Clinical Thermometry with Glass Thermometers
Fundamentals of Mercury-in-Glass Thermometers
Mercury-in-glass thermometers rely on the principle of thermal expansion. Mercury, a metallic element, expands predictably with increasing temperature. This expansion is visible within a calibrated glass tube, allowing for a direct reading of temperature.
Components of a Clinical Glass Thermometer
- Bulb: The reservoir at the bottom of the thermometer containing the mercury.
- Stem: The narrow glass tube extending from the bulb, with a fine bore (capillary) through which the mercury rises.
- Scale: A calibrated temperature scale etched onto the glass, typically in degrees Celsius (°C) or Fahrenheit (°F).
- Constriction: A narrowing of the bore just above the bulb. This constriction prevents the mercury from automatically receding after measuring a temperature, allowing the user to record the reading accurately after removal from the measurement site.
Measurement Techniques
Oral Temperature Measurement
Place the thermometer bulb under the tongue, ensuring contact with the sublingual pocket. Instruct the individual to close their mouth and breathe through their nose. Leave the thermometer in place for a minimum of 3-5 minutes to ensure accurate reading.
Axillary (Armpit) Temperature Measurement
Ensure the armpit is dry. Place the thermometer bulb in the armpit, ensuring good contact with the skin. Instruct the individual to hold their arm firmly against their body. Leave the thermometer in place for a minimum of 5-10 minutes.
Rectal Temperature Measurement
This method is typically used for infants and young children, and when oral or axillary measurement is not possible. Lubricate the bulb of the thermometer with a water-soluble lubricant. Gently insert the thermometer approximately 1-2 inches into the rectum. Hold the thermometer in place for 2-3 minutes. This method typically yields the most accurate core body temperature.
Reading and Interpreting the Temperature
After the recommended measurement time, remove the thermometer and hold it horizontally at eye level. Rotate the thermometer until the mercury column is clearly visible. Read the temperature at the point where the mercury column ends, noting the units (°C or °F).
Resetting the Thermometer
After each use, the mercury must be forced back into the bulb. This is achieved by firmly shaking the thermometer downwards with a snapping motion, away from the body and hard surfaces, until the mercury column recedes below the lowest reading on the scale.
Safety Precautions and Handling
- Glass thermometers are fragile and can break if dropped or subjected to sudden temperature changes.
- Mercury is a hazardous substance. If a thermometer breaks, avoid direct contact with the mercury. Contain the spill immediately and follow local guidelines for mercury spill cleanup and disposal.
- Never use a mercury thermometer rectally on a child without proper training and supervision.
- Do not sterilize mercury thermometers in boiling water or autoclaves, as this can damage the glass and potentially release mercury.
- Store thermometers in a safe place, out of reach of children.
Alternatives to Mercury Thermometers
Due to the environmental and health concerns associated with mercury, digital thermometers and infrared thermometers are increasingly used as alternatives. These options offer faster readings, are less prone to breakage, and do not contain mercury.