A thermometer is a device or instrument used for measuring temperature. There are many different types of thermometers; however each one is based on a physical property of a thermometric (temperature measuring) substance that differs in a measurable way with temperature. Some of the physical properties that vary with temperature are volume, resistance and color. A physical property that increases or decreases with temperature can be used to measure temperature. This is called a thermometric property.
There are two types of liquid-in-glass thermometers: 1) Mercury-In-Glass Thermometers 2) Alcohol-In-Glass Thermometers.
A liquid in glass thermometer uses mercury or alcohol as its thermometric substance. Its thermometric property is volume and it is used to measure the temperature by allowing the mercury or alcohol to expand or contract in the capillary tube as temperature rises or decreases. I.e. an increase in temperature equals an increase in volume while a decrease in temperature equals a decrease in volume. A mercury-in-glass thermometer has a range of -39oC to 360oC while an alcohol-in-glass thermometer has a range of -112o C to 78o C.
Here are some advantages and disadvantages of mercury-in-glass thermometers:
* It doesn’t cling to the sides of the tube.| * It’s expensive.| * It expands uniformly.| * Its freezing point is high at -39oC| * It has a visible surface that makes it easily seen in fine capillary tubes.| * It’s poisonous.| * It is a good conductor of heat, which makes it reach the exact temperature faster.| * It cannot expand a great deal.| * Its boiling point is high at 375o C.| |
Here are some advantages and disadvantages of alcohol-in-glass thermometers:
* Its expansion is uniform.| * It sticks to the tube.| * It can expand a great deal.| * It reaction to changes in temperature is slow.| * It’s cheap.| * It needs to be dyed as it is transparent.| * It’s a safe liquid.| * It’s boiling point is low at 78oC| * Its freezing point is low at -115oC.| |
Liquid-in-glass thermometers can be used as laboratory thermometers for use in schools and labs and as clinical thermometers for use in the measuring of body temperature.
A thermocouple thermometer consists of two wires made of different metals such as iron and copper. As shown above, the ends of the wires are joined together to form two junctions. These two different metals, maintained at different temperatures, cause a net voltage (e.m.f.) to be generated across the junctions. This voltage generated (once not large) is directly proportional to the difference in temperature between the junctions and so, when measured, can be used to find out an unknown temperature. The most commonly used pair of metals is platinum (Pt) and Rhodium (Rh), which is an alloy of platinum. The voltage is measured with a galvanometer for less accurate readings or a potentiometer for very accurate readings. Due to the junction being made of metal thermocouples can be used to measure high temperatures.
Its thermometric substances are two different metals and its thermometric property is electrical conductivity. Its range is -183oC to 1127oC.
Some advantages of thermocouple thermometers are:
* The wire junctions may be very small therefore it’ll need little heat to warm up and respond quickly to temperature changes. * The output of this thermometer is an electrical signal that can be used to operate equipment capable of giving warnings of sudden temperature changes or of keeping records of temperature. * By choosing particular pairs of metals A and B, temperatures up to about 1500oC can be measured.
A disadvantage of thermocouple thermometers is:
* It is less accurate than constant gas and platinum resistance thermometers.
Constant Gas Thermometers
In a constant gas thermometer the bulb contains a fixed amount of hydrogen, helium or nitrogen gas. When the temperature of the gas increases the pressure of it increases as well. A larger height h difference is needed to keep the gas at its fixed volume. The pressure of the gas is equal to ρgh plus atmospheric pressure. Its thermometric substance is the gas and its thermometric property is the pressure on a fixed mass. Its range is -270oC to 1477oC.
Advantages of a constant gas thermometer are:
* It has a wide range.
* It is accurate and sensitive.
* It is used as a standard
Disadvantage of a constant gas thermometer are:
* The reading taken does not come directly from the thermometric substance.
* Its response is slow.
* It is fragile.
Platinum Resistance Thermometers
A platinum resistance thermometer works by having a platinum coil that has an increase in resistance as the temperatures raises and a decrease in resistance as the temperature decreases. The resistance of the coil is measured accurately with a circuit that makes use of balanced PDs. The platinum resistance thermometer’s thermometric substance is platinum and is thermometric property is resistance. Its range is from -248oC to 1477oC.