| There are several types of thermometers or temperature sensors used to determine the temperature for weather calculations. The three most common types are: bulb thermometers, bimetallic strip thermometers, and electronic thermometers
The simplest and oldest form of the three is the bulb thermometer. This is the common glass thermometer most people are familiar with. The thermometer contains some type of fluid, generally mercury. Bulb thermometers rely on the simple principle that a liquid changes its volume relative to its temperature. Liquids take up less space when they are cold and more space when they are warm. In order to take best advantage of this effect, bulb thermometers use a narrow tube attached to a relatively large bulb. While the bulb is significantly larger than the tube, in size it is quite small, so that the liquid reaches the outside temperature quickly. As the volume of the liquid increases, the liquid is pushed up the narrow tube. As the volume decreases, it falls back into the bulb. Any liquid would work; however, mercury is used because it has a lower freezing point and a higher boiling point than other, more common liquids. This allows a much greater range of measurable temperatures.
Another form of thermometer, often used in thermostats (devices which measure and control temperature) is the bimetallic strip thermometer. As the name suggests, this thermometer consists of a two metal strips bonded to each other. The strip is used so that as the temperature changes, the strip flexes and opens or closes a contact. Bimetallic strips are also coiled into long spirals and used in outdoor thermometers. As the strip increases in length, it becomes more sensitive to small temperature changes.
The third type of thermometer we will be discussing is the electronic thermometer. The most common of this type is a thermoresistor (or thermistor), which changes resistance with changes in temperature. An internal circuit measures the resistance and then converts it to the corresponding temperature.
Temperature sensors for weather stations and stand-alone sensors usually use a thermistor to measure the temperature. Remote sensors then relay the information to the base display. RadioShack sells several thermometers (and thermometer systems) as stand-alone weather sensors. |
