Wet Bulb Temperature: Heat Stress Monitor for India

Wet bulb temperature is the most critical metric for human survival in extreme heat — and India is facing rising wet bulb temperatures year after year. When wet bulb temperature exceeds 35°C, the human body cannot cool itself through sweating, even in shade with unlimited water. This guide helps you build a practical wet bulb temperature monitor that could genuinely save lives during Indian heatwaves.

What Is Wet Bulb Temperature

Wet bulb temperature is measured by wrapping a wet cloth around a thermometer bulb and allowing air to flow over it. As water evaporates, it cools the thermometer. The greater the evaporation (drier the air), the lower the wet bulb reading. In saturated air (100% RH), the wet bulb equals the dry bulb temperature.

The wet bulb temperature encapsulates both heat and humidity in a single number. A dry 45°C day in Rajasthan with 15% humidity has a wet bulb of about 24°C — uncomfortable but survivable. A 38°C day in Kolkata with 85% humidity has a wet bulb of about 35°C — potentially lethal for prolonged outdoor exposure.

Wet Bulb Globe Temperature and Heat Stress in India

The Wet Bulb Globe Temperature (WBGT) is the occupational health standard for heat stress. It combines three measurements:

• Wet bulb temperature (Tw) — 70% of the index. Measures evaporative cooling potential.
• Globe temperature (Tg) — 20% of the index. Measures radiant heat from sun and surfaces.
• Dry bulb temperature (Td) — 10% of the index. Plain air temperature.

Formula: WBGT = 0.7 × Tw + 0.2 × Tg + 0.1 × Td

India experiences some of the highest WBGT readings globally. In May 2024, Nagpur and Delhi recorded wet bulb temperatures exceeding 32°C — affecting over 100 million outdoor workers including construction labourers, farmers, and delivery personnel. Building a monitoring system is a genuinely important contribution to community safety.

Sensors and Components Required

For a complete WBGT monitor, you need:

• Arduino Uno or ESP32 (WiFi recommended for remote alerts)
• DHT22 or BME280 sensor for dry bulb and humidity (to calculate wet bulb)
• DS18B20 waterproof probe wrapped in wet muslin for direct wet bulb measurement
• DS18B20 inside a black-painted copper sphere (150mm diameter) for globe temperature
• Small 5V fan for ventilation over the wet bulb sensor
• OLED display, buzzer, and LED for alerts
• Solar panel and battery for field deployment

Building the Wet Bulb Assembly

The wet bulb sensor assembly requires:

1. Wrap the waterproof DS18B20 probe in medical gauze or cotton muslin.
2. Feed the wick end into a small water reservoir (a 50ml plastic bottle works).
3. Position a 5V fan to blow air across the wet wick at 2-3 m/s.
4. Shield the assembly from direct sunlight with a louvred radiation shield (Stevenson screen design).

For the globe thermometer, paint a 150mm hollow copper or aluminium sphere matte black. Insert a bare DS18B20 sensor into the centre. The sphere absorbs radiant heat and equilibrates to the radiant temperature in about 20 minutes.

Arduino Code for WBGT Calculation

If you do not have a direct wet bulb sensor, you can estimate wet bulb from dry bulb temperature and relative humidity using Stull’s formula (accurate to ±1°C for typical Indian conditions):

#include <DHT.h>
#include <OneWire.h>
#include <DallasTemperature.h>

#define DHTPIN 2
#define GLOBE_PIN 3
DHT dht(DHTPIN, DHT22);
OneWire oneWire(GLOBE_PIN);
DallasTemperature globe(&oneWire);

// Stull's approximation for wet bulb temperature
float calcWetBulb(float T, float RH) {
  return T * atan(0.151977 * sqrt(RH + 8.313659))
       + atan(T + RH)
       - atan(RH - 1.676331)
       + 0.00391838 * pow(RH, 1.5) * atan(0.023101 * RH)
       - 4.686035;
}

void setup() {
  Serial.begin(9600);
  dht.begin();
  globe.begin();
}

void loop() {
  float dryBulb = dht.readTemperature();
  float humidity = dht.readHumidity();
  globe.requestTemperatures();
  float globeTemp = globe.getTempCByIndex(0);

  float wetBulb = calcWetBulb(dryBulb, humidity);
  float wbgt = 0.7 * wetBulb + 0.2 * globeTemp + 0.1 * dryBulb;

  Serial.print("Dry: "); Serial.print(dryBulb);
  Serial.print(" Wet: "); Serial.print(wetBulb);
  Serial.print(" Globe: "); Serial.print(globeTemp);
  Serial.print(" WBGT: "); Serial.println(wbgt);

  delay(5000);
}

Heat Stress Alert Thresholds

WBGT thresholds for work-rest cycles (based on ISO 7243 and NDMA India guidelines):

• Below 25°C WBGT — Safe for all activity levels. Normal work schedule.
• 25-28°C WBGT — Caution for heavy physical work. 45 min work, 15 min rest per hour.
• 28-30°C WBGT — High risk. 30 min work, 30 min rest. Mandatory water breaks.
• 30-32°C WBGT — Extreme risk. 15 min work, 45 min rest. Only essential outdoor work.
• Above 32°C WBGT — Danger. Suspend outdoor work. Heat stroke risk is very high.

Deployment for Outdoor Workers and Athletes

Deploy the monitor at construction sites, agricultural fields, sports grounds, and factory floors. Mount the sensor assembly at 1.5 metres height (breathing zone) in an open area. The ESP32 version can push alerts via WiFi to a central dashboard or directly to workers’ phones via SMS/WhatsApp using the Twilio API.

For sports applications (cricket, football, athletics), mount the system at the venue and display WBGT on a large screen. The BCCI and SAI now mandate heat stress monitoring at major sporting events — your DIY system can fill this need at the grassroots level.

India-Specific Heat Safety Guidelines

India’s National Disaster Management Authority (NDMA) has issued heatwave guidelines that reference WBGT. Key facts:

• India records over 25,000 heat-related deaths annually (likely underreported)
• States most affected: Rajasthan, Madhya Pradesh, Maharashtra, Andhra Pradesh, Telangana
• Heatwave declared when temperature exceeds normal by 4.5°C or absolute 45°C
• Wet bulb temperatures above 32°C are now being recorded in parts of India during May-June

Your monitoring system contributes to a growing citizen science network that can provide hyperlocal heat safety data where official weather stations are sparse.