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Thermometers - General Information

Thermometers - General Information
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Data sheet INFO THERMOMETER Date Page GENERAL INFORMATION ABOUT THERMOMETERS INTRODUCTION The name is referred generally to an instrument for the measurement of temperature, but in practice it is an instrument to measure temperature changes by means of dimension variation (bimetal thermometer) or pressure variations (gas expansion thermometer) or electric resistance variations (resistance thermometers) resulting from temperature changes. GAS EXPANSION THERMOMETER. The measuring principle of these thermometers is based on pressure variations caused by the expansion of a fluid or gas into a closed enclosure. The closed sensing element is made of a Bourdon tube pressure gauge connected to the thermometric bulb containing the liquid or the captor gas through a flexible capillary. Using a gas (Nitrogen or Helium), the temperature-volume-expansion is linear. BIMETAL THERMOMTER. The bimetal temperature gauge relies on the difference between the thermal expansion of two different metals which are welded at one end and laminated together in a wound cylindrical or flat spiral. DEFINTIONS THE BULB is the vessel containing the sensing element (liquid, gas, bimetal) and it is put into the heat source. THE WELL is used to contain the bulb to protected it against corrosion, abrasion, bending and breakage and to separate the process fluid from the ambient. SPIRAL BOURDON SPRING is the indicator element able to pick up pressure variations caused by the thermal expansion of intermediary fluid contained in the bulb. FLEXING CAPILLARY TUBE is the transmission and connection element between bulb and spiral Bourbon tube. It is used to read temperature remote from the heat source. _______________________________________________________________________________________________ PCI (Process System Solutions) Ltd, Unit 7 Slaidburn Crescent, Southport, Merseyside, PR9 9YF, UK, Tel: +44 (0) 1704 509090, Fax: +44 (0) 1704 509091, email: sales@pci-instruments.com, Website: www.pci-instruments.com

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Data sheet INFO THERMOMETER Date Page THERMOMETER SELECTION GUIDE The following requirements must be considered when selecting a thermometer: 1. 2. 3. Position Working conditions Accuracy These will be analyzed with the possible options for each requirement Panel mounting through a 3 hole rear flange Local mounting that is directly on the pipe Panel mounting through a 3 hole front flange or rear bracket Instrument diameter must be chosen in relation to distance between installation point and reading position to ensure the operator can see the graduated scale reading. This is a...

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Data sheet Date Page Fluid pressure Ambient temperature Fluid pressure in the temperature sensors must be considered for the bulb joints in order to prevent leaks. Average error due to external pressure on inert gas thermometers is +/- 1º every 40 bar. The ambient temperature during the calibration of the instrument is 20°C +/- 2°C, so every ambient temperature variation from this value will influence the precision of the thermometer. In inert gas or liquid filled thermometers the quantity of gas or liquid contained in the capillary and spiral Bourdon tube reacts to the ambient temperature...

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