Natural Wet Bulb Thermometer

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Natural wet bulb thermometer (ta, wb):

Natural Wet Bulb Thermometer instrument consists of mercury-in- glass thermometer, whose bulb is covered by a highly absorbent fabric wick, woven cotton being the most suitable. A recommended range for the thermometer is -5°C to 50°C, with O.5°C graduations. The accuracy should be at least +0.5°C.

The wick extends over the stem of the thermometer about one additional bulb length (or about 3 cm) and it must be long enough so that

(a) its other end can be immersed in distilled water, which is kept in a container underneath, and

(b) about 2.5 cm of wetted wick remain exposed to the air (between the top of the container and the bottom of the bulb).

During the operation, the wick must be kept wet to its top, which is usually achieved by keeping its loose end in the distilled water.

However, under too hot and/or too dry conditions, this may become difficult and additional wetting of the wick may be required (by means, for example, of a washing bottle or similar). The natural wet bulb thermometer is exposed to natural air movement Contrary to the set-bulb thermometer there is not artificial air movement over its bulb. Also, the natural wet bulb thermometer is not shielded from radiant heat.

Natural Wet Bulb Thermometer

Procedure of Natural Wet Bulb Thermometer:

The steps to followed for the determination of WBGT are The wick must thoroughly wetted with distilled water about thirty minutes prior to the measurement. This accomplished by means of a syringe. Washing bottle of similar container with distilled water.

The loose end of the wick is kept in touch with the distilled water in the open container; so that it will be kept wet all the time through capillarity. In very hot dry environments, capillarity may not be enough to maintain the wick wet and it tends to become dry. In such situations, it is necessary to wet the wick. At intervals, by means of a syringe or washing bottle containing distilled water.

The natural wet bulb thermometer is then placed in a. vertical position, at the measuring point, as well as the globe thermometer.

When the reading of the natural wet bulb thermometer stabilizes, which usually takes about 25 minutes, it gives the natural wet bulb temperature tn wb.

When the reading of the globe thermometer stabilizes (in about 20-25 minutes) it gives the globe temperature, t8. If a dry bulb thermometer used, its reading, when stabilized, gives the air temperature ta. The WBGT can then calculated, through an equation. As a guideline for estimating the time required for a survey, it should be kept in mind that, for each set of measurements, at least 30 minutes are necessary. Therefore, under the best conditions and assuming that distances between sampling sizes are not great; a maximum of 15 or 16 determinations can expected to be carried out during 8 hour working day, for each set of instruments.


  • The thermometer must kept in a vertical position
  • The wick for the natural wet bulb thermometer must kept clean, and should changed and washed, as necessary.
  • There should be no restriction to the air flow around the bulb.

Fundamental Equation of Natural Wet Bulb Thermometer:

WBGT = 0.7 tnwb + 0.3 (tg – ta) Where, Tmwb =natural wet bulb temperature tg = globe temperature. ta = dry bulb temperature

For outdoor exposures with solar load

Minard has proposed a simplified formula which has adopted, for instance; by the American Conference of Governmental Industrial Hygienists (Threshold Limit Values) as follows WBGT = 0.07Tnwb+0.2 tg +0.1 ta

Indoor exposure or outdoor exposures with no solar load

WBGT = 07 Tnwb + 0.3 tg Time weighted average WBGT (WBGT ). When environment conditions vary significantly, or the workers move through places and jobs that cause different levels of heat stress, a time weighted average WBGT should calculated as follows:

WBGT = (WBGT1)b + (WBGT1)b + ………+ (WBGTp)tp / t1+t2+………+tn

WBGT = WBGT determined for situation or location 1

WBGTn = WBGT determined for situation or location n

t1. t2…….tn= time spent respectively, in location 1,2…

Natural Wet Bulb Thermometer

Time weighted average WBGT values must be calculated on an hourly basis if-the heat exposure is continuous, and not for a 8 hour period. A very extreme heat exposure for over 1hour might cause health impairment. An inappreciable exposure for the rest of the shift might bring the calculated time weighted average WBGT to a value even below the permissible level but damage would already have done to the worker’s health. For intermittent heat exposure, this time weighted average can calculated on a 2 hourly basis.

In some cases, where the worker exposed to a wide and variable range of environmental conditions, it may very difficult to determine a meaningful time weighted average WBGT.

Permissible Heat Exposure:

The permissible heat exposure threshold limit values recommended for use in the United States given in the following table:

Permissible heat exposure threshold limit values

(Values are given in °C WBGT)

Work- Rest RegimenLightMode- rateHeavy
Continuous Work30.026.725.0
75%Work 25 % rest Each hour30.6 30.628.025.9
50%Work 50 % rest Each hour31.4 31.429.427.9
25%Work 75 % rest Each hour32.2


The determination of the WBGT index is very simple and relatively inexpensive. The items of equipment required usually found on the market. Auxiliary personnel can trained to carry out the measurements but must surprised by an experiences professional.


Legal Provision on Natural Wet Bulb Thermometer:

Recalling the importance of the problems arising out of the industrial heat, and its relevance to health and safety consequences of the workmen in the industry, the section 13 of the Factories Act 1948(modified up to 1987) interiliac states that

Effective and suitable provision shall made in every factory for securing and maintaining in every work-room

(a) adequate ventilation by the circulation of fresh air, and

(b) such a temperature as will secure to workers therein reasonable conditions of comfort and prevent injury to health”

Natural Wet Bulb Thermometer

Model Rule 22 prescribed under the section 13 on Ventilation and Temperature’ stipulated that in any factory the maximum wet-bulb temperature of air in a work-room at a height of 1.5 meter above the floor level shall not exceed 3()OC and adequate air movement of at least 30 meters per minute shall provided, and in relation to dry bulb temperature, wet bulb temperature in the work-room at the said height shall not exceed that shown in the schedule detailed below:

Natural Wet Bulb Thermometer

TemperatureWet-Bulb Temperature
35°- 39°C28.5°C
40°- 44°C28.0°C
45°- 47°C27.5°C

This means to adopted under above provisions call for the following measures that

  • Walls and roof should be of such materials which will prevent undue materials which will prevent undue rise I temperature.
  • The process responsible for production of excessive industrial heat should be as for as possible segregated from other departments.
  • The hot surface should insulated.
  • Methods such a White-washing, spraying or insulating and screening the outside walls, etc. raising the level of the roof, or insulating the roof either by ‘providing double-roofing or by the use of insulating roof-materials should be introduced.

Scientific Measures for Natural Wet Bulb Thermometer:

When the environmental temperature beyond the permissible limits; certain control ‘measures need to taken to improve the working ‘conditions for optimizing efficiency and productivity. The following are a few of them:

Use of suitable thermal barriers:

When it is difficult to control the process temperature at source due to operational requirement, the best method that could be visualized is the use of some suitable thermal barriers between the heat source and the workers to cut down the radiant heat. Findings of a scientific study in an engineering plant in U.K. have shown that the globe temperature readings, could b brought down from 71.70C to 43.30C by use of some reflective shielding make of aluminum sheet metal coated ‘with aluminum paint. A recent study in a Railway Smith shop in India reported remarkable reduction in radiant heat load by 40 to 50 percent and the physiological strain by around 30 percent when Calcium Silicate-insulation block’ used.

Personal protective clothing for Natural Wet Bulb Thermometer:

The use of personal protective clothing seems to be last resort for providing comfort to the workmen as there is hardly any suitable protective clothing for the tropical workers. Findings of a recent study in forging-units in western India reported favorable results while ‘Aluminum Fabrics-suit’ used by the Furnace workers. The physiological strain reduced by 50%.

Reduction of metabolic workload:

Since the physical work adds to the problem of heat stress, any manual task, particularly of severe nature, should always avoided, if possible, by adoption of partial mechanization. Scientific studies have reported considerable reduction in cardiovascular strain among the workers after introduction of partial mechanization in some tasks.

Provision of air conditioned rest room:

A satisfactory recovery can observed when the rest taken; in a comfortable place after the cessation of work in heat. Findings of a study in an aluminum company in Canada; revealed that both cardio-vascular strain and body temperature remained well within the limits of safety; when the rest taken to the comfortable rest-rooms.

Provision of cold drinking water:

The workers should educated and advised to take plenty of cold water very frequently to replenish the fluid loss due to heavy sweating during work in heat. Provision should, there, made for easy availability of cold water very close to shop floors.

Selection criteria for Natural Wet Bulb Thermometer:

The heat stress is likely to in pose undue stress on the cardio-vascular system. Thus workers with strong body build and high levels of physical fitness should selected for hot operations. Persons reported to be suffering any cardiac diseases should be avoided for hot-tasks. Findings of a study in a steel mill in 1975 in USA gave fairly; established the correlation between the ‘physical fitness levels’ and ‘heat tolerance limits’ Performance decrement reported more among the workers; with relatively low levels of fitness than those with high degree of fitness.

Periodical check-up:

Frequent periodical check-up is also necessary to assess the physical fitness levels of individuals exposed; to hot environments to ensure whether they are still fit to work under such environment.  

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