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This article looks at the use of whirling hygrometers to measure and assess heat and humidity in the mining environment.
The mine environment exposes workers to a wide range of obvious dangers: falls caused by unstable ground, contact with moving parts of machinery and inhalation of dusts to name but a few. A less noticeable, but nonetheless potential life threatening danger, however, is exposure to hot and humid conditions.
Access to future reserves has required mine shafts and workings to become deeper and more extensive. These factors, combined with the use of heavy-duty equipment, have resulted in mine workings becoming hotter. The use of water, and on occasion the natural presence of water in the mine, has also led to humid atmospheres being encountered during normal mining operations. In the event of a fire or spontaneous combustion, these conditions can become more hazardous to persons operating within them, especially when workers are required to wear breathing apparatus.
Defining heat and humidity
Heat is one of the primary sensations produced by contact with, or nearness to, fire or any other source of a high temperature. In mines this is produced from depth of workings, oxidation of coal, machinery, people working and friction due to mine ventilation coming into contact with roadway roof, sides and floor.
Humidity refers to the degree of moisture found in the atmosphere. The air around us can absorb water. A simple example of this principle is the ‘drying’ of a mopped floor. The water on the floor eventually evaporates and is contained within the air. When air contains moisture it is said to be humid. The amount of moisture that can be absorbed by the surrounding air is limited – think of air as being a sponge capable of absorbing water. When air reaches the point at which it can hold no more water the atmosphere is referred to as being totally saturated.
The combination of heat and humidity can be defined, therefore, as a warm environment in which the air contains a certain amount of moisture. The moisture content may vary until the saturation point is reached.
Monitoring instruments
As mine atmospheres contain varying amounts of moisture that can be present at varying temperatures due to the prevailing conditions encountered, it then follows that their presence can have important effects on a person’s safety and their ability to perform hard work.
Over the years, extensive trials have been carried out to determine the effects of working in hot and humid conditions. From the results obtained, ‘safe working periods’ have been devised for use by rescue workers wearing breathing apparatus within these conditions. To determine the safe working period for a rescue operative, it is necessary to measure the actual heat and humidity present in the areas in which teams are currently working – think of it as risk assessing the working environment.
The instrument used to measure the heat and humidity present in the mine atmosphere is a whirling hygrometer. The results obtained from the whirling hygrometer have to be interpreted and this is achieved using a set of hygrometric tables. These are sometimes referred to as the Linds chart , so named after the person who devised them.
Whirling hygrometer
The relative humidity of air is readily ascertained from hygrometrical tables when the readings from a wet and dry bulb hygrometer are known. This instrument simply consists of two thermometers. One thermometer has its bulb exposed to the air and is known as the dry bulb. The dry bulb will measure the temperature of the surrounding atmosphere. The second thermometer has its bulb wrapped up in a piece of muslin, which dips into a reservoir of distilled water. Evaporation of the water from the wet muslin into the surrounding atmosphere reduces the temperature of the of the wet bulb thermometer in direct proportion to the dryness of the surroundings. The two readings obtained are plotted on a hygrometrical table for the breathing apparatus being worn and a safe working time will be determined.
The whirling hygrometer is a hand-held instrument approximately 23cm long and 5cm wide, excluding the handle. The main frame is made of wood or plastic. A handle passes through the frame at the top, supported by bearings which, when spun, allow the frame to revolve freely. Both thermometers are cushioned and firmly fixed within the frame. As mentioned previously, the dry bulb is open to the atmosphere and the wet bulb is covered by a fine muslin sleeve, which is kept moist with distilled water. A screwed cap is fitted on one end of the reservoir to prevent loss of water when the hygrometer is rotated.
Pre-use examinations
1. Before use, ensure that the thermometers are: • Mercury based • Secure in the frame • Unbroken and intact • Easily readable • Calibrated in degrees Celsius and both of the same type, to include a range of -5 to 50ºC
2. Check that the handle is secure and that the frame revolves freely.
3. Examine the muslin sleeve and check that it covers the bulb completely, that it is in good condition and reasonably clean.
4. Check to ensure that the water vessel has been charged with distilled water and that the end cap is in position and secured.
Using the whirling hygrometer
The above checks should be made on the surface so that any defects or damage can be made good. At this point it is also advisable to check that the hygrometric chart is carried.
When a reading is required, the team member who is to obtain the reading should separate himself, preferably upstream of the ventilation, to ensure that the team and the apparatus do not influence the results.
The two thermometers should be checked to ensure they are in a readable position. The operator should then commence to revolve their hygrometer at arms length, taking note of the time and also ensuring that the instrument is clear of all obstructions while being spun, at the same time traversing the whole cross-section of the roadway. The hygrometer should be spun at arms length at a speed of about 200 revolutions per minute for one full minute. This would produce a simulated air speed of 3.5 metres per second. This rapid airflow is fundamental to the operation of the instrument and the speed at which the wet bulb is influenced.
As soon as the hygrometer comes to rest, the air surrounding the wet bulb will pick up moisture from the wet muslin and the humidity of that small pocket of air will be artificially increased, which in turn will slightly alter the reading indicated on the wet bulb. For this reason, the wet bulb should be read before the dry bulb at the precise moment the rotation ceases.
It is important to avoid contact with the thermometers when reading the temperatures obtained as all sources of heat, such as the operator’s hands or cap lamp, can influence the thermometer readings. The wet bulb reading should never be in excess of the dry bulb reading.
Interpreting results
Using the hygrometric chart, we can see that should an operator obtain a wet bulb reading of 30ºC and a dry bulb reading of 34ºC, then the safe working time for a team wearing BG4 breathing apparatus will be 42 minutes.
Example 1 A team is being deployed to work on a return stopping. The travelling distance to the stopping is 20 minutes. Hygrometer readings in this roadway indicate a safe working time of 120 minutes. At a junction leading into stopping area the safe working time is 40 minutes. The team, therefore, has a 40 minute safe working time in the hot and humid atmosphere and two 20 minute periods for travelling.
Example 2 A team has left the fresh air base (FAB) and has taken a hygrometer reading at the first sample point.
From the readings obtained the operational time is 120 minutes. After travelling for 10 minutes the team passes a junction and a second sample is taken at the second sample point. The operational time from the reading obtained is now 90 minutes. After a further 10 minutes the team takes a hygrometer reading at the third sample point, where the safe working time is now 40 minutes. The captain should ascertain that the team has a 40-minute wearing period beyond sample point three. On return to sample point three, the team has 20-minutes of travelling time to FAB in improved environmental conditions.
Human heat regulation
Heat and humidity can have a major impact on mining rescue operations. In order to appreciate their significance it is necessary to look at the human body and how rises in temperature can affect its operation.
The optimum temperature at which the human body performs is within the range of 36-38°C. To preserve this, the body must maintain a balance between heat gain and heat loss, and a ‘thermostat’ within the brain regulates this. The body’s metabolic system produces heat through the conversion of food into energy and also by muscular activity. Certain illnesses can affect body temperature, as can the environment to which people are exposed. If body heat is not dissipated then there are potential health risks from heat exhaustion and heat stroke.
The human body can lose heat in the following ways.
Heat may be lost to: • Cool surrounding air – by radiation and by evaporation from the skin, and in the breath • Cool objects in contact with the skin, which provide a pathway for heat to escape
In hot conditions, the body attempts to lose heat by: • Dilating blood vessels that lie in or near the skin • Activating sweat glands • Increasing the rate and depth of breathing – warm air is expelled and cool air is drawn in to replace it, cooling the blood as it passes through the lungs
Effects of extreme heat
When the atmospheric temperature is the same as your body temperature, the body cannot lose heat by radiation or by evaporation. If there is also a humid atmosphere then it may affect the evaporation of sweat, as the surrounding air may not be able to absorb it, thereby impeding the body’s cooling process. These circumstances, coupled with strenuous exercise during which the body will generate more heat, can lead to heat exhaustion and heat stroke.
Heat exhaustion This condition usually develops gradually and is caused by loss of salt and water from the body through excessive sweating.
As the condition develops the sufferer may experience: • Headaches, dizziness and confusion • Loss of appetite and nausea • Sweating, with pale, clammy skin • Cramps in the arms, legs or abdominal wall • Rapid, weakening pulse and breathing
Heat stroke A failure of the ‘thermostat’ in the brain causes heat stroke. The body overheats due to high fever or prolonged exposure to heat. In some cases, it follows heat exhaustion when sweating ceases, and the body cannot be cooled by evaporation. Heatstroke can occur suddenly, causing unconsciousness within minutes. This may be signalled by the casualty feeling uneasy and ill.
As the condition develops it may cause: • Headaches, dizziness and discomfort • Restlessness and confusion • Hot, flushed and dry skin • A rapid deterioration in the level of response • A full, bounding pulse • Body temperature to rise above 40°C
Working out operational times is straightforward when teams are operating in environments in which the heat and humidity temperatures are fairly constant. On some occasions, a team may travel through mine workings in which the temperatures encountered can be variable. In such circumstances, team captains must pay particular attention to the condition of team members and the interpretation of results obtained using a whirling hygrometer.
Published: 04th Mar 2015 in AWE International
