Question

1. A temperature of the metal in liquid state need to be measured using thermo-electricity principle. Choose a thermometer which is working under this principle and justify with its three basic engineer clarify how these challenges can be resolved? (10 marks)

Answer 1

Thermo-electricity principle states that it is possible to convert heat energy into electric energy or vice versa.Thermocouple is the temperature measuring equipment which works under this principle.Working of thermocouple can be explained with three basic effects of thermocouple,i.e:Seebeck effect,Peltier effects and Thomson effect.

Seebeck effect:If a connection made between two dissimilar metal wire and its junctions at different temperatures ,a voltage will be produced such that it will be proportional to the temperature change between two metal junctions.Hence we can measure a temperature difference here rather than the temperature.To get a absolute temperature value we need to know the reference value.

Seebecte Loefficient V = x (AT) Temperature difference Voltage / Emf.

Peltier effects:It stated that if current passes through the thermocouple circuit heat transfer occurs from one junction to the other,also direction of heat transfer reverses if current direction is reversed.Heat transfer between the junctions is because of temperature change.

Electric current A B from Q = (TA-To) I AB Peltien Loefficients at conductors it f o Heat transfer

Thomson effect:It represented the heating or cooling of the homogeneous conductors with change in temperature gradients.He related reversible thermal gradient and voltage in a homogeneous conductor as follows.

Q =-K.IDT IT = J = Temperature gradient current density = 3 - 2 K = Thomson constant Q = Heat transfer per unit volume.

By considering all the above effects we design the Thermocouple for temperature measurement.A schematic representation as follows.

Junction. CONDUCTOR » Reference voltage Measuring Junction. B

Challenges faced in using thermocouple and resolving techniques.

Challenges	Solution
Less sensitive-Sensitivity(seebeck coefficient) of the thermocouple we use is very low.Hence it may require relatively high temperature to get a considerable voltage output.	Use E,J or K type thermocouple with more sensitivity. Use amplifiers to amplify the output signal.
Thermocouple aging-Since thermocouple deals with high temperatures as it becomes old seebeck coefficients and conductor properties may change ,Which will result in the inaccurate temperature measurement.	Calibrate the instrument regularly. use the high size(diameter) wire for long life.
Referance junction temperature-Measuring the reference temperature is a difficult task.	Use ice bath to make the referance temperature as 0 deg.But it is only possible for some systems. We can use a different highly temperature sensitive sensor(made of ICs ) to measure referance temperature.
Green rot-It is the process of corrosion when the Conductors are subjected to very high temperature.It is because at very high temperature oxygen content is low.	Supply oxygen.