Non-Dispersive Infrared sensors provide stable, low-maintenance hydrocarbon gas detection for industrial safety, process monitoring, storage, and environmental applications.
A hydrocarbon sensor is normally calibrated and linearized for a specific target gas, such as propane. However, many hydrocarbons absorb infrared energy within similar wavelength bands. As a result, a propane-calibrated NDIR sensor may also respond to other gases.
Understanding these relative responses is essential when using cross-reference factors, interpreting readings, or configuring a sensor for a different hydrocarbon.
How NDIR Hydrocarbon Gas Detection Works
An NDIR hydrocarbon sensor measures the amount of infrared energy absorbed as light passes through a gas sample.
The main components include:
- A broadband infrared source
- An optical gas chamber
- Wavelength-specific filters
- An active detector
- A reference detector
- Signal-processing electronics
When hydrocarbon gas enters the optical path, it absorbs infrared energy within a characteristic wavelength band. As concentration increases, the signal reaching the active detector decreases.
The sensor electronics compare the active and reference signals and use the stored calibration curve to calculate the gas concentration.
Many hydrocarbons share overlapping absorption features within the mid-infrared spectrum. This allows a sensor calibrated for propane to respond to gases including butane, pentane, hexane, ethylene, ethane, ethanol, and methanol.
What Is Hydrocarbon Sensor Cross-Sensitivity?
Cross-sensitivity describes a sensor’s response to a gas other than the one for which it was originally calibrated.
In some applications, this response can be useful. If the gas composition is known and stable, a cross-reference factor may allow a propane-calibrated sensor to provide an indicative reading for another hydrocarbon.
The host instrument can store different factors and offer users a menu of selectable gas types. This can increase the flexibility of a single gas detector without requiring a separate sensor for every hydrocarbon.
Cross-sensitivity also creates an important limitation. If several hydrocarbons are present simultaneously, the sensor cannot identify how much of the signal comes from each gas. The resulting concentration reading becomes indeterminate.
Why Propane Is Used as a Calibration Baseline
Dynament hydrocarbon sensors are commonly characterized across a 0–2% volume propane range.
Propane provides a useful calibration reference because:
- It produces a strong infrared response: Its absorption characteristics align effectively with the sensor’s optical detection band.
- It provides a suitable response across the required range: The resulting calibration curve can support hydrocarbon monitoring across relevant flammable-gas concentrations.
- Several hydrocarbons produce comparable response shapes: For compatible gases, multiplication factors or alternative curve-fit data can be used to estimate concentration.
Methane is an important exception. Its response is not sufficiently similar to propane to support reliable cross-referral using the same method. A dedicated methane configuration should be used when methane measurement is required.
Using Multiplication Factors
For selected hydrocarbons, a linear multiplication factor can be applied to the propane reading to estimate the concentration of another gas.
Examples from the Premier Sensor Relative Response Calculations datasheet include:
| Gas | Multiplication factor |
| Butane | 0.97 |
| Pentane | 0.89 |
| Hexane | 0.80 |
| Ethylene | 3.43 |
| Propylene | 1.69 |
| Ethane | 1.01 |
| Cyclopentane | 1.62 |
| Chloromethane | 4.97 |
These multiplication factors apply only when concentrations are expressed as percentage by volume. According to the supporting data, reasonable accuracy can be maintained to at least the 50% LEL-equivalent value for the listed gases.
Additional guidance factors are available for substances including ethanol, methanol, toluene, isopropanol, acetone, xylene, and ethyl acetate. These values should be treated as indicative because vapor-generation methods and temperature can introduce additional uncertainty.
Cross-referred measurements should always be verified using an appropriate test gas wherever possible.
Multiplication Factor or Re-Linearization?
A multiplication factor provides a relatively simple way to estimate the concentration of another hydrocarbon. However, a single linear factor may not provide the required accuracy across the sensor’s complete measurement range.
For compatible Dynament hydrocarbon sensors, an alternative curve-fit can provide improved linearity for a selected gas.
This process requires:
- Access to the relevant curve-fit data
- Compatible configuration equipment
- Correct programming of the sensor
- Zero calibration
- Span calibration using a suitable target gas
- Verification across the required measurement range
Re-linearization requires more technical work than applying a multiplication factor, but it can provide a more representative output where improved accuracy is needed.
Users should consult the Dynament technical team before reconfiguring a sensor for a different target gas.
How Temperature Affects Cross-Referred Readings
Temperature compensation is based on the gas for which the sensor was originally calibrated, typically propane.
When the sensor is cross-referred to another hydrocarbon, differences between the gases may create additional error as ambient temperature moves away from the calibration temperature.
The relative-response datasheet includes temperature graphs for several gases. In one example, a propane-calibrated sensor exposed to 0.7% volume butane maintains the cross-referred reading within approximately ±10% of the applied value from −20 to +50 °C.
Performance will vary according to:
- The target hydrocarbon
- Its spectral similarity to propane
- Ambient temperature
- Gas concentration
- Sensor tolerances
- Calibration and test conditions
Testing with certified gas is particularly important in safety-critical applications.
Important Limitations in Mixed-Gas Environments
Cross-reference calculations are most useful when the target gas is known and the composition remains stable.
An NDIR hydrocarbon sensor cannot distinguish between multiple hydrocarbons that absorb within the same optical band. If a gas mixture is present, the reported value represents a combined optical response rather than the exact concentration of each component.
Cross-referred readings should therefore be treated as guidance rather than absolute measurements.
For flammable-gas safety applications involving potentially variable hydrocarbon mixtures, a propane-range sensor with appropriately established alarm thresholds may provide broad hazard detection. However, the sensor configuration, alarm strategy, and test requirements must be assessed for the specific application.
Certified test gases should be used to confirm that the complete detector responds appropriately to every relevant hazard.
Applications for Relative Hydrocarbon Responses
Hydrocarbon cross-reference data can support sensor configuration in applications including:
- Process lines containing a known hydrocarbon
- Fuel and solvent storage
- Confined-space monitoring
- Industrial leak detection
- Petrochemical equipment
- Portable gas detectors
- Environmental monitoring instruments
- OEM safety systems
The method is most effective when the expected gas is known, interfering gases are controlled, and measurement performance is verified under representative conditions.
Reliable Hydrocarbon Detection with Dynament
Understanding relative response is essential when using a propane-calibrated sensor to detect another hydrocarbon. Multiplication factors and alternative linearization curves can extend sensor flexibility, but they must be applied with careful consideration of temperature, gas mixtures, calibration, and the required measurement accuracy.
Dynament Platinum hydrocarbon sensors provide integrated NDIR measurement with linearized, temperature-compensated outputs for industrial and OEM gas detection applications.
Review the Premier Sensor Relative Response Calculations datasheet for the complete factors, response graphs, and curve-fit data.
Contact our technical team for guidance on gas selection, cross-referencing, sensor configuration, and application testing.
