A Comparison Guide

A comparison of the industry's legacy technology to NevadaNano's next-generation, cutting-edge approach, the MPS™ Flammable Gas Sensor

Introduction

The Molecular Property Spectrometer™ (MPS) represents the first completely innovative technology for flammable gas detection in over 40 years, and was designed to overcome the shortcomings of existing technologies. This guide provides a categorical comparison of the MPS vs. existing technologies, including their sensing methods and corresponding advantages and limitations.

Catalytic (“Cat”) Bead Sensors

How they work: A pair of small beads—one coated with a chemical catalyst, the other with an inert material—are both heated to a high temperature (400 500°C) using heaters (e.g., coiled platinum wire) built into their cores. In the presence of a flammable gas, the catalyst-coated bead produces an exothermic reaction, causing it to heat up more than the reference bead. This temperature difference can be measured using a resistance bridge circuit, the output of which is proportional to the concentration of the flammable gas present.

Key advantages:

  • Low cost
  • Detect full range of flammable gases (from hydrogen to heavy hydrocarbons)
  • Built-in environmental compensation

Key limitations:

  • Common chemicals—including silicones, chlorine, and acidic gases-deactivate, or “poison,” the catalyst bead. This can happen gradually, or within minutes, depending on the environment.
  • Flammable gases at high concentrations can “burn up” the catalyst, deactivating the sensors.
  • Not fail-safe. Poisoned or burned-out sensors appear to be operating normally. Once discovered (via bump check or re-calibration, e.g.) the sensor must be replaced.
  • Every gas heats the catalytic bead differently, so calibration to a single gas (e.g. methane) means the sensor will output inaccurately for all other gases. (See Figure 1.)
Non-dispersive Infrared (NDIR) Sensors

How they work: Infrared light is passed through a small chamber exposed to the air. Flammable gas molecules absorb certain wavelengths of this light more than others. Using a pair of detectors with different filters, the intensity of light detected in the wavelength range a flammable gas absorbs is compared to the intensity from a range it does not absorb. The difference is proportional to the concentration of flammable gas present.

Key advantages:

  • Long life
  • Resistant to contamination and poisoning
  • Fail-safe (built-in sensor diagnostics detect inoperable sensors)
  • Lower power
  • Built-in environmental compensation

Key limitations:

  • Hydrogen cannot be detected (because it does not absorb infrared light).
  • The open chamber can allow in humidity, fog, and ambient IR light, all of which cause interference.
  • Susceptible to moderate changes (0.6 to 2.0 °C/min) in temperature/humidity (e.g. moving from
    freezing cold outdoors to warm, humid indoors during winter). Some products freeze their output
    during temperature transitions.
  • Every gas has a unique absorption profile, so calibration to a single gas (e.g. methane) means the
    sensor will output inaccurately for all other gases. (See Figure 1.)
MPS™ Flammable Gas Sensors

How they work: A micro-electromechanical system (MEMS) transducer—comprising an inert, micrometer-scale
membrane with an embedded heater and thermometer—measures changes in the thermal properties of the air and
gases in its proximity. Multiple measurements, akin to a thermal “spectrum,” as well as environmental data are
processed to classify the type and concentration of flammable gas(es) present.

Key advantages:

  • Long life
  • Resistant to contamination and poisoning (the measurement is purely physical, not a chemical reaction)
  • Fail-safe (built-in sensor diagnostics detect inoperable sensors)
  • Lower power
  • Built-in environmental compensation
  • Detects full range of flammable gases (from hydrogen to heavy hydrocarbons)
  • Accurate to 12 flammable gases with a single calibration to methane. See Figure 1. (To achieve this with
    Cat Bead or NDIR sensors, the user would need to deploy detectors for every gas of interest.) Gases are
    automatically classified into one of the following categories: hydrogen; hydrogen-containing mixtures;
  • methane (or natural gas); light, medium or heavy gases/mixtures.
Comparison Matrix

 

Comparison Matrix

Table 1: Relative performance in key categories of the three main sensor types.

Conclusion

The new MPS Flammable Gas Sensor delivers accurate flammable gas measurement without the limitations
inherent to catalytic bead and NDIR flammable gas sensors. MPS Flammable Gas Sensors open the opportunity to
upgrade existing detectors and to introduce new applications where low-maintenance, accurate measurement of
multiple gases, stability over broad environmental conditions, and low power are critical to the application.

 

MPS CAT beadtypical NDIR

Figure 1: The delivered vs. reported concentrations of selected flammable gases, for each of the three sensor
Conclusion types when calibrated to methane.