As seen earlier, Thermocouple Instruments as two parts,
1- Thermo-electric element which consists of heater element and a Thermocouple.
2. Indicating instrument whic consists of sensitive instrument like PMMC instrument which can sense e.m.f produced which is of the order of few millivolts.
1.3 Heater Element
The Thermocouple Instruments are practically used to measure voltages and currents at very high frequencies. Hence the heater element must be such that it should not produce frequency errors such as errors due to skin effect. At high frequencies, the current has tendency to flow through outer surface of the conductor as inductance hence the reactance of inner part of conductor is very high at high frequencies. This effect due to which current flows along the skin of the conductor is called skin effect. This increases the overall resistances of the conductor. The heating element must be free from skin effect.
Practically fine wire of non magnetic materials like constantan or platinum-irridium alloy having small area of cross section is used for the heating element to minimize the skin effect
The fine wire of suitable size is used for small currents upto 3 A while thin walled tube is preferred for the current above 3 A, at high frequencies.
there are various types of thermoelements used in te thermocouple instruments.
1. Contact Cype : in this type, the hot junction of the thermocouple is in direct contact wit the heater element. This is shown in the Fig 3.
such type of the thermoelement has a drawback that te variations in the ambient temperature affect the indication. The direction of flow of current through the heating element also affects te indication.
2. Compensated Type : To avoid the drawbacks of the contact type thermoelemen, the compensated thermoelement is used. The arrangement is shown in the fig 4. There are two terminal blocks which carry the current via two parallel strips P' and Q'. The conductor P and Q are welded in the centre of the parallel strips. This forms two thermocouples PP' and QQ'. The termocouple R is insulated electrically from the terminal block. It produces thermo-electric e.m.f. opposes the thermo-electric e.m.f.s produced by the main thermocouples. This compensates the increase in e.m.f.s. of main termocouples due to increase in temperature of hot junction than the cold junction due to continuous flow of current. Such instruments are not dependent on the direction of flow of current under measurement.
3. Non contact Type : In some instruments, the thermocouple is electrically insulated from the heating element. the heat is transferred by conduction. This is shown in the Fig 5.
As heat is conducted across the insulation, the response of such instruments is slow and sluggish. The instruments are less sensitive than contact type. It is possible to increase the efficiency and sensitivity of such instruments by placing heating element and thermocouple in an evacuated glass tube. This avoids cooling for heater due to the convection air currents.
This is called vaccum thermoelement and is shown in the Fig 6.
For the currents upto 100 mA the response of these instruments is same as that of contact type. These instruments are used for the current range of 2-500 mA and suitable for frequencies upto 100 MHz or more.
4. Bridge Type : In this type, the bridge circuit is formed using thermocouples as shown in the Fig 7.
The current directly passes trough the termocouples and no separate heating elements are used. The temperature of thermocouples increases proportional to I2R.
The bridge is embedded in the baklite base plate. The cold junctions indicated as 'c' are at te pins while the hot junction indicated as H are midway between the pins. The termocouples are oriented in a specific manner so that d.c. potential difference is available across A-B where PMMC instrument is connected. The bridge is balanced by keeping resistances of all four arms equal and hence no a.c. current flows trough the instrument.
The advantages of bridge type thermoelement are,1. Greater output as much as 25 mV.
2- Much robust and can withstand mechanical shocks as well as overloading.
3. Sensitivity is greater.
4. The operating range from 100 mA to 1 A.
The thermocouple voltmeters are available for ranges upto 500 V witj tje sensitivities of 100 - 500 Ω/V.
1.5 Actions in The Thermocouples Instruments.
The various actions in thermocouples instrument are,
1. The electrical energy is converted to heat energy by the heating element.
2. The heat energy is given to the junction and converted back to an electrical energy due to seebeck effect.
3. The electrical energy is given to the PMMC instrument where it is converted to the mechanical energy utilised to deflect the pointer.
Due to all these energy transfers, the overall efficiency of such instruments is very less.
1.6 Advantages
The advantages of thermocouple instrument are,
1. Irrespective of the waveform, it indicates accurately the r.m.s. value of the voltage or current.
2. The capacitance and inductance of these instrumetns is very small hence unaffected by the frequency.
3. As free from frequency errors, can be used over very wide frequency range upto 50 to 100 MHz. Due to this, the instruments are classified as radio frequency (RF) instruments.
4. Have very high sensitivity.5. Can measure currents in the range 0.5 to 20 A.
6. Superior over any other instruments with respect to accuracy and frequency range.
7. Very useful as transfer instruments to calibrate d.c. instruments using potentiometer and standard cell.
8. Not affected by stray magnetic fields.1.7 Disadvantages
The disadvantages of termocouple instruments are,
1. The overload capacity is small. This is because of square law response. At high current, heat produced is so large which may increase the temperature upto limiting temperature.
2. Thermal conversion efficiency is poor.3. Considerable power losses.
4. The PMMC instruments used must be sensitive and delicate and hence require careful handling.