pressure_thermometer, Schemes and Mind Maps of High Pressure Vessels and Boilers

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PRESSURE

THERMOMETER

INTRODUCTION

The pressure thermometer temperature measurement is based on fluid expansion.Pressure thermometers have now been superseded by other alternatives in most applications, but they still remain useful in a few applications such as furnace temperature measurement when the level of fumes prevents the use of optical or radiation pyrometers. Examples can also still be found of their use as temperature sensors in pneumatic control systems. The sensing element in a pressure thermometer consists of a stainless steel bulb containing a liquid or gas. If the fluid were not constrained, temperature rises would cause its volume to increase. However, because it is constrained in a bulb and cannot expand, its pressure rises instead. As such, the pressure thermometer does not strictly belong to the thermal expansion class of instruments but is included because of the relationship between volume and pressure according to

Boyle’s law: PV = KT

CONSTRUCTION

The change in pressure of the fluid is measured by a suitable pressure transducer such as the Bourdon tube. This transducer is located remotely from the bulb and connected to it by a capillary tube as shown in Figure of previous slide. The need to protect the pressure-measuring instrument from the environment where the temperature is being measured can require the use of capillary tubes up to 5 m long, and the temperature gradient, and hence pressure gradient, along the tube acts as a modifying input that can introduce a significant measurement error.

OPERATION

The sensing bulb is introduced into the medium those which temperature is to be measured. The difference of temperature between environment and medium to be measured will increase or decrease the pressure in the bulb. This change in pressure of the fluid in the bulb is transmitted to the bourdon tube. The transmission of pressure takes place through the capillary tube. Thus, the pressure change will be taken as the indication of temperature change. Normally, before the pressure change in the bourdon tube, the tube is in elliptical shape. After change in pressure, it tends to become a circle. So, this small displacement is amplified using the links, and the pointer will reflect for small displacement. This new position of the pointer on the calibration scale is the indication of temperature.

RANGES

The covered ranges for different systems are:

● Liquid filled system: 150⁰F to 750⁰F for xylene

-380⁰F to 1100⁰F for mercury.

● Gas filled system: -400⁰F to 1200⁰F ● Vapor filled system: -400⁰F to 600⁰F

Thermoelectric

Thermometry

● Thermoelectric thermometry has advantages over other temperature measurement techniques, such as its ability to measure temperature without the need for direct contact with the material being measured. ● However, thermoelectric thermometry also has limitations, such as the need for careful calibration to ensure accurate temperature measurement and the potential for external factors to affect the response of the thermocouples. ● Despite these limitations, thermoelectric thermometry remains a widely used and reliable technique for temperature measurement in a variety of applications. ● Some examples of applications for thermoelectric thermometry include high-temperature environments, medical applications, and industrial processes. ● In conclusion, thermoelectric thermometry is a useful and versatile method of temperature measurement that can provide accurate and reliable results in a wide range of settings.