SCIE 4001
2.4 Thermometers
 

Thermometers

Liquid in Glass.

Laboratory thermometers are usually liquid in glass. Common liquids used are mercury and alcohol (with a coloring).

thermometer  
Mercury in glass laboratory thermometer
 

Structure of the laboratory thermometer

The bulb

— is relative larger than the bore in the stem to contains a reservoir of liquid which can expand into the bore to provide readings (large bulb improves the sensitivity of the thermometer). Most of the liquid is in the bulb.
— is made of glass with very thin walls to allow the heat from sample being measured to reach the liquid quickly for fast response and quick reading (improves the quickness of response by the thermometer).
— is elongated to provide a large surface area through which heat can be conducted to the liquid ((improves the quickness of response by the thermometer).

— The Stem

— is made of thick glass (capillary tube) and may be round or lens shaped to magnify the thin column of liquid. this
— has a narrow bore to improve accuracy and sensitivity (the thinner the bore the more the liquid moves).
— has a uniform bore to improve accuracy (amount of moment is the same for a temperature change).
— has a vacuum above the liquid column so there is no resistance to the expanding liquid.
— has a scale to provide reading.

— Liquid

— must have uniform expansion over the range used. Mercury is good but alcohol is less suitable.
— mercury is limited to measuring -39°C to 357°C (freezing to boiling points). Good for extremely high temperatures.
— alcohol is limited to measuring -75°C to 120 °C (freezing to boiling points). Good for extremely low temperatures.
— mercury is a better conductor of heat and thus will respond quicker ((improves the quickness of response by the thermometer).

Clinical Thermometer.

The clinical thermometer is designed to measure the body temperature and so incorporates special elements in its design to accommodate the following needs:

— body temperature varies by just a few degrees so it is important to have a small range (34°C -42°C ) with accuracy and sensitivity.
— It may be inserted into body cavities so it must be easily sterilized and not be toxic if broken. Alcohol is used as mercury may prove toxic if the thermometer is broken.
— Its reading must be captured and not deviate as it is removed from a body cavity. There is a constriction which captures the reading and the thermometer has to be shaken to return the mercury after using.

clinical thermometer — small range (34°C -42°C )
— constriction to capture reading
very thin walled bulb - quick response
— very narrow uniform bore for accuracy
— may have a lens shape to help magnify the column
Clinical Thermometer
 

Other kinds of Thermometers

 

bimetallic thermometer

Bimetallic thermometer.

— made up of a coil of two different metals (bimetallic coil of the thermometer) and a dial on which the temperature can be read. 
— bends due to the different expansion rates of the two metals of the coil. 
— the temperature on the dial is determined by the amount that the bimetallic strip bends.
— Used in devices like ovens.

resistance  thermometer

Resistance Thermometer

— designed to measure temperatures between -250 and 700°C by noting the resistance change . T
— the temperature gauge in the car is connected to one of these in the engine compartment.

thermocouple

Thermocouple Thermometer

— a current flows between hot and cold junctions through the in the attached probes.
— very sensitive model that can provide accurate temperature results between -250 and 1600 °C.

infra red thermometer

Infrared Thermometer

— can measure very high or low surface temperatures without the need to make contact with the item in question.
— can have large errors from surroundings.

liquid crystal thermometer

Liquid Crystal

— change colors due to the liquid crystals in the face.

gas thermometer

Gas Thermometer

— a very common type of industrial thermometer used to keep the temperature of stored gas constant within a confined area.

— the pressure of this gas is measured to determine the temperature inside.

     
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Concept by Kishore Lal. Programmed by Kishore Lal... Copyright © 2015 Kishore Lal. All rights reserved.