Temperature measurement

Question 1

Difference between peripheral and core body temp

Where to measure peripheral

Answer 1

Difference between core and peripheral temps gives an indication of adequacy of perfusion. Peripheral temp is usually ~1-2 degrees C lower

Core temperature reflects temperature of internal organs.

Peripheral temperature may be measured at:
* Skin (most common)
* Axilla
* Groin

Question 2

Where to measure core body temp

Answer 2

May be measured at:
* Oesophagus (lower 1/3, to avoid inaccuracies due to cooling effects of inspired gas)
* Tympanic membrane (~brain temp)
* Rectum (NB not usually used during anaesthesia due to slow response time)
* Bladder (may be affected by urine flow)
* Pulmonary artery

Question 3

Kelvin vs celsius

SI units, relative scales, key reference points

Answer 3

Kelvin (K)
* The SI unit of temperature
* Referenced to its lowest point, or absolute zero, which is -273.15 degrees C

Celsius
* Categorized by the freezing (0) and boiling (100) points of water

Absoloute magnitude of 1 degree C i s the same as that of 1K. So if a pateints temperature rises by 1 degree C, has also rised by 1K

Question 4

How to convert from Kelvin to degrees C

Answer 4

K = degrees C + 273.15

Degrees C = K - 273.15

Question 5

Temperature measuring devices, e.g. probes, use different technologies, including (6)

Answer 5

• **Thermocouples **- found in most thermometers
• Thermistors - in most thermometers attached to catheters
• Resistance wire thermometers - mostly used in industry to measure very high temps
• **Radiation thermometers **- used in handheld tympanic membrane thermometers
• Mercury thermometers - not used due to danger of toxicity
• **Liquid crystal thermometers **- strips that alter colour at specified temperature, available commercially, usually placed on forehead, band of temperature strip lights up to indicate range of temp.

Question 6

Thermocouples

Principle, mechanism, structure

Answer 6

Make use of the Thermoelectric effect or Seebeck effect: **two different metals in contact generate a voltage which is temperature dependent. **

Thermocouple
* Hot junction and reference junction against which the voltage is measured
* Reference junction is maintained at a constnat temperature that is colder than the hot junction
* The **voltage is directly proportional to the temperature (linear relationship) **
* Different metal groups are used, e.g copper-constantan, which is a combination of copper and nickel. Two wires of 0.4mm-2mm in diameter are combined by welding or soldering
* Thermopile = multiple thermocouples linked in series, to improve sensitivity

Question 7

Thermistor

Structure, mechanism, relationship with temperature, advantages/uses

Answer 7

A type of resistor that has a resistance that changes with temperature
* Made of semiconductor beads
* Oxides including chromium, cobalt, iron, manganese may be used

Relationship between resistance and temperature
* Negative temperature coefficient usually present: as temperature rises, resistance of thermistor falls
* NB some can have a positive temperature coefficient
* The relationship is **non-linear **

Advantages
* Accurate to an order of 0.1 degrees C
* Relatively cheap
* Can be made very small - i.e. suitable for use at end of pulmonary artery catheters, or oesophageal temperature probes

Question 8

Resistance wire thermometer

Answer 8

Aka resistance thermometers
* Like thermistors, use principle that resistance changes with temperature
* Number of forms. Usually use platinum wire (chemically inert, has a linear resistance-temperature relationship)

Advantages:
* Stability
* High accuracy and repeatability with low drift
* Wide operating range
* Often used in industry to measure v high temperatures

However, disadvantages compared to thermistors:
* Less sensitive to small changes in temperature
* Slower response time
* Require a power supply
* Hysteresis (cased by change in characteristics of the wire owing to exposure to varying temperatures) is a common source of error

Question 9

Radiation thermometer

Relationship between radiation and temperature

Answer 9

• The rate of radiation emitted by an object is proportional to temperature to the 4th power
• Radiation thermometers convert infrared radiation to an electrical signal -> temperature of an object can be measured
• NB infrared region lies between 0.72 and 1000 microm on the electromagenetic spectrum