LEL-MPS Sensor Technical Notes

MOLECULAR PROPERTY SPECTROMETER TECHNOLOGY

The latest innovation in combustible gas detection is a new LEL sensor utilizing Molecular Property Spectrometer (MPS) technology. This new LEL-MPS sensor has a myriad of benefits including:

  • Five-year expected lifetime
  • Classifies gases into one of six categories
  • Environmental compensation for temperature, pressure, and humidity
  • Immune to poisoning
  • Low power consumption
  • Excellent response time
  • Fail-safe self-diagnostic capability

This support article is a quick reference for some key topics related to the introduction of this sensing technology. For an overview of combustible gas detection as well as detailed information on the LEL-MPS sensor refer to Blackline’s comprehensive eBook being published in July 2020.

CALIBRATION VERIFICATION

By default, the calibration process for an LEL-MPS sensor will validate and ensure accuracy without adjusting the sensor readings. This sensor is factory calibrated for optimal accuracy and we recommend using the factory calibration for the lifetime of the sensor. Advanced users can perform a full calibration with a span adjustment by configuring this in Blackline Live which may negatively impact the accuracy of other gases.

GAS CYLINDER SELECTION

When bump testing or calibrating cartridges containing this sensor, Blackline recommends using a gas mixture containing at least 18% oxygen (O2). Less oxygen than this may impact the MPS sensor’s reading.

Gas cylinders often contain a “balance” gas.  This is done to allow the cylinder to be filled with a specific volume of test gas, then the reminder of the cylinder can be topped-up with a balance of other gas to fill the rest of the space and keep the measurements precise.  A balance of air means the cylinder is topped up with roughly the same mix of gases in the air we breathe. Around 78% nitrogen, 20.9% oxygen and 1% argon. The LEL-MPS sensor is designed for optimal accuracy in real world environments and therefore depends on some oxygen in the atmosphere to function. When applying gas to a device with an LEL-MPS sensor the gas cylinder should contain at least 18% oxygen (O2). If you apply gas using a cylinder balanced with nitrogen (N2) the lack of oxygen may impact the MPS sensor's reading and a power cycle of the device is recommended.

Typical quad gas mixes (H2S, CO, O2 and LEL methane) are balanced with nitrogen (N2), but contain 18% Oxygen. This mix will work for bump testing or calibrating LEL-MPS sensors because there is sufficient oxygen (O2) in the gas. Due to the chemical composition of some gases, nitrogen is required to keep the calibration gas stable in the cylinder. Some examples include NCH and NH3. These situations are rare, and a power cycle after a bump test is all that is required.

TROUBLE SHOOTING

If a cylinder balance with nitrogen (N2) and less than 18% oxygen (O2) is used to bump or calibrate an LEL-MPS sensor it will typically read 10% - 30% LEL after the operation even though it is in a clean air environment. The device will go into high gas alarm and remain this way for hours unless the device is power cycled. After power cycling the issue is resolved and the sensor will read accurately.

INERT ENVIRONMENTS AND LOW OXYGEN

The LEL-MPS sensor is designed for optimal accuracy in real world environments and therefore depends on some oxygen in the atmosphere to function accurately.

 Using LEL-MPS sensors when oxygen levels are below 18% may negatively impact accuracy. Furthermore, Blackline does not recommend use using MPS technology when oxygen levels are below 10%. This limitation is similar to that of pellistor/catalytic bead sensor technology.

When using a G7 monitor in inert environments with less than 10% oxygen, Blackline recommends using LEL-IR (infrared) sensor to detect combustible gases.

METHANE COMPARISON (2.5% VS. 2.2%)

As with all Blackline LEL supported sensors, 2.5 %vol methane (CH4) is considered 50 % ±2 %LEL (As per EN60079). Customers applying 2.2% methane will read as 44% LEL.

For customers who prefer 2.2%vol methane to read 50% ±2 %LEL can do this by first changing the device’s default configuration profile in Blackline Live to enable span adjustment. Then they can perform a calibration using 2.2% vol methane and the readings will be adjusted to display 50% ±2 %LEL.