gradual gas loss becomes apparent only when the PLC alarm is reached.
The 4–20 mA signal is scaled incorrectly
The PLC uses a different measuring range or unit from the sensor.
Incorrect registers are used with Modbus
Gas pressure, temperature or density are interchanged or read using the wrong data type.
The sensor is isolated from the gas compartment by a closed valve
The measured value appears stable even though the gas density in the switchgear is changing.
Warning and shutdown contacts are interchanged
The installation indicates the wrong condition or locks out only at an excessively low limit.
Switching points are adopted from another installation
The filling pressure, gas mixture and permissible density ranges do not match the specific switchgear.
A sensor replaces the independent gas density monitor without an assessment
If the power supply, communication or PLC fails, no independent limit alarm remains available.
No leak-tightness test is performed after installation
The newly installed connection itself becomes the source of gas loss.
Practical example: Monitoring an SF6 switchgear installation
A gas-insulated high-voltage switchgear installation has several separate SF6 gas compartments. Gas density monitors with two contacts are currently installed on each gas compartment:
- first stage: warning,
- second stage: switching lockout.
The contacts are already processed in the station control system. The operator additionally wants to detect gradual density losses and plan maintenance more effectively.
The existing gas density monitors remain in place as independent protection and alarm components. An electronic gas density sensor is additionally connected to each gas compartment.
Modbus RTU is selected for transmission because the gas density, pressure, temperature and diagnostic data are all to be recorded centrally.
The PLC stores the following for each gas compartment:
- current density value,
- 24-hour average,
- change compared with the previous month,
- communication status,
- date of the most recent maintenance or refilling.
After several months, one gas compartment shows a slow but continuous decrease. The fixed contacts of the gas density monitor have not yet responded.
The operator can now carry out a planned service visit, locate and repair the leak before the warning stage is reached.
At the same time, the existing contacts remain available as an independent fallback level if the sensor, bus communication or control system fails.
Which system is suitable for the application?
Product selection begins with the required monitoring function.
| Requirement | Suitable solution |
|---|---|
| Local indication only | Gas density indicator |
| Local indication and fixed alarm contacts | Gas density monitor |
| Simple binary limit signal without a scale | Gas density switch |
| Continuous measured value to the PLC | Gas density sensor with 4–20 mA |
| Digital transmission of density, pressure and temperature | Gas density sensor with Modbus |
| Local indication, contacts and continuous data | Hybrid gas density monitor with transmitter |
| Independent protection function and trend monitoring | Gas density monitor plus separate sensor |
At least the following information is required for the specific design:
- switchgear manufacturer and installation type,
- SF6 or the gas mixture used,
- filling pressure or nominal density,
- reference temperature,
- minimum permissible operating density,
- warning and shutdown points,
- minimum and maximum process pressure,
- ambient temperature,
- process connection and valve concept,
- number and type of switching contacts,
- 4–20 mA or Modbus,
- supply voltage,
- degree of protection and outdoor installation,
- requirements for redundancy and diagnostics,
- testing and calibration concept.
Which products are suitable?
Gas density monitors
The gas density monitors category includes various WIKA versions for monitoring sealed SF6 gas compartments in medium- and high-voltage installations.
Depending on the model, available options include:
- local temperature-compensated indication,
- one or more switching contacts,
- versions with reference chambers,
- integrated or attached transmitters,
- analogue or digital output signals,
- calibration and combination valves,
- versions for different gas mixtures.
WIKA GDM-100 gas density monitor
The WIKA GDM-100 is intended for local monitoring of sealed gas compartments in medium- and high-voltage installations.
Its main features include:
- local indication of the gas condition compensated to 20 °C,
- electrical switching contacts for defined alarm limits,
- temperature-compensated and hermetically sealed measuring system,
- versions for SF6 and gas mixtures,
- calibration valve for testing without complete removal.
The GDM-100 is particularly suitable for independent warning, alarm and lockout functions directly at the switchgear.
WIKA GD-20 gas density sensor
The WIKA GD-20 gas density sensor is used for continuous monitoring of gas density, temperature and pressure in sealed insulating-gas compartments.
Depending on the version, it provides:
- digital Modbus RTU communication via RS-485,
- alternatively, an analogue 4–20 mA output,
- output of the SF6 gas density or compensated pressure,
- configuration for different defined gas mixtures,
- compact design and high degree of protection,
- continuous integration into PLC and control systems.
The GD-20 is particularly suitable where long-term trends, centralised measured-value recording and early leak detection are required.
Druck UPS4E current-loop calibrator
The Druck UPS4E is suitable for commissioning and troubleshooting analogue 4–20 mA measuring circuits.
Among other functions, it provides:
- measurement and generation of 0 to 24 mA,
- 24 V loop power supply,
- percentage indication of the measuring range,
- step and ramp testing,
- testing of PLC inputs and alarm limits,
- data recording.
The UPS4E can be used to verify whether the gas density sensor outputs the expected current and whether the PLC, scaling, warning and alarm limits respond correctly.
Conclusion: Switching contacts provide protection – continuous measured values provide transparency
A gas density monitor is suitable for robust and largely independent warning, alarm and lockout functions. Its temperature-compensated indication allows direct checking at the switchgear, while its electrical contacts switch at defined density limits.
A gas density sensor, by contrast, transmits continuous measured values to a PLC or control system. A selected measured variable can be integrated easily into existing automation systems using 4–20 mA. Modbus additionally enables several gas-condition and diagnostic values to be transmitted.
Continuous measurement provides the basis for trend analyses and can detect gradual gas losses before a fixed alarm contact is reached. However, the values must be stored, checked for plausibility and evaluated systematically.
For safety-related applications, a combination is often appropriate: a gas density monitor provides independent limit indication, while a gas density sensor supplies condition data for the control system and maintenance team.
The correct filling and limit data for the specific switchgear, the gas mixture used, the gas-tight process connection and traceable electrical and software integration are decisive.
Frequently asked questions about gas density monitors and gas density sensors
What is the main difference between a gas density monitor and a gas density sensor?
A gas density monitor indicates the gas condition locally and switches at fixed limits. A gas density sensor provides continuous electronic measured values to a PLC or control system.
Why is a conventional pressure gauge not sufficient for SF6?
Gas pressure changes with temperature. A conventional pressure gauge therefore cannot reliably distinguish between a temperature change and an actual gas loss.
Can a gas density sensor locate a leak?
No. It can detect a decreasing density value and therefore a possible gas loss. The leaking point must subsequently be located using a suitable SF6 leak-detection instrument.
Which measured values does a gas density sensor provide via Modbus?
Depending on the instrument, gas density, compensated pressure, current gas pressure, temperature and diagnostic information can be transmitted.
Does a 4–20 mA sensor transmit density, pressure and temperature simultaneously?
A single current loop normally transmits only one configured measured variable. A digital interface such as Modbus is more suitable for several values.
Can a gas density sensor completely replace the switching contacts?
Technically, limits can be generated in a PLC. However, for independent or safety-related protection functions, it must be assessed whether the power supply, sensor, PLC, software and output stage provide the required reliability.
Why should gas density values be stored over the long term?
A trend reveals slow changes that have not yet triggered a fixed alarm contact. This allows maintenance work to be planned before a critical condition is reached.
What must be considered when retrofitting a gas density sensor?
Important factors include a suitable gas-tight connection, the valve position, gas mixture, measuring range, additional dead volumes, electrical power supply and approval for the specific switchgear installation.
How is a 4–20 mA output tested?
The actual sensor output can be measured using a current meter or current-loop calibrator. Defined mA values can also be injected into the PLC input to test the scaling and alarm limits.
Which information does ICS Schneider require for selection?
The required information includes the installation type, gas or gas mixture, filling pressure or nominal density, warning and shutdown values, temperature range, process connection, required switching contacts, output signal, power supply, degree of protection and requirements for trend monitoring, redundancy and calibration.