Temperature Measurement

 Temperature is a measure of degree of hotness or coldness of an object.

Units of Temperature                                                                           The two most common temperature scales are Fahrenheit (°F) and Celsius (°C).

Water freezes at 32°F and boils at 212°F. The Celsius scale uses the same reference points only it

defines the freezing point of water as 100°C and the boiling point as 100°C.

We see from these definitions that a Fahrenheit degree is 9/5 of a Celsius degree and is offset by

32°. To convert between these scales we use the following formulas:

Temperature is most commonly measured by Resistance Temperature Devices (RTD) and Thermo- couples, and to a lesser degree Infrared (IR).

Thermocouples

Thermocouples are fabricated from two electrical conductors made of two different metal alloys. At one end of the cable the two conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple, the hot or sensing junction, is thermally attached to the object to be measured.

The other end, the cold or reference junction is connected to a measurement system.

Thermocouples generate an open-circuit voltage, called the Seebeck voltage that is proportional to the temperature difference between the sensing (hot) and reference (cold) junctions.

The relationship between the temperature difference and the voltage produced has been

documented for several commonly used combinations of metals.

Junctions

Since thermocouple voltage is a function of the temperature difference between junctions, it is

necessary to know both voltage and reference junction temperature in order to determine the

temperature at the hot junction. Consequently, a thermocouple measurement system must either

measure the reference junction temperature or control it to maintain it at a fixed, known

temperature. Most controllers today measure the temperature of the reference junction.

                                         VS =V (THot -TRef )

                                         TRef =Temperature of the Reference Junction

                                         THot =Temperature of the Sensing Junction

                                         V      =Proportionality Constant

                                         VS    =Seebeck Voltage

  

                                                                                                               

Different methods of temperature measurement

The different methods of temperature measurement are:

1. Mechanical  2. Electrical.

Mechanical methods:

1. Mercury in glass thermometers: This consists of a glass tube of very fine bore joined to a reservoir at the bottom and sealed at the top. A measured quantity of mercury is the enclosed. When the thermometer is heated the mercury expands much more than the glass and is therefore forced to rise up in the tubing A scale is fixed at the side.

2. Bimetallic Thermometer: Two metals whose coefficient of linear expansion is different are welded and rolled together to the desire thickness. The actual movement of a bimetal is its flexivity with one end fixed, a straight bimetal strip deflects in proportion to its temperature, to the square of its length and inversely with its thickens.            

3.Pressure Spring Thermometers: There are four classes of pressure spring thermometers.

1. Liquid filled      = class 1

2. Vapor pressure = class 2

3. Gas filled          = class 3

4. Mercury filled    = class 4

Liquid filled & Mercury filled:

Both type; operate on the principle of thermal expansion. Where the bulb is immersed in a heated substance. The liquid expands causing the pressure spring to unwind. The indicating, recording or controlling mechanisms are attached to pressure spring.

Compensated Thermometer System:

Compensations are provided in order to nullify the effect of changes in ambient temperature. The compensation in liquid filled expansions thermal system consists of the second tubing and helical element, both liquid filled. The two elements are so constructed that the measuring helical floats on a movable base the position of which is governed by the compensating helical. The two tubing and helical are matched in volume so that variation in temperature at the instrument case and along the capillary tubing produce equal motion from both helical. Such motion nullity each other so that only motion produced by varying the bulb temperature actuates the recorder pen.

Gas filled Thermometers:

This type depends upon the increase in pressure of a confirm gas (constant volume) due to temperature increase. The relation between temperature and pressure in this kind of system follow Charles law and may be expressed.

                                                        P₁ /P2  =   T_{1 /T2}

                                                       

The system is filled under high pressure. The increase pressure for each degree of temperature rise is therefore greater than if the filling pressure were low. Nitrogen the gas most after used for such systems, because it chemically insert and possesses a favorable coefficient thermal expansion.

Vapor - Pressure Thermometers:

Vapor pressure thermometers depend upon vapor pressure of liquid, which only partially fills the system. At low temperatures the vapor pressure increase for each unit temperature charge is small, at higher temperature the vapor pressure change is much greater.