A building near or on a gas-producing site needs permanent gas monitoring that alarms before methane approaches dangerous levels. This post describes a multi-point sampling system designed for a commercial building in Ireland: 11 internal sample heads, methane, oxygen and hydrogen sulphide analysers, an audible alarm set at 5% of the lower explosive limit (LEL), and a bound certification pack.
The challenge: protect people inside a building
The system was for a building at a business park, one of several phases monitored by the same supplier under an earlier contract. The consultants asked for a defined test regime, a single bound document containing certificates, drawings and test certificates, and the full installation, labelled and protected.
The approach
What is measured
| Gas | Range | Detection level |
|---|---|---|
| Methane | 0 to 5% | 0.02% |
| Carbon dioxide | 0 to 3% | 0.01% |
| Hydrogen sulphide | 0 to 20 ppm | 0.1 ppm |
| Oxygen | 0 to 25% | 0.1% |
Carbon dioxide was in the first version and removed in the revision, as was a set of sample heads under the concrete slab (cutting the count to the 11 internal heads) and the number of protective covers dropped from 15 to 11.
How it is built
- Each sample head connects to the analysis board with 8 mm nylon tubing. Runs were calculated from the project drawing.
- Each head has a durable plastic cover with fine mesh filtration. Heads must remain accessible for maintenance.
- A switching mechanism accepts up to 24 sample tubes. Gas is filtered and conditioned before the analysis bench.
- Two sample pumps, a needle valve for flow adjustment, and a de-watering system.
- Two IP55 instrumentation cabinets, and an analogue board that passes data to the existing central data logger.
- One audible alarm on the ground floor, factory-set at 5% LEL in line with the guidance in Waste Management Paper No. 27, and operator adjustable.
An analyser for hydrogen sulphide came with noise attenuation, since pumps and analysers can be audible in occupied spaces. The scope excluded conduit through walls and assumed one mobilisation visit.
Why it works
A single analysis bench with a switching manifold is cheaper and easier to calibrate than a separate detector per room. The trade-off is that points are read in sequence, so response time depends on tube length and sweep rate. Using existing data logging avoids a second control suite.
Commercial note
The client asked for a review of two price items. The supplier declined to review them in isolation, pointing out that its prices had not changed significantly since the project began and offering a full price-basis review instead. That is a reminder that price changes belong in a whole-contract conversation.
What to consider for your own project
- Decide where points go, including below slabs, with a marked-up drawing.
- Set alarm levels deliberately. 5% LEL is a common first alarm.
- Specify documentation up front.
- Keep sensors accessible.
- Check the current guidance. Waste Management Paper No. 27 has been superseded; see the Environment Agency’s landfill gas guidance.
FAQ
What does 5% LEL mean?
Five per cent of the concentration at which the gas becomes explosive. It gives a margin before danger.
How many points can one system serve?
This switching mechanism accepts up to 24 tubes.
Monitoring a building?
Send us the plans. Related: well monitoring.