When digging into the ground, trapped gases can be partially released into the air. When this occurs underground, air is confined in tunnels and a dangerous situation in terms of poisonous or explosive gas can result if a suitable ventilation system is not in place.
In South Africa, a mine can be closed if these standards are not adhered to. There are two parts of ventilation monitoring. The first involves measuring the concentration of the most commonly occurring gases in a particular mine; e.g. in a coal mine the concentration of methane in the air may be monitored. Secondly, the amount of air flowing in the shafts must be monitored to ensure that there are no too-high concentrations of gas.
Vortex flowmeters for mine ventilation
This kind of monitoring can be very tricky considering some mines are 4,000m underground. Human activity takes place on quite a few levels and trains run underground to transport the mined commodities. There are doors at certain shafts to prevent ‘ventilation shorting’. This works by the train opening the doors and closing them again after passing, in order to have enough ventilation left for deeper levels.
It is here that the SAFDY solid-state airflow sensor is useful. Based on the vortex method, which is proven to measure the airflow very accurately, the sensor is independent of small particles present in the air stream, such as water droplets.
Vortex-principle airflow sensors
When you stand on a bridge across a river, you can see very clearly that after the pillar the water shows ‘vortices’ which move downstream. When you can count those vortices you have an accurate measure of the water speed.
Exactly the same happens in airstream. Our instrument counts those vortices with a sound of 150kHz, which travels through the air medium and is modulated by the vortices. Small parts of dust or water have no influence on the vortices.
Solid-state airflow sensor
The SAFDY solid-state airflow sensor is a proven way to accurately measure airflow in mines, independent of small particles. It was originally designed for use in South African diamond and coal mines to monitor the airflow underground and to determine whether the ventilation system – crucial for human life – was functioning.
Available in fixed-mount and hand-held models, the system has a remote-sensing head to continuously monitor airflow and is designed to be interfaced to a remote telemetry system. The output of the SAFDY airflow sensor can easily be displayed as trend graphics on a SCADA software programme.
The SAFDY airflow meter has been found to be the most suitable for underground application from a durability and applicability point of view.
Airflow sensor technical data
- Supply voltage: 7V-24V DC
- Power consumption: 5mA + loop current (4mA-20mA)
- Normal range: 0.4m/s-15m/s
- Other ranges: DIL switch selectable, 0.4m/s-6m/s, 0.4m/s-2.5m/s, 0.4m/s-2m/s
- Output: 4mA-20mA or 0.1mA-1mA
- Housing: stainless-steel, powder-coated orange box
- Explosion protection: according to SABS Ex ib I/II T6
- Dimension: 250mm x 140mm x 75mm
- Weight: 1.6kg
Mine airflow sensor advantages
SAFDY airflow systems are intrinsically safe for use in coalmines, and highly accurate with a wide turn-down ratio and linear output. The systems have rapid response, excellent repeatability and rugged construction, with no moving parts or calibration. They are highly reliable and producible with a low lifecycle cost. The lifespan of the airflow detector can be in excess of ten years.
Other advantages include:
- Airflow speed maximum range of 40m/s or 144km/hr
- Linear output proportional to airspeed
- Highly accurate, dependant on the width of the ‘accentuatar’ bar
- Calibration is a function of the width of the bar
- Rapid response of change of airflow
- Excellent repeatability inherent to the vortex principles
- Very low lifecycle costs due to no moving parts
- Insensitive to small particles, dust or water droplets in the airstream
- Internal DIL switch for different ranges of airspeed
Handheld air velocity sensor
The Airwolf II is a handheld air velocity sensor which has been tested and proven over a long time in harsh South African underground mining conditions. It uses the ‘vortex shedding’ principle to accurately measure air velocity. There are no moving parts, making the instrument highly insensitive to dust, temperature and humidity. The unit comes with a leather pouch for easy belt attachment.