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Methane Detector Lag Times. Methane Detectors Do Not Give Instantaneous Measurement And Reporting Of Actual Methane Concentrations. Grosvenor Inquiry Seems To Have Been Misinformed Of How Methane Detectors Operate

Methane Detector Lag Times. Methane Detectors do not give Instantaneous Measurement and Reporting of Actual Methane Concentrations. Grosvenor Inquiry seems to have been misinformed of how Methane Detectors Operate

NSW Response-times-of-gas-detectors Sept 2017

Can we please dispense with the falsehood or misinformation concerning methane monitors providing instantaneous readings of the actual methane concentration at the Grosvenor Inquiry?

It is creating totally false impression at the Grosvenor Inquiry about when the methane detectors will trip power for >2.5% CH4.

The only part of the sensors operation that is “Instantaneous” is the information it is registering being relayed.

That is why the  Australian Standard AS/NZS 60079.29.1. is worded the way it is

To achieve registration, a methane detector must comply with the requirements of Australian Standard AS/NZS 60079.29.1.

This standard requires the time for a sensor to reach 50% of the test gas concentration (t(50)) of no greater than 10 seconds and the time for the sensor to reach 90% of the test gas concentration (t(90)) of no greater than 30 seconds.

 

This has been long known.

In 2000 the Queensland Mines Department issued Mines safety bulletin no. 18 | 17 July 2000 | Version 1 Authorised by Roger Billingham – Deputy Chief Inspector of Mines

It concludes

It is essential that this delay in recording the gas concentrations is taken into account when considering the risks associated with flammable gas hazards.

https://www.rshq.qld.gov.au/safety-notices/mines/methane-gas-monitor-response-times

In some applications this delay may be significant. For example the methane monitors on the cutting heads of continuous miners in coal mines, which are required to be set to trip power to the machine at a maximum of 2%v/v to prevent exposure of the machine electric’s to explosive mixtures, cannot be relied upon to prevent frictional ignition where sufficient methane is released from the coal by the cutting action. Under these circumstances an accumulation of flammable gas could occur in the time taken for the monitor to record its presence and trip power to the machine.

There is a lag time from the sensor head reaction commencing/changing till it actually registers 90% of the actual methane concentration

This is a safety alert issued by NSW in 2017

NSW Response-times-of-gas-detectors Sept 2017

Incident

Following entry into an underground roadway at a coal mine, a longwall deputy’s gas detector alarmed at 2% methane.  The gas detector peaked at 3.5% during the deputy’s retreat from this roadway. This incident occurred on 5 September 2017. 

Circumstances

An LHD vehicle hit a water trap on the methane drainage line underground causing the water trap to break off at a ‘T’ piece. The operator of the vehicle was unaware of the damage. The gas drainage plant had been shut down for maintenance activities resulting in the gas drainage range being pressurised. The damage to the drainage pipe range led to methane gas issuing under pressure into the underground roadway.

Gas detector requirements
While inspecting roadways inbye of the damaged water trap, a deputy’s gas detector alarmed at 2% methane. He retreated from the roadway with his gas detector peaking at 3.5%. The deputy had entered a methane concentration greater than 2% without being aware. This was because of the inability of the gas detection instrument to instantaneously measure the actual concentration of methane in this situation.

Methane detectors for underground coal mines in NSW are required to be design registered. To achieve registration, a methane detector must comply with the requirements of Australian Standard AS/NZS 60079.29.1.

This standard requires the time for a sensor to reach 50% of the test gas concentration (t(50)) of no greater than 10 seconds and the time for the sensor to reach 90% of the test gas concentration (t(90)) of no greater than 30 seconds.

The link below is to a power point presentation entitled

“Are Your Gas Detectors Actually Working” by Tim Wylie, Chief Technical Officer AMP Control

Ampcontrol https://ampcontrolgroup.com/wp-content/uploads/2019/08/Gasguard-2.pdf

t90 Response Time Test

t90 is a critical parameter in ensuring the intended safety function of real time monitoring of gas
concentrations.
Detector response times are defined as time taken to get to 90% of final value from an initial step input.
Gasguard 2 has an automated feature allowing ease and accuracy of this measurement.

 

Mines safety bulletin no. 18 | 17 July 2000 | Version 1

Methane gas monitor response times

All gas detecting instruments have an inbuilt delay, or response time, between being introduced to a gas and recording the gas concentration. Response time is usually referred to as the t(90) time.

The standard AS/NZS 61779 ‘Electrical apparatus for the detection and measurement of flammable gas’ defines the response time as being the time from when a gas sensing element in clean air is suddenly exposed to a prepared mixture of gas in air having a concentration corresponding to 100% of full scale gas concentration.

The t(90) time is the time taken for the instrument to record 90% of full scale gas concentration. To meet the requirements of AS/NZS 61779 an instrument must have a t(90) time of less than 30 seconds.

A practical test on a machine mounted methane monitor commonly used in the coal industry was recently carried out This instrument was suddenly introduced to a 4%v/v methane at 0.5L/min and the time taken for the instrument to read 1.25% and 2% recorded. The test was repeated a number of times and the average time was:

  • 1.25% – 8.22 seconds
  • 2% – 10.5 seconds

The 4%v/v methane was chosen as being as close as was practically available to an explosive mixture of 5%v/v methane.

In some applications this delay may be significant. For example the methane monitors on the cutting heads of continuous miners in coal mines, which are required to be set to trip power to the machine at a maximum of 2%v/v to prevent exposure of the machine electric’s to explosive mixtures, cannot be relied upon to prevent frictional ignition where sufficient methane is released from the coal by the cutting action. Under these circumstances an accumulation of flammable gas could occur in the time taken for the monitor to record its presence and trip power to the machine.

It is essential that this delay in recording the gas concentrations is taken into account when considering the risks associated with flammable gas hazards.

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