The Bimetallic Thermograph is designed for measuring & recording atmospheric temperature as function of time on recording chart. It is used in the meteorological service as also in cold stores, spinning mills, harmaceutical factories, telephone exchanges & pollution control equipment.
In Bimetallic Thermograph bimetal. strip is used as sensor for air temperature
measurement. It consists of to layers of metals having different coefficient of thermal expansion. This strip is wound in helical coil form to give more magnification. This coil is fixed on a holder at one end while other end is
connected to the link mechanism, Due to temperature change bending stress arises in bimetal strip, this stress causes a rotation of the free end of helical coil. This rotary movement is transferred to the pen arm. The recording chart is fixed on a drum. This drum is put in to motion by precision clockwork mechanism. As a result of these to motions graph of time against air temperature is drawn on the chart.
The arms of recording pens are supported in articulated joints with slant axis that makes possible uniform & constant pressure of the pen tip on the chart drum. Precision clockwork is set inside the recording drum. The precise make of all components of clockwork & recording mechanism ensures high accuracy and longer field-life.
Bimetallic element of thermograph is designed to give linear indication range from -20°C to +60°C. Normally daily minimum to maximum temperature difference is not more than 25°C to 30°C. As such to give better & magnified record, range of graph is kept 40°C only. ‘Floating Zero’ adjustment is provided to utilize & accommodate full working range of bimetallic element i.e. – 20°C to + 60°C. ‘Floating Zero’ means that the base line of graph can have different temperature value as per seasonal temperature change.
In the thermograph this-adjustment is provided with the help of:
i) Rapid range setting by pen arm setting screw.
ii) Fine adjustment by spring loaded radial arm & zero setting screw.
iii) In the chart blank boxes are kept to mark temperature range values.

Technical Specification :

Sensing Element:             Bi-metallic Strip
Range:                                -20 C to +60 C
Accuracy:                           ± 1 C
Recording Resolution:    0.5 C
Drum Diameter:               933 mm
Housing Material:            FRP/ Aluminium enclosure with window
Recording Mechanism:   Drum driven by Quartz Clock