Degassed Cation Conductivity Explained
Prosoft Engineering
Technical Compliance Specialist
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Introduction
Accurate steam and water chemistry monitoring is essential for protecting boilers, HRSGs, turbines, condensers and other components of a power-generation water-steam cycle. Among the most important measurements used in high-purity cycle chemistry is cation conductivity.
However, dissolved gases—particularly carbon dioxide—can influence conductivity measurements. In applications where a more specific assessment of ionic contamination is required, degassed cation conductivity can provide additional information.
A properly designed SWAS Panel can incorporate degassed cation conductivity along with specific conductivity, cation conductivity, pH, dissolved oxygen, silica and sodium measurements.
This article explains what degassed cation conductivity means, how it differs from conventional cation conductivity, why degassing is used, how the measurement system works, and what should be considered when designing and maintaining an Online SWAS Panel.
What Is Degassed Cation Conductivity?
Degassed cation conductivity is a conductivity measurement performed after:
The sample passes through a cation-exchange system.
Dissolved gases—particularly carbon dioxide—are removed or reduced.
The conductivity of the treated sample is measured.
The simplified measurement sequence is:
Process Sample
↓
Pressure Reduction
↓
Cooling
↓
Cation Exchange
↓
Degassing
↓
Conductivity Measurement
The exact arrangement can vary depending on the analyzer manufacturer and SWAS design.
The objective is to reduce the influence of dissolved carbon dioxide on the conductivity measurement so that the remaining conductivity provides a better indication of ionic contamination.
Why Is Degassing Necessary?
Carbon dioxide can dissolve in water and form carbonic-acid-related species.
The simplified reaction is:
CO₂ + H₂O ⇌ H₂CO₃
Carbonic acid can dissociate to produce hydrogen and bicarbonate/carbonate species.
These ionic species contribute to conductivity.
Consequently, a cation conductivity measurement can include a contribution associated with dissolved carbon dioxide.
If the objective is to distinguish potentially significant strong-acid contamination from the conductivity contribution associated with carbon dioxide, degassing can provide additional diagnostic information.
Cation Conductivity vs Degassed Cation Conductivity
The fundamental difference is the additional degassing step.
ParameterCation ConductivityDegassed Cation ConductivityDirect conductivity measurementNoNoCation exchangeYesYesDegassingNoYesCO₂ influenceCan contributeReduced after effective degassingMain purposeDetect ionic contamination after cation exchangeFurther evaluate contamination with reduced CO₂ contributionSystem complexityModerateHigherMaintenance requirementSensor + cation exchangerSensor + cation exchanger + degassing systemTypical applicationCycle chemistryDiagnostic/high-purity cycle chemistry
The two measurements should therefore be considered complementary.
Understanding the Measurement Principle
Consider a water sample containing sodium chloride.
Initially:
NaCl → Na⁺ + Cl⁻
When the sample passes through the cation exchanger, sodium is exchanged for hydrogen:
Na⁺ → H⁺
The chloride remains in solution.
The resulting hydrogen/chloride ionic contribution produces increased conductivity.
Now consider dissolved carbon dioxide.
Carbon dioxide can contribute to conductivity through the formation of carbonic-acid-related species.
If the sample is subsequently degassed, a significant portion of dissolved CO₂ can be removed.
The resulting conductivity can therefore be lower than the non-degassed cation conductivity when CO₂ is contributing significantly.
The difference between the two measurements can provide useful diagnostic information.
What Does Degassed Cation Conductivity Tell You?
Degassed cation conductivity can help distinguish between different contributors to cation conductivity.
For example:
High Cation Conductivity + Lower Degassed Cation Conductivity
This can indicate that dissolved CO₂ is making a meaningful contribution.
High Cation Conductivity + High Degassed Cation Conductivity
This may suggest that ionic contamination remains significant even after degassing.
Potential sources can include:
Chloride
Sulphate
Other strong-acid anions
Cooling-water contamination
Condenser leakage
Makeup-water contamination
Process contamination
The actual diagnosis must always consider other plant chemistry measurements and operating conditions.
Where Is Degassed Cation Conductivity Used?
Degassed cation conductivity is particularly useful in applications where very low levels of contamination need to be monitored.
Typical applications can include:
Thermal power plants
Combined-cycle power plants
HRSG systems
High-pressure boiler systems
Steam turbine cycles
Condensate systems
Feedwater systems
High-purity water systems
It can be especially useful when operators need to investigate whether an increase in cation conductivity is associated primarily with dissolved carbon dioxide or with other ionic contaminants.
Degassed Cation Conductivity in a SWAS Panel
A properly engineered SWAS Panel integrates the degassed conductivity system into the overall sample-conditioning architecture.
A typical arrangement may include:
Process Sample
→ Sample Isolation
→ Pressure Reduction
→ Sample Cooler
→ Temperature Measurement
→ Flow Control
→ Cation Exchanger
→ Degassing System
→ Conductivity Cell
→ Drain
The actual sequence may differ according to the selected analyzer and application.
The sample-conditioning system must be designed as a complete system rather than simply connecting a conductivity sensor to the process sample.
Sample Extraction Is Critical
The quality of the measurement begins at the sample point.
The sample should be:
Representative of the process stream
Extracted safely
Transported with minimal delay
Compatible with the sample-line material
Properly pressure-reduced
Properly cooled
Delivered at the required flow
A poor sample point can produce misleading results regardless of how sophisticated the conductivity analyzer is.
Pressure Reduction
Power-plant samples can originate from high-pressure and high-temperature locations.
The sample must therefore be brought into an appropriate operating range before entering the analyzer.
Pressure-reduction equipment can include:
Pressure-reducing valves
Needle valves
Regulators
Back-pressure control
Multi-stage pressure reduction where required
The selected arrangement should account for:
Upstream pressure
Downstream pressure
Sample temperature
Required flow
Flashing risk
Valve material
Pressure rating
There is no universal SWAS pressure setting. The design must be based on actual process conditions and analyzer requirements.
Sample Cooling
Sample cooling is normally required before conductivity measurement when the source sample is too hot for the analyzer.
A sample cooler may use plant cooling water to reduce the sample temperature.
Important design factors include:
Sample inlet temperature
Required outlet temperature
Sample flow
Cooling-water temperature
Cooling-water flow
Heat-transfer duty
Cooler material
Pressure rating
The basic heat-transfer relationship can be expressed as:
Q = m × Cp × ΔT
where:
Q = heat-transfer duty
m = mass flow
Cp = specific heat
ΔT = temperature change
Actual cooler sizing should be based on engineering calculations and the manufacturer's requirements.
Cation Exchange System
The cation exchanger is central to the measurement.
Its purpose is to exchange dissolved cations for hydrogen ions before conductivity measurement.
Important design considerations include:
Resin type
Resin capacity
Column volume
Sample flow
Pressure drop
Temperature
Resin condition
Column accessibility
Replacement procedure
The cation exchanger should be installed in a way that allows inspection and maintenance without unnecessarily disturbing the rest of the SWAS system.
Degassing System
The degassing stage removes dissolved gases from the sample before the final conductivity measurement.
Different analyzer manufacturers may use different approaches.
The system may involve:
Gas-permeable membrane technology
Vacuum-assisted degassing
Gas stripping
Controlled sample exposure
Other proprietary arrangements
The exact technology should be selected according to the analyzer manufacturer's design.
The important engineering objective is to achieve repeatable removal of dissolved gases without introducing contamination into the sample.
Why Carbon Dioxide Matters
CO₂ is particularly important because it can enter the water-steam cycle from several sources.
Possible sources include:
Air ingress
Condenser vacuum-system leakage
Makeup-water chemistry
Atmospheric exposure
Decomposition of carbonate/bicarbonate species
Other process-related sources
CO₂-related conductivity can therefore complicate the interpretation of conventional cation conductivity.
Degassed cation conductivity helps provide another diagnostic perspective.
Comparing Three Conductivity Measurements
It is useful to understand the relationship between:
Specific Conductivity
Measured directly on the sample.
It provides an overall indication of ionic conductivity.
Cation Conductivity
Measured after cation exchange.
It increases sensitivity to certain anionic contaminants.
Degassed Cation Conductivity
Measured after cation exchange and subsequent degassing.
It reduces the contribution associated with dissolved CO₂ and can therefore provide additional diagnostic information.
These measurements should not be interpreted independently.
Using Conductivity Measurements Together
A modern Online SWAS Panel can trend multiple conductivity measurements together.
For example:
Specific Conductivity
Cation Conductivity
Degassed Cation Conductivity
pH
Dissolved Oxygen
Silica
Sodium
This combination can provide much better diagnostic information than any single measurement.
Operators can examine trends rather than relying only on instantaneous readings.
Example of Diagnostic Interpretation
Consider a situation where cation conductivity increases unexpectedly.
Step 1 — Check Specific Conductivity
If specific conductivity also increases, an actual increase in ionic contamination may be occurring.
Step 2 — Check Cation Conductivity
If cation conductivity increases significantly, investigate possible anionic contamination.
Step 3 — Check Degassed Cation Conductivity
If degassed cation conductivity remains relatively low compared with cation conductivity, CO₂ may be contributing to the difference.
Step 4 — Check Other Parameters
Review:
pH
Sodium
Silica
Chloride, if available
Sulphate, if available
Dissolved oxygen
Condenser operating condition
Makeup-water quality
Step 5 — Check the SWAS System
Before concluding that the process chemistry has changed, verify:
Sample flow
Temperature
Pressure
Cation resin
Conductivity sensor
Sample tubing
Drainage
Electrical connections
This process helps distinguish a real chemistry event from a sample-system or analyzer problem.
Degassed Cation Conductivity and Condenser Leakage
Condenser leakage is a major concern in many power plants.
Cooling water can contain dissolved salts and other ionic contaminants.
If cooling water enters the condensate, conductivity may increase.
Cation conductivity can be particularly useful for detecting contamination associated with strong-acid anions.
Degassed cation conductivity provides additional information by reducing the influence of dissolved CO₂.
However, no single conductivity measurement should be used as the sole proof of condenser leakage. Plant chemistry trends and other process information should also be evaluated.
Degassed Cation Conductivity in HRSG Systems
Combined-cycle power plants use Heat Recovery Steam Generators to recover energy from gas-turbine exhaust.
The water-steam cycle typically includes:
Condenser
Condensate system
Feedwater
Economizer
Evaporator
Steam drum
Superheater
Reheater
Because modern HRSG systems can operate with high-purity chemistry requirements, continuous monitoring can be important.
A SWAS Panel India solution for an HRSG application may integrate:
Specific conductivity
Cation conductivity
Degassed cation conductivity
pH
Dissolved oxygen
Silica
Sodium
Other required parameters
The exact parameter list should be established from the plant chemistry program and equipment requirements.
Temperature Control for Degassed Cation Conductivity
Temperature stability is important because conductivity is temperature-dependent.
The system should therefore provide appropriate:
Sample cooling
Temperature measurement
Temperature compensation
Flow control
The analyzer's specified operating range should always be followed.
Unstable sample temperature can appear as an unstable chemistry reading even when the process itself is stable.
Sample Flow Requirements
Stable flow is important for the complete measurement chain.
Flow affects:
Sample transport time
Cooling performance
Cation-exchange performance
Degassing performance
Conductivity-cell response
Measurement stability
There is no universal SWAS sample flow rate.
The required flow should be determined from:
Analyzer manufacturer's specification
Cation-column requirements
Degassing system requirements
Cooler design
Tubing dimensions
Sample pressure
Process conditions
Sample-Line Material Selection
Sample-line materials should be selected to minimize contamination and maintain sample integrity.
Common materials can include:
Stainless steel
PTFE
PFA
Other application-compatible polymers
The final material should be selected according to:
Water chemistry
Temperature
Pressure
Required cleanliness
Sample conductivity
Analyzer requirements
For very-low-conductivity applications, contamination from inappropriate materials or fittings can become significant.
SWAS Panel Maintenance for Degassed Cation Conductivity
An effective SWAS Panel Maintenance program should cover the entire measurement chain.
Cation Exchanger
Check:
Resin condition
Resin capacity
Column condition
Flow
Leakage
Replacement requirement
Degassing System
Check:
Degassing performance
Membrane/system condition where applicable
Vacuum or gas system where applicable
Connections
Leakage
Flow
Conductivity Cell
Check:
Cell condition
Cleanliness
Cable
Connector
Calibration/verification
Sample System
Check:
Pressure
Temperature
Flow
Cooler
Filters
Tubing
Valves
Drain
Common Problems and Troubleshooting
Problem 1: Cation Conductivity High, Degassed Value Normal
Possible causes include:
Increased dissolved CO₂
Air ingress
Condenser-related conditions
Makeup-water changes
Verify the process chemistry and sample system before drawing a conclusion.
Problem 2: Both Cation and Degassed Cation Conductivity High
Possible causes include:
Ionic contamination
Chloride/sulphate contamination
Condenser leakage
Makeup-water contamination
Chemical contamination
Review other chemistry parameters and plant operating conditions.
Problem 3: Degassed Conductivity Unstable
Check:
Sample flow
Degassing system
Temperature
Pressure
Cation resin
Conductivity sensor
Electrical connections
Problem 4: Conductivity Suddenly Drops
Possible causes include:
Change in process chemistry
Flow interruption
Cation-resin condition
Degassing-system change
Sensor problem
Sample-line problem
Calibration and Verification
Conductivity systems should be calibrated or verified according to the manufacturer's procedure.
Before performing calibration:
Check sample flow.
Check temperature.
Inspect the conductivity cell.
Verify the cation exchanger.
Check the degassing system.
Confirm the correct calibration standard.
Perform calibration according to the instrument manual.
Verify the result.
Record the activity.
For low-conductivity measurements, the cleanliness of calibration equipment and standards is particularly important.
DCS and SCADA Integration
A modern Online SWAS Panel can transmit degassed cation conductivity to the plant automation system.
Typical signals may include:
Conductivity value
Cation conductivity
Degassed cation conductivity
Temperature
Flow
Analyzer status
Alarm status
Calibration status
Maintenance status
Communication can use:
4–20 mA
Modbus RTU
Modbus TCP
Ethernet
Other plant-approved protocols
Historical trending can help operators identify gradual changes before they become major chemistry events.
Alarm Management
Potential alarms include:
High cation conductivity
High degassed cation conductivity
High specific conductivity
Low sample flow
High sample temperature
Analyzer fault
Sensor fault
Cation resin service
Degassing system fault
Communication failure
Alarm limits should be determined by the plant chemistry program and applicable equipment/OEM requirements. Generic values should not automatically be applied to every plant.
Maintenance Records
Good documentation is an important part of SWAS Panel Maintenance.
Records can include:
Conductivity calibration
Cation-resin replacement
Degassing-system maintenance
Sensor replacement
Sample cooler maintenance
Flow checks
Temperature checks
Alarm testing
Analyzer fault history
DCS/SCADA verification
Historical records make it easier to identify gradual degradation and recurring problems.
Selection Checklist for Degassed Cation Conductivity
When specifying a system, consider:
Process Data
Sample source
Pressure
Temperature
Flow
Sample chemistry
Measurement Requirements
Specific conductivity
Cation conductivity
Degassed cation conductivity
Required accuracy
Measurement range
Sample Conditioning
Pressure reduction
Cooling
Flow control
Filtration
Cation exchange
Degassing
Mechanical Design
Tubing material
Valve material
Panel construction
Drainage
Accessibility
Automation
PLC/HMI
DCS/SCADA
Communication protocol
Alarm requirements
Data logging
Maintenance
Resin replacement
Sensor maintenance
Degassing-system maintenance
Calibration
Spare parts
Dedicated vs Multiplexed Measurement
For plants with several sample points, degassed cation conductivity can be configured using dedicated or multiplexed arrangements.
Dedicated System
Each sample has a dedicated measurement channel.
Advantages:
Continuous monitoring
Faster response
No switching delay
Lower cross-contamination risk
Multiplexed System
One analyzer sequentially monitors several samples.
Advantages:
Lower analyzer count
Lower initial investment
Compact system
However, multiplexed systems require careful engineering of:
Switching valves
Flushing
Sample transport time
Cross-contamination
Measurement sequence
Response time
Critical applications may favor dedicated measurement.
Best Practices
For reliable degassed cation conductivity monitoring:
Select a representative sample point.
Engineer pressure reduction correctly.
Provide suitable sample cooling.
Maintain stable flow.
Use appropriate sample-line materials.
Maintain the cation exchanger properly.
Maintain the degassing system.
Keep the conductivity cell clean.
Prevent air ingress.
Verify temperature measurement.
Follow analyzer calibration procedures.
Trend specific, cation and degassed cation conductivity together where appropriate.
Integrate alarms into the DCS/SCADA system.
Investigate sample-system problems before replacing an analyzer.
Maintain adequate critical spares.
How Prosoft Automation Can Help
Prosoft Automation can support customers in designing application-specific SWAS Panel systems for steam and water chemistry monitoring.
Depending on the application, a system can integrate:
Specific conductivity
Cation conductivity
Degassed cation conductivity
pH
Dissolved oxygen
Silica
Sodium
Phosphate
Sample coolers
Pressure-reduction systems
Flow-control systems
Cation-exchange systems
PLC/HMI
DCS/SCADA integration
Alarm and data-logging functions
The final configuration can be engineered around the customer's sample points, process conditions, required parameters and plant automation philosophy.
Get a free site-application consultation to discuss your SWAS requirements, sample conditions and required online chemistry measurements.
Benefits of Degassed Cation Conductivity Monitoring
A properly designed system can help plants:
Improve cycle-chemistry diagnostics
Reduce the influence of dissolved CO₂
Investigate conductivity excursions
Identify potential contamination
Support condenser-leakage investigations
Improve steam-water chemistry monitoring
Protect boilers and turbines
Improve trend analysis
Support preventive maintenance
Provide additional information for plant operators
Frequently Asked Questions
What is degassed cation conductivity?
It is conductivity measured after the sample has undergone cation exchange and subsequent degassing to reduce the contribution from dissolved gases, particularly CO₂.
What is the difference between cation conductivity and degassed cation conductivity?
Cation conductivity is measured after cation exchange. Degassed cation conductivity adds a degassing step before the final conductivity measurement.
Why is CO₂ important?
Dissolved CO₂ can form carbonic-acid-related ionic species that contribute to conductivity. Removing dissolved CO₂ can therefore provide additional diagnostic information.
Is degassed cation conductivity better than cation conductivity?
Not necessarily. They serve complementary purposes. Degassed cation conductivity provides additional information that can help interpret the cation conductivity result.
Is degassed cation conductivity required in every SWAS Panel?
No. Its requirement depends on the plant chemistry program, process conditions and monitoring objectives.
Where is degassed cation conductivity commonly used?
It is particularly relevant to high-purity steam-water cycle applications such as thermal power plants, combined-cycle plants and HRSG systems.
Does degassed cation conductivity require a cation exchanger?
Yes. Cation exchange is normally performed before the degassing and conductivity measurement stages.
Can degassed cation conductivity be connected to a DCS?
Yes. Depending on the analyzer and plant architecture, the measurement can be integrated through analog and/or digital communication.
How often should the cation resin be replaced?
There is no universal replacement interval. Resin life depends on sample chemistry, loading, flow and system design. Follow the manufacturer's recommendations and monitor performance.
What should I check if the measurement is unstable?
Start with the complete sample path:
Process → Sample Point → Pressure → Cooling → Flow → Cation Exchanger → Degassing → Sensor → Analyzer → Signal → DCS/SCADA
This approach helps separate actual process chemistry changes from sample-system or instrument problems.
Conclusion
Degassed cation conductivity is an advanced and useful measurement for steam-water cycle chemistry applications where operators need additional information beyond conventional conductivity and cation conductivity.
The key distinction is simple:
Specific Conductivity: Direct conductivity measurement.
Cation Conductivity: Conductivity after cation exchange.
Degassed Cation Conductivity: Conductivity after cation exchange and removal/reduction of dissolved gases such as CO₂.
When properly integrated into an Online SWAS Panel, these measurements can provide valuable information for detecting and investigating contamination, condenser-related problems and changes in high-purity cycle chemistry.
Reliable performance depends not only on the conductivity analyzer but also on representative sampling, pressure reduction, cooling, flow control, cation exchange, degassing, sensor condition, calibration and proper SWAS Panel Maintenance.
For thermal power plants, combined-cycle plants, HRSG systems and other industrial applications, Prosoft Automation can help develop a SWAS solution based on actual process conditions and monitoring requirements.
Get a free site-application consultation to discuss your application and SWAS Panel requirements.