Instrumentation

Temperature Gauge

Temperature gauges are used to indicate temperature of any body, Line or Process.

Temperature gauges are filled with inert gases. Mercury is generally used. When temperature increases it expands and pointer moves.

  • A Bourdon Tube is a thin metal – usually brass or copper – tube that is filled with an easily vaporized fluid, typically alcohol. It is sealed at both ends. At the gauge end, it is formed into a circle or spiral with its end attached to the indicating needle by some form of linkage. The other end is fitted to a water-tight connector that is in direct contact with the coolant in the engine.
  • As the coolant warms up the alcohol in the Bourdon tube expands. The expansion transfers its force to the coiled end of the tube inside the gauge. As the coil or spiral unwinds it pulls the linkage on the needle, which in turn shows a temperature reading on the gauge face. The gauges are calibrated during the manufacturing stage and are not adjustable afterward.
  • Since the Bourdon Tube design is purely mechanical the gauge will continue to read some temperature level even after the engine is shut off. As the engine cools the gauge’s needle will return to its rest position.
  • Bourdon Tube gauges aren’t used anymore because of cost and convenience factors. The tubes are delicate and must be carefully routed from the dash to the appropriate fitting on the engine. The gauges themselves are far more expensive than electric or electronic gauges and if the tube is kinked or split the entire gauge assembly must be replaced.

 

Bimetallic Gauges

bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases steel and brass. The strips are joined together throughout their length by riveting, brazing or welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.

Types of Temperature Gauge

Temperature Gauges are of three types.

  1. Fixed Head Temperature Gauge.
  2. Flexible Head Temperature Gauge.
  3. Capillary tube Temperature gauge.

Fixed Head

Fixed Head temperature Gauge having its Head Fixed. Its head is fixed at one position.

Flexible Head

Variable Head temperature Gauge Having it’s Head Flexible, it can be move in any direction where we want. It is easy to calibrate than a fixed head temperature Gauge.

Capillary Type

This type of Gauge is used when in a tank or in a closed container another type of gauges not give a proper reading. An expansion temperature gauge consists of a temperature sensor, a capillary and a Bourdon tube. The measuring system itself is filled with a liquid. If the temperature changes, the internal pressure of the thermometer will change as well. The pressure is transferred via a tube to a pointer shaft and thus the temperature value is indicated on the scale. Using capillaries from 500 to 10,000 mm long, measurements can also be taken from remote measuring points. The scale ranges for expansion temperature gauges.

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