Temperature, heat and humidity Flashcards
What is the definition of specific heat capacity?
The heat energy required to raise a unit mass of a substance by one degree of temperature
Typically expressed in J.kg⁻¹.K⁻¹
I.e. the energy required to raise the temperature of 1kg of a substance by 1K
Water has a high specific heat capacity of 4184J.kg⁻¹.K⁻¹
What are the routes of heat loss from the body?
Roughly how much in % does each contribute?
- Radiation (40%) - Thermal energy emitted as electromagnetic radiation
- Convection (30%) - The transfer of heat by the gas / liquid which surrounds it. Heat from the body warms the surrounding air, which then becomes less dense and rises, being replaced by cooler air
- Evaporation (20%) - By sweat
- Respiration (10%) - Humidification and warming of inspired air
- Conduction (minimal) - Transfer of heat energy to another object in direct contact with it
Hoe does a platinum resistance thermometer work?
The resistance of a thin platinum wire has a positive linear relationship with temperature. As temperature increase, resistance increases. Resistance can be measured and then converted to temperature using a calibration equation
How does a thermistor work?
The resistance of a thermistor varies with changes in temperature. The relationship is non-linear and negative - as temperature increases, resistance decreases. A Wheatstone bridge circuit is used to measure the resistance accurately
How does a thermocouple work?
Thermocouples rely on the Seebeck effect. When a junction is formed between two dissimilar metals a small voltage is produced across the junction, which varies with temperature
V = α.(T₁ - T₂)
α = Seebeck coefficient
T₁ and T₂ are the temperatures at the two junctions
One junction is the temperature being measured, and the other is held at a known ‘reference’ temperature, or may be a thermistor which accurately senses the ambient temperature
How does an infrared tympanic thermometer work?
The sensor in this type of thermometer is called a thermopile. Infrared radiation is emitted from the eardrum and converted to electrical energy by the sensor. The radiated power of the infrared energy provides an indication of the temperature at the tympanic membrane - which is similar to that of the body’s thermoregulatory centre at the hypothalamus
What are the laws of thermodynamics?
- First law - The change in internal energy of a system is equal to the heat added minus the work done by the system (i.e. conservation of energy - cannot be created or destroyed)
- Second law - Heat cannot flow naturally from a material at lower temperature to a material at a higher temperature
- Third law - The temperature absolute zero (0K) is unattainable
What is the Joule-Thomson effect?
The change in temperature that accompanies the rapid change in pressure of a gas without production of work or transfer of heat
Typically when a gas is compressed the temperature will rise
List non-electrical methods of temperature measurement
Liquid thermometer
Bourdon thermometer
Bimetallic strip
Liquid crystal thermometer
List electrical methods of temperature measurement
Platinum resistance wire
Thermistor
Thermocouple
What are the NICE recommendations for peri-operative temperature management?
Pre-operative:
* Measure and document
* Warm if <36
* Walk to theatre where able
Intra-operative:
* Measure and document every 30mins
* Induction of anaesthesia only if >36
* Any more than 500mls IV fluids should use fluid warmer
* Forced air warmer for surgery >30mins
* Irrigation fluids should be warmed
Post-operative:
* Measure and document every 15mins
* Do not discharge to ward until >36
* Measure 4-hourly on ward
Why is hot air less dense?
As the temperature of water increases, the saturated vapour pressure also increases - so more water molecules evaporate and enter the air as water vapour
The number of particles in air must remains the same due to Avogadro’s hypothesis (at equal volumes, temperature and pressure). Therefore as the amount of water vapour increases the density of air must reduce - because water vapour has a lower molecular mass than other constituents of air
This less dense air will then rise, and will start to expand as the effect of gravity reduces. The process of expansion requires energy, and so the air will cool. The saturated vapour pressure will therefore be lower and some water will condense out and form clouds. The remaining air will have less humidity and therefore will become more dense and stop rising - resulting in an evaporation / condensation equilibrium
What are the definitions of absolute and relative humidity?
Absolute humidity = The mass of water molecules per unit volume, commonly expressed as grams per cubic metre
Absolute humidity increases with temperature
Relative humidity = The ratio of water vapour present in air compared to the maximum amount of water vapour the air could hold at that temperature
= actual vapour density / saturation vapour density
Expressed as a % - i.e. 100% relative humidity = air completely saturated with water vapour. This is equivalent to the saturated vapour pressure at the same temperature. Excess water condenses out of vapour phase to form liquid - dew
How does a wet and dry bulb hygrometer work?
This hygrometer consists of two glass thermometers:
1) Dry bulb - Standard thermometer measures ambient temperature
2) Wet bulb - Thermometer bulb is covered in a wet wick. The water in this wick will evaporate at a rate determined by the surrounding air temperature and relative humidity. As this occurs, the temperature of the thermometer will drop due to latent heat of vaporization. The difference in temperature between the two thermometers allows a measurement of relative humidity to be obtained by using a specialized chart
The temperature drop seen on the wet bulb thermometer will be lower if the relative humidity of surrounding air is higher, due to the reduced concentration gradient / ability of the air to take up further water vapour. At 100% relative humidity both thermometers will read the same
How does a hair hygrometer work?
A hair is held under tension, connected to a spring which will move a needle as it changes in length.
When the relative humidity is higher the hair will lengthen as it takes up water. The amount of lengthening depends on the relative humidity, and therefore can be used to give a measurement of relative humidity once calibrated
How does Regnault’s hygrometer work?
Regnault’s hygrometer is a type of dew point hygrometer
This hygrometer consists of a silver tube containing ether, as silver is a good conductor. Air is bubbled through the ether to cause vaporization, and therefore cool the tube due to latent heat of vaporization. The temperature at which condensation occurs on the outside of the tube is the ‘dew point’ - i.e. saturated vapour pressure. The dew point is higher when humidity is higher
Using the ambient temperature and the known saturated vapour pressure at that temperature, the relative humidity can be obtained using reference tables
