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Temperature measurement: electrical techniques

Created: 2/6/2005

 

There are three principal techniques for measuring temperature:

 Resistance thermometer
 Thermistor
 Thermocouple
 

Resistance thermometer

A temperature change causes a change in electrical resistance, which is detected using a Wheatstone bridge circuit and galvanometer. This type of thermometer can measure a very small temperature change (0.0001°C) and there is a linear relationship between temperature and resistance over the range 0-100°C.
The Platinum resistance thermometer is commonly used because platinum has a large temperature coefficient of resistance and resists corrosion.
 

Advantages

  • Accurate

Disadvantages

  • Large
  • Fragile
  • Slow response time

Wheatstone bridge circuit

The circuit we now know as the Wheatstone Bridge was actually first described by Samuel Hunter Christie (1784-1865) in 1833. However, Sir Charles Wheatstone invented many uses for this circuit once he found the description in 1843. As a result, this circuit is known generally as the Wheatstone Bridge.

To this day, the Wheatstone bridge remains the most sensitive and accurate method for precisely measuring resistance values.





Thermistor

Thermistors are semiconductors in which the resistance changes with temperature. Although the change is non-linear, it can be manufactured so that it is almost linear within the working range (body temperature plus or minus 25°C ).
They are composed of fused heavy metal oxides including manganese, nickel, zinc, iron and cobalt. A greater temperature coefficient makes thermistors more sensitive than wire resistance thermometers.
 

Advantages

  • Small
  • Rapid response time

Disadvantages

  • Drift
  • Resistance changes over time
  • Need recalibration
Resistance changes with temperature


Thermocouple

The thermocouple relies on the Seebeck effect. In 1821, Estonian-German physicist Seebeck demonstrated the electrical potential in the juncture points of two dissimilar metals, the magnitude of which changes with temperature. If the circuit is completed by a reference junction of which the temperature is known, the temperature of the measuring junction can be measured.
Metals commonly used include copper and constantan (alloy of copper and nickel).

Advantages

  • Small
  • Accurate
  • Rapid response times
  • Relatively cheap

Disadvantages

  • Needs signal amplification and processing




y = Junction potential mV
x = Temperature



The Adult Skin Temperature Sensor can be used in all clinical settings requiring routine monitoring surface temperature. The hypoallergenic adhesive is less traumatic to patients skin. There is a positive locking hub to secure cable connections. This highly accurate skin sensor provides reliable surface temperature readings. It is fully compatible with most monitoring equipment.





The General Purpose Temperature Probe is used for routine monitoring of patient’s core temperature. Its versatile design allows for oral, nasal or rectal placement. There is a low friction surface for ease of insertion. The completely enclosed sensor wires improves stability and hygiene. Non-toxic, non-porous material for patient safety. Fully compatible with most monitors.


 



The Tympanic Temperature Probe is designed for atraumatic measurement of patient temperature from the aural canal. The contoured ear piece ensures simple, secure placement and the soft cotton tip minimises trauma.







The Oesophageal Stethoscope with temperature sensor is designed for use in surgical settings to monitor heart and respiratory sounds and to measure core body temperature of anaesthetised patients.
Provides heart and respiratory sound transmission. Available with and without temperature monitoring capability. Fully compatible with most monitors. Atraumatic, low friction surface for ease of insertion. Complete line includes all popular French sizes.




The Foley Catheter with temperature sensor is used for urinary drainage and simultaneous monitoring of bladder temperature.Dual-purpose design can be used before, during and after surgery. Colour coded hubs for precise size identification. Complete range of sizes: 8-18 French.

 








ArticleDate:20050602
SiteSection: Article
 
   
    
                                            
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