Thermocouple A thermocouple or thermocouple

thermometer is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control[1] and can also be used to convert heat into electric power. They are inexpensive[2] and interchangeable, are supplied fitted with standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy: system errors of less than one kelvin (K) can be difficult to achieve

Applications Thermocouples are suitable for measuring over

a large temperature range, up to 2300 °C. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications thermistors and resistance temperature detectors are more suitable. Applications include temperature measurement for 

kilns,  gas turbine exhaust,  diesel engines, and other industrial processes

Types of Thermo-couples Type K (Chromel / Alumel)  - Type E (Chromel / Constantan) Type J (Iron / Constantan) Type N (Nicrosil / Nisil) Type B (Platinum / Rhodium) Type R (Platinum / Rhodium) Type S (Platinum / Rhodium)

SpecificationsThermocouple type Overall range °C 0.1°C resolution 0.025°C resolution

B 20 to 1820 150 to 1820 600 to 1820

E -270 to 910 -270 to 910 -260 to 910

J -210 to 1200 -210 to 1200 -210 to 1200

K -270 to 1370 -270 to 1370 -250 to 1370

N -270 to 1300 -260 to 1300 -230 to 1300

R -50 to 1760 -50 to 1760 20 to 1760

S -50 to 1760 -50 to 1760 20 to 1760

T -270 to 400 -270 to 400 -250 to 400

Resolutions in high range

E J K

SR

B

N

T

Resolution in Low range

Precautions and Considerations for Using Thermocouples

Connection problems. Many measurement errors are caused by unintentional thermocouple junctions. Remember that any junction of two different metals will cause a junction. If you need to increase the length of the leads from your thermocouple, you must use the correct type of thermocouple extension wire (eg type K for type K thermocouples). Using any other type of wire will introduce a thermocouple junction. Any connectors used must be made of the correct thermocouple material and correct polarity must be observed.

Precautions …… Lead Resistance. To minimise thermal shunting and improve

response times, thermocouples are made of thin wire (in the case of platinum types cost is also a consideration). This can cause the thermocouple to have a high resistance which can make it sensitive to noise and can also cause errors due to the input impedance of the measuring instrument. A typical exposed junction thermocouple with 32 AWG wire (0.25 mm diameter) will have a resistance of about 15 Ohms / meter. The Pico TC-08 has an input impedance of2 MΩ so will have an error of less than 0.01% for 12 meters of such cable. If thermocouples with thin leads or long cables are needed, it is worth keeping the thermocouple leads short and then using thermocouple extension wire (which is much thicker, so has a lower resistance) to run between the thermocouple and measuring instrument. It is always a good precaution to measure the resistance of your thermocouple before use.

Precautions ……

Decalibration is the process of unintentionally altering the makeup of thermocouple wire. The usual cause is the diffusion of atmospheric particles into the metal at the extremes of operating temperature. Another cause is impurities and chemicals from the insulation diffusing into the thermocouple wire. If operating at high temperatures, check the specifications of the probe insulation.

Precautions ……

Noise. The output from a thermocouple is a small signal, so it is prone to electrical noise pick up. Most measuring instruments (such as the TC-08) reject any common mode noise (signals that are the same on both wires) so noise can be minimised by twisting the cable together to help ensure both wires pick up the same noise signal. Additionally, the TC-08 uses an integrating analog to digital converter which helps average out any remaining noise. If operating in an extremely noisy environment, (such as near a large motor) it is worthwhile considering using a screened extension cable. If noise pickup is suspected first switch off all suspect equipment and see if the reading changes.

Resistively of Material

R = ρ L / A         (1)

where R = resistance (ohm) ρ = resistivity coefficient (ohm m) L = length of wire (m) A = cross sectional area of wire

(m2

dR / Rs = α dT         (5)

where dR = change in resistance (ohm) Rs = standard resistance

according reference tables (ohm) α = temperature coefficient of

resistance dT = change in temperature (K)

Material          

Resistivity (Ω·m) at 20 °C          

Temperature coefficient* [K−1]          

Silver 1.59×10−8 0.0038

Copper 1.72×10−8 0.0039

Gold 2.44×10−8 0.0034

Aluminium 2.82×10−8 0.0039

Calcium 3.36x10−8 ?

Tungsten 5.60×10−8 0.0045

Zinc 5.90×10−8 ?

Nickel 6.99×10−8 ?

Iron 1.0×10−7 0.005

Tin 1.09×10−7 0.0045

Platinum 1.06×10−7 0.00392

Lead 2.2×10−7 0.0039

Manganin 4.82×10−7 0.000002

Constantan

4.9×10−7 0.000 008

Mercury 9.8×10−7 0.0009

Nichrome[5] 1.10×10−6 0.0004

Carbon[6] 3.5×10−5 −0.0005

Germanium[6]

4.6×10−1 −0.048

Silicon[6] 6.40×102 −0.075

Assignment No 4

T J K

SR

N

E

BT

•Which of type is best for measuring range of 0 to 10000C ?•Draw a circuit diagram for quarter bridge arrangements.

Crystal oscillator

 crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency

The most common type of piezoelectric resonator used is the quartz crystal, so oscillator circuits designed around them were called "crystal oscillators".

Operation

When a crystal of quartz is properly cut and mounted, it can be made to distort in an electric field by applying a voltage to an electrode near or on the crystal. This property is known as piezoelectricity. When the field is removed, the quartz will generate an electric field as it returns to its previous shape, and this can generate a voltage. The result is that a quartz crystal behaves like a circuit composed of an inductor, capacitor andresistor, with a precise resonant frequency. (See RLC circuit.)

Oscillations

MechanicalElectrical

Electrical ModelA quartz crystal can be modelled as an electrical network with a low impedance (series) and a high impedance (parallel) resonance point spaced closely together. Mathematically (using the Laplace transform) the impedance of this network can be written as:

OR

where s is the complex frequency (s = jω), ωs is the series resonant frequency in radians per second and ωp is the parallel resonant frequency in radians per second

Temperature effects A crystal's frequency characteristic depends on

the shape or 'cut' of the crystal. A tuning fork crystal is usually cut such that its frequency over temperature is a parabolic curve centered around 25 °C. This means that a tuning fork crystal oscillator will resonate close to its target frequency at room temperature, but will slow down when the temperature either increases or decreases from room temperature. A common parabolic coefficient for a 32 kHz tuning fork crystal is −0.04 ppm/°C².

Applications

This frequency is commonly used: to keep track of time (as in 

quartz wristwatches),to provide a stable clock signal for digital

integrated circuits, and to stabilize frequencies for 

radio transmitters and receivers. (145-146 MHz crystal FM Transmitter)

quartz wristwatches

You can use to produce a 32.768 KHz square wave from a common watch crystal. The output can be fed to a 15 stage binary counter to obtain a 1 second square wave. The circuit on the left using the 4069 inverter is recommended over the transistor circuit and produces a better waveform. The single transistor circuit produces more of a ramping waveform but the output swings the full supply voltage range so it will easily drive the input to a CMOS binary counter.

Clock for digital-integrated circuits

74HCU04 is a chip that was made for this purpose, HCT may not work for such a circuit. C1 and C2 can go to upto 33pF and R2 can be increased to make R2 * C2 = t.Time constant much less than the period T of the crystal T = 1/F . This is to remove higher frequency components in the Oscillator.