Thermal Dispersion Mass Flow Meters and Switches

A thermal mass flow meter is an instrument used to measure mass flow by utilizing heat transfer principles and the thermal properties of the media. The technology is based on L.V. King's Law, which states that the heat transfer from a heated sensor to a flowing gas is directly proportional to the mass flow rate.

Thermal mass flow meters operate by utilizing a flowing gas to cool a heated sensor where the gas absorbs heat through convection. As the gas flow increases the amount of heat removed from the sensor increases. There are two main measurement concepts utilized to relate the cooling effect to the flow rate, constant temperature differential or constant power.

Constant Temperature Differential: This method measures the mass flow rate by maintaining a constant temperature differential between a heated sensor and the process media. In this scheme there are two sensors, a measurement RTD and a reference RTD. The reference RTD measures the temperature of the process media and the measurement RTD is constantly heated to maintain a steady temperature differential between the process media and the sensor. The power needed to maintain the temperature differential is directly proportional to the mass flow rate.

Constant Power: For this method, the power supplied to the heated sensor remains constant. The temperature differential between two separate temperature probes is used to compute the mass flow, where a higher differential temperature indicates a higher mass flow.

In practice there are two designs of thermal mass flow meters utilizing these principles: thermal dispersion and bypass meters.

Thermal Dispersion: This design utilizes two RTD sensors in the measuring probe, one which is heated and one which measures the ambient media temperature. KOBOLD's thermal dispersion flow meters, such as the KEC and KET, employ the constant differential temperature method to compute the mass flow. This design offers both inline and insertion style meters.

Bypass: This design utilizes a flow bypass measuring system where a constant amount of heat is transferred to a subsection of the media creating a temperature differential between two RTD coils. The heating is done under constant power, and the resulting temperature differential is used to compute the mass flow rate. A differential pressure between the laminar flow element and the bypass tube ensures a defined, reproducible flow of gas through the bypass tube. The KOBOLD MAK is an example of the bypass design for thermal mass flow meters. This style is only offered as an inline flow meter.

Irrespective of the measurement design, thermal mass flow meters share the following characteristics, making them ideal for measuring gases:

• Provide a precise, true mass flow measurement
• Utilize heat transfer principles to determine the fluid flow rate
• The measurement is independent of temperature and pressure variations

Thermal flow meters provide continuous measurement of the mass flow rate with output signals like 4-20 mA or digital communication protocols for monitoring and process control. Thermal flow switches are discrete devices that trigger an on/off signal when flow reaches a predetermined setpoint, commonly used for alarm conditions or basic flow indication.

Thermal Flow Meter Features:

• Continuous flow rate measurement
• Analog or digital outputs (some have multiple outputs that also include a switching output)
• Process monitoring and data logging
• KOBOLD Products: KET, KEC, KME, MAK

Thermal Flow Switch Features:

• Binary on/off switching output
• Setpoint-based activation
• Alarm and safety applications
• Often more economical for simple monitoring
• KOBOLD products: KAL, KAL-K, KAL-L, KAL-D, KAL-A

Both technologies use thermal principles to determine flow but serve different control and monitoring needs.

Thermal mass flow meters are primarily designed for gas applications, where they excel at measuring argon, nitrogen, natural gas, compressed air, and other industrial gases due to the technology's ability to provide direct mass flow measurement without compensation for temperature or pressure. The KEC, KET, KME, and MAK represent KOBOLD's lineup of thermal mass flow meters designed for gas mass flow measurement.

Gas Applications (Thermal Mass Flow Meters):

• Compressed air systems
• Natural gas measurement
• Process gas monitoring
• HVAC air flow

Thermal flow meters are generally not used for liquids because of the differences in how liquids absorb and transfer heat compared to gases. However, specialized thermal flow sensors, such as the KOBOLD KAL series, are available for water-based liquids and offer dependable performance for specific liquid applications.

Liquid Applications (Thermal Flow Sensors):

• Water-based process fluids
• Cooling water circuits
• Food and beverage sanitary applications

These sensors offer switching outputs and basic indication of flow velocity, not continuous signals or mass flow rates. They are designed for and work best with clean, water-based media and are not recommended for use with oils or fuels.

A calorimetric flow meter is a specific type of thermal flow meter utilizing the bypass measurement method where the media is heated and temperature sensors upstream and downstream of the heating element measure a temperature differential that is proportional to the mass flow rate.

This term calorimetric is sometimes used with thermal dispersion and thermal mass flow meters as a description of the thermal measurement principle in general and is not to be confused with the specific calorimetric flow meter. The term calorimetric references calorimetry, the science of measuring heat transfer.

Ultimately, all variations of thermal flow meters including calorimetric flow meters operate on the same fundamental principle of measuring flow by quantifying the heat transfer between a heated element and the flowing media, despite their slight differences in measurement approaches.

No, thermal mass flow meters provide direct mass flow measurement and do not require pressure and temperature compensation. This is a significant advantage over volumetric flow measurement technologies that measure volume and must correct for changing pressure and temperature conditions for gases.

Direct Mass Measurement Benefits:

• Accurate readings regardless of pressure variations
• No correction needed for temperature fluctuations
• Simplified installation with fewer instruments
• Reduced calibration complexity
• Lower total system cost

The thermal dispersion principle directly measures the molecular mass flowing past the sensor. Since mass remains constant regardless of pressure or temperature changes, thermal flow meters deliver consistent, reliable measurements across varying process conditions.

Inline thermal flow meters are installed directly in the pipe as a spool piece, with the flow passing through the meter body. Insertion thermal flow meters use a probe that extends into the existing pipe through a fitting, measuring flow at a specific point in the cross-section.

Inline Configuration:

• Complete pipe section replacement
• Best for smaller pipe diameters (typically under 4")
• Higher accuracy across full pipe cross-section
• More expensive for large pipes

Insertion Configuration:

• Probe inserted through pipe wall
• Economical for large diameter pipes
• Easier retrofit installation
• Requires proper positioning for representative measurement

Both styles use thermal dispersion measurement principles. Selection depends on pipe size, installation constraints, accuracy requirements, and economic considerations for your specific application.

Thermal mass flow meters offer accuracy consistent with other advanced flow measurement technologies. The exact accuracy depends on the meter design, application conditions, and the calibration. Accuracies for thermal mass flow meters are reported as either the full scale accuracy, the reading accuracy, or as a combination of a full scale and reading accuracy. Reading accuracies typically range from ±1% to ±5%, full scale accuracies generally range from ±1% to ±3%. High-performance industrial models such as the KEC can achieve ±(1% reading + 0.3% full scale) accuracy with the precision calibration option.

• Gas-specific calibration improves precision
• Flow velocity within optimal range
• Stable process conditions
• Proper installation and positioning
• Regular maintenance and verification

Additional Performance Characteristics:

• High repeatability (typically ±0.2% to ±0.5%)
• Wide range of flow and turndown ratios
• Fast response time to flow changes between 1 to 3 seconds depending on the model

Thermal mass flow meters are highly capable of operating under a variety of demanding conditions to provide reliable measurement of gases even with variations to process pressure or temperature. There are a number of circumstance where thermal mass flow meters would not be the ideal choice.

• Liquid Measurements: Thermal mass flow meters are intended primarily for mass flow measurements of gases. The KAL series is, however, available for water based media for basic flow indication and detection but does not give a true mass flow rate output.
• Contaminated Gases: These meters are not suited for dirty gases containing solid particles or other contaminants that can coat the sensors.
• Moisture: Condensation, aerosols, or gases that drop below their dew point during the measurement can cause significant errors as the thermal properties of liquids vary significantly from the gases. This can lead to high measurements or false readings. Small amounts of vapor or normal air humidity are usually acceptable and only have a minimal impact on the readings.
• Gas Composition: Changes in the gas composition or the type of gas altogether can cause measurements errors due to the different thermal properties of gases. Thermal mass flow meters are calibrated for a specific gas or gas composition, and measurements of different gases requires either a correction factor or recalibration.

In addition, attention should be paid to the limitations to the meter in terms of process conditions. Thermal mass flow meters are intended to operate within defined pressure, temperature, and flow ranges and operating the meter outside of these ranges is not recommended.

Thermal mass flow meters and switches serve diverse industrial sectors requiring reliable gas and liquid flow measurement. The technology's ability to measure low flows, handle varying pressure conditions, and provide mass flow data makes it valuable across multiple industries.

Primary Industries:

• Oil & Gas (upstream, midstream, downstream operations)
• Power Generation (boiler feed, flare gas, emissions monitoring)
• Chemical Processing (reaction control, batch processes)
• Wastewater Treatment (aeration monitoring, biogas measurement)
• HVAC (air flow monitoring, energy management)
• Pharmaceuticals (clean gas measurement, process control)
• Electronics Manufacturing (low flowrate, ultrapure gases)
• General Manufacturing (industrial process gases, compressed air consumption, leak detection)

Each industry leverages specific advantages of thermal dispersion technology, whether for energy management, process optimization, regulatory compliance, or equipment protection applications.

Thermal mass flow meters excel in applications requiring direct mass measurement of gases. Their robust design and pressure and temperature independent measurement make them particularly suitable for challenging industrial gas applications.

Ideal Applications:

• Compressed air consumption monitoring and leak detection
• Flare gas and vent gas monitoring for emissions reporting
• Biogas and landfill gas measurement in renewable energy
• Stack gas and combustion air in power plants
• Aeration control in wastewater treatment facilities
• Process gas monitoring in chemical plants
• HVAC air flow measurement and control

The technology performs exceptionally well in applications with varying pressure conditions, low flow rates requiring sensitive detection, or where pressure compensation would add complexity and cost to volumetric measurement solutions.

Yes, thermal mass flow meters are excellent choices for compressed air measurement and are widely used for energy management and leak detection programs. These applications leverage the technology's sensitivity to low flows and ability to measure mass flow without pressure compensation.

Compressed Air Measurement Benefits:

• Direct mass flow measurement unaffected by pressure fluctuations
• High sensitivity detects small leaks
• Wide turndown ratio measures both demand and baseline consumption
• No moving parts for reliable performance
• Lower pressure drop than mechanical meters

Leak Detection Applications:

• Baseline flow monitoring during off-hours
• Plant or machine-level sub-metering
• Compressed air system audits
• Maintenance prioritization based on consumption data

With compressed air representing significant energy costs in industrial facilities, thermal flow meters provide the accurate, reliable data needed for optimization programs.

Selecting the appropriate thermal mass flow meter requires evaluating several application parameters to ensure optimal performance. Start by clearly defining your media type, flow range, process conditions, and output requirements.

Key Selection Criteria:

• Media Type: Gas type or composition (gas mixtures)
• Flow Range: Minimum, nominal, and maximum flow rates expected
• Process Conditions: Pressure and temperature ranges
• Accuracy Requirements: Process control vs. monitoring needs
• Pipe Size: Pipe diameter to determine suitability for inline for insertion style
• Output Signals: Analog, digital, switching, or combination
• Installation Constraints: Available space, piping configuration

Additional Considerations:

• Calibration for specific gas
• Hazardous area certifications if needed
• Materials of construction for media compatibility

KOBOLD application engineers are available to help match your requirements to the right thermal flow meter solution for reliable, long-term performance. Schedule a free engineering consultation today.

Thermal mass flow meters and thermal flow switches require minimal maintenance due to their design with no moving parts and robust materials. The sensor construction ensures reliable operation with infrequent service needs, reducing lifecycle costs.

Routine Maintenance Tasks:

• Periodic verification of calibration accuracy
• Inspection of sensor for fouling or deposits (if applicable to media)
• Electrical connection verification
• Zero-point check for drift

Maintenance Frequency:

• Annual calibration verification for most applications, shorter for critical applications
• More frequent inspection for dirty gases
• Minimal intervention for clean media applications

Advantages of Thermal Mass Flow Technology:

• No mechanical wear components
• Resistant to vibration and shock
• Extended service intervals compared to mechanical meters

Yes, the meter must be configured for the actual gas or gas mixture used in your application for accurate measurement. If feasible, the flowmeter can be calibrated with the specific gas intended for measurement. If that is not possible it may be possible for the flowmeter to calibrated with a standard calibration gas, such as air or nitrogen, and configured with correction factors to the specific gas to be measured if the flowmeter supports that.

Calibration Options:

• Factory calibration for specified gas
• Factory calibration with standard gas and use of correction factor to configure the measurement for the intended gas
• Field or factory recalibration for gas composition changes
• Calibration certificates for traceability

Gas Property Considerations:

• Thermal conductivity varies by gas type
• Specific heat capacity affects heat transfer
• Gas mixture composition impacts measurement

Changing gas composition without recalibration or correction will affect measurement accuracy, so inform your supplier of the exact gas or mixture for proper meter configuration.

Thermal mass flow meters offer exceptional turndown ratios, typically ranging from 50:1 to 100:1, with some advanced designs achieving ratios of 1000:1. This wide operating range allows a single meter to accurately measure both very low and high flow rates.

Turndown Ratio Benefits:

• Single meter covers broad flow range
• Accurate measurement of process variations
• Reduced inventory of different meter sizes
• Better process visibility across operating conditions

High turndown capability makes thermal dispersion flow meters especially valuable in applications where flow conditions change significantly. The meter maintains accuracy throughout the range without requiring multiple instruments or meter sizing compromises typical of more limited technologies.