Which of the following thermometers is used to measure the low air velocity rather than the cooling power of the air?

Explanation

## Correct Answer: D. Kata thermometer The **Kata thermometer** is a specialized instrument designed specifically to measure **air velocity** (wind speed) rather than temperature or cooling power alone. It consists of an alcohol-filled thermometer with a bulb at each end—one filled with alcohol and one empty. The principle is based on the **evaporative cooling rate**: when air moves across the thermometer, it accelerates evaporation from the bulb surface, causing a faster temperature drop. The time taken for the thermometer to cool from 38°C to 35°C (or 40°C to 35°C) is inversely proportional to air velocity. This cooling time is then converted to air velocity using a calibration chart. The Kata thermometer is particularly useful in occupational health settings in India—textile mills, foundries, and mining operations—where assessing air movement is critical for worker safety and heat stress prevention. Unlike globe thermometers (which measure radiant heat) or wet bulb thermometers (which measure humidity and cooling power), the Kata thermometer isolates air velocity as the primary variable affecting cooling rate, making it the gold standard for low-velocity air measurement in industrial hygiene surveys. ## Why the other options are wrong **A. Globe thermometer** — The **globe thermometer** measures **radiant heat** and **mean radiant temperature** (MRT), not air velocity. It consists of a thermometer inside a 15 cm black copper sphere and is used to assess thermal comfort by accounting for radiation from hot surfaces—critical in foundries and steel plants in India. It does not measure air movement directly. **B. Dial thermometer** — A **dial thermometer** is a simple temperature-measuring device with a dial face, used for general temperature recording in clinical and environmental settings. It measures absolute temperature, not air velocity or cooling dynamics. It has no mechanism to detect or quantify air movement. **C. Wet globe thermometer** — The **wet globe thermometer** (wet bulb globe temperature, WBGT) combines wet bulb, dry bulb, and globe readings to measure **cooling power** and **thermal stress index**—used in India for heat illness prevention in outdoor workers and athletes. It assesses the combined effect of temperature, humidity, and radiation, not specifically air velocity. ## High-Yield Facts - **Kata thermometer** measures **air velocity** by timing evaporative cooling from 38°C to 35°C (or 40°C to 35°C), with cooling time inversely proportional to wind speed. - **Globe thermometer** measures **mean radiant temperature (MRT)** and radiant heat exposure—essential in foundries, steel plants, and hot occupational environments. - **Wet bulb globe temperature (WBGT)** combines wet bulb (humidity), dry bulb (temperature), and globe (radiation) to assess **thermal stress index** and heat illness risk. - **Air velocity measurement** is critical in Indian occupational health surveys (textile mills, mines, foundries) to prevent heat stress and ensure worker safety under RNTCP/occupational health guidelines. - Kata thermometer cooling rate is converted to **air velocity (m/s)** using a **calibration chart** specific to the instrument—not a direct temperature reading. ## Mnemonics **KATA = Air Velocity** **K**ata measures **A**ir velocity (not **T**emperature or cooling power **A**lone). The cooling TIME is the key—faster cooling = faster air movement. **Thermometer Trio in Occupational Health** **Dry Bulb** = temperature; **Wet Bulb** = humidity + cooling power; **Globe** = radiation; **Kata** = air velocity. Each answers a different question about thermal environment. ## NBE Trap NBE may pair "cooling power" with globe or wet bulb thermometers to distract from the fact that Kata specifically isolates **air velocity** as the independent variable. Students who confuse "cooling" with "measuring what cools" fall into this trap. ## Clinical Pearl In Indian textile mills and foundries, occupational health officers use the Kata thermometer during heat stress surveys to quantify air velocity in poorly ventilated work areas—a low reading (<0.5 m/s) signals stagnant air and high heat illness risk, prompting immediate ventilation improvements under factory safety regulations. _Reference: Park's Textbook of Preventive and Social Medicine, Ch. 10 (Environment and Health); KD Tripathi Essentials of Medical Pharmacology (occupational health section)_