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A hygrothermograph is a device that is used to simultaneously record temperature and humidity on a line chart. It might be either battery-powered or spring-wound. The device records data, in the form of a line, on a roll of paper that is mounted on a revolving drum, which turns at a predetermined rate. This system provides a visual representation of fluctuations in temperature and humidity over a given time period. Lines on the chart are drawn by two separate pens, one attached to a temperature-sensing element and the other attached to a humidity-sensing element. Hygrothermographs are often used in areas where it is important to track fluctuations in both humidity and temperature, such as laboratories, museums and agricultural storerooms.
The temperature-sensing element in a hygrothermograph is a bimetallic strip. This component, which is also used in thermostats and other mechanical instruments, consists of two separate metals welded together. The two metals expand and contract at different rates with changes in temperature, causing the strip to become displaced in a sideways direction. A lever system that is attached to the strip magnifies the displacement and moves the arm of the upper pen up and down across the roll of paper, creating a record of temperature changes in the form of a line graph.
The hygrothermograph senses humidity using a bundle of human hair that has been specially treated for the purpose. As the humidity in the atmosphere fluctuates, the hair expands or contracts. The movement is transferred mechanically to the lower pen arm, which moves up and down to record the humidity.
The rate of rotation of the drum determines the length of time that the hygrothermograph records. Many hygrothermographs can be set for different time intervals, depending on how much data is required. A faster rotation might cover one day, and a slower rotation might allow the graph to span an entire month. Spring-loaded hygrothermographs generally have a limit of about seven days before they must be reset.
If there is poor ventilation in the room where the device is being used, false readings might occur. An axial fan, which is optional as an add-on in many hygrothermograph models, can help correct this problem. The fan works by actively drawing air from the room into the device, allowing for faster and more accurate recording of data. The fan requires electricity to run, so it can be used only if there is battery power or another power source nearby.