Temperature Sensors

As-Salaam-Alaikum, Welcome to my blogger "World of Knowledge".Here we discuss the topic Temperature Sensors.

Points to be discuss in this Article are given below:

• Temperature Sensors
• Bimetallic Thermometer
• Construction & Operation of Bimetallic Thermometer
• Liquid Crystals Thermometer
• Infrared Thermometers (Non contact Sensors)
• Measurement Principles
• Infrared Thermometers
• IR Thermometer Advantages
• Resistance Thermometers
• Thermister
• Thermocoupes
• Operation of Thermocouples
• IC Temperature Sensor

Temperature Sensors:

A measure of Substance’s internal K.E,The degree of hotness or coldness of a substance

How to Sense it?

Changes in,

• Physical dimensions (Bimetallic Thermometers, Filled bulb and glass stem)
• Electrical properties (IC Transistors and Diodes, RTDS, Thermistor, Thermocouples)
• Chemical Phase (Quartz Crystal)
• Emitted Thermal Radiation (Radiation and Infrared Pyrometers)

Bimetallic Thermometer:

• Two metals with different coefficients of thermal coefficients are bonded together.
• If one end is fixed, other end displaces in response to temperature changes.
• A metal tends to undergo a volumetric  dimensional change (expansion/ contraction) according to change in temperature.
• Bimetal strips can be fabricated into coils, spirals, and disks. Frequently used in On/Off temperature control (thermostats)
• Range -65 to 630C
• Accuracy +- 0.5 to 12 C

Advantages:

• Low cost
• Negligible maintenance
• Stable

Construction of Bimetallic thermometer:

Figure 1

Figure 2

Operation of Bimetallic Thermometer: 

• A change in temperature causes the free end of the strip to expand or contract due to different coefficients of expansion of the two metals.
• This movement is linear to the change in temperature and the deflection of the free end can be read out by attaching a pointer to it.
• This reading will indicate the value of temperature.

Liquid Crystals Thermometer:

•  It posses the mechanical properties of a liquid, But have the optical properties of a single crystal.
• Temperature change can affect the color of a liquid crystal, which makes them useful for temperature measurement.

Range and resolution is adjusted by formulation.

•  RANGE 0 TO SEVERAL HUNDRED
• Resolution 1C to 0.5C     Special types: 0.1C

A liquid crystal thermometer, temperature strip or plastic strip thermometer is a type of thermometer that contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change colour to indicate different temperatures.

Infrared Thermometers (Non contact Sensors):

•  IR wavelength between 0.7 to 1000 microns
• All objects warmer than absolute zero (0K) emit energy in IR Range.
• IR thermometers sense this energy.
• Most IR sensors can measure between 0.7 and 20 micron; are not sensitive enough to measure small energies outside the range.

Figure 3

Measurement Principles:

• Energy transmitted as Electromagnetic waves or photons travel at the speed of light.
• Radiation striking at the surface is partially reflected, absorbed and transmitted.
• Radiation heat transfer

Infrared Thermometers:

• Lens to focus energy from object onto a Detector that converts the thermal energy into electrical signal.
• Signal conditioning(filters and amplifiers) and emissivity adjustment to correct various surface finishes and materials.

Figure 4

IR Thermometer Advantages:

• IR thermometers can measure objects that move, rotate or vibrate.
• They can measure temperature>1500C
• They do not damage or contaminate the surface of the object of interest (food, painted surface)
• Response Time is in Millisecond

Resistance Thermometers:

•  RTD (Resistance Temperature Detector) is a metal conductor, has a positive coefficient of resistance. R increases with T
• Thermister, made from semiconductor materials, have a large negative coefficient of resistance. R decreases with T
• Metals have linear Relationship with temperature
• Each metal has a specific resistivity, which varies with temperature.
• R=ρL/A
• ρ_T=ρ_o[1+a(T-T_o)]

Figure 5

Thermister:

• High Resistance 1kὨ to 100KὨ
• Highly nonlinear resistance to temperature relationships.
• Mostly NTC (Metal oxides) but PTC(barium and strontium titanate mixtures) models available.
• Small physical size, Fast response time
• Lower cost that RTD’s
• Very high sensitivity and resolution: upto 1000 times more sensitive than RTD’s

Figure 6

Thermocouples:

• Operate under the principle that the junction of two dissimilar metals produces a measurable voltage when two ends of the thermocouple are at different temperatures.
• They are inexpensive, small in size, rugged and remarkably accurate

Figure 7

Operation:

• When two dissimilar wires joined and a junction is formed an emf voltage will be generated. 

Voltage or emf produced depends on:

• Type of material used, A and B
• Temperature of the junctions, measuring and reference

IC Temperature Sensor:

An IC Temperature Sensor is a two terminal integrated circuit temperature transducer that produces an output current proportional to absolute temperature. The sensor package is small with a low thermal mass and a fast response time.

Figure 8

Figure 9