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a) The process of determining the degree of hotness or coldness of an object .

b) Controlling temperature is important in many process like mixing or separating materials .

c) Right temperature keeps machine working safely and properly

TEMPERATURE SCALE

1) Celsius (C)

-Commonly used around the world

-water freezes at 0 C and boils at 100 C

2) Fahrenheit (F)

-Mainly used in the united states

-water freezes at 32 F and boils at 212 F

3) Kelvin ( S.I )

-Use in science and engineering

-Water freezes at 273 K and bpoils at 373 K

RELATIVE AND ABSOLUTE TEMPERATURE

1) Relative

- Zero on the scale doesn't mean no thermal energy

- uses unit like Celsius (C) or Fahrenheit (F)

- The scale is based on reference points , not actual energy

2) Absolute

- Zero temperature means no thermal energy

- uses units like Kelvin (K) ae Rankine (R)

- The scale measures energy directly

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Comparison of the three temperature scale

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Comparison of the three temperature scale

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Understand the Thermocouple Circuit Working Principle and Its Applications

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What is a Thermocouple?

• A temperature sensor made by joining two dissimilar metals.

• When the junctions are at different temperatures, a voltage (emf) is produced.

• Voltage corresponds to the temperature difference between junctions.

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Working Principle

• Based on the Seebeck effect: emf is generated when heat flows from hot to cold junction.

• Thermocouple has:

• Hot Junction – Exposed to the process temperature.

• Cold Junction – Reference point (often 0°C or electronically compensated).

Formula:

emf = b × (T1 - T2)

Where:

• b = Seebeck coefficient (V/K)

• T1 = Temperature at hot junction

• T2 = Temperature at cold junction

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Output Characteristics

• Output is in millivolts.

• Non-linear relationship between voltage and temperature.

• Low sensitivity: ~50 µV/°C.

• Needs amplification and filtering.

Thermowell

• A protective metal tube for thermocouples.

• Functions:

• Shields sensor from damage/corrosion.

• Allows sensor removal/replacement without process interruption.

• Commonly used with high-pressure or hazardous fluids.

Advantages

• Low cost

• Simple and rugged

• Fast response time

• Wide temperature range (-200°C to 1700°C)

• Works in harsh environments

Disadvantages

• Accuracy is lower than RTDs

• Output is not linear

• Sensitive to electrical noise

• Requires cold junction compensation

• May degrade over time, especially in corrosive environments

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Important Effects

• Seebeck Effect: Voltage from heat difference.

• Peltier Effect: Heat absorbed/released at junction when current flows.

• Thomson Effect: Additional emf in single wire with a temperature gradient; must use homogeneous wires to avoid error.

Cold Junction Compensation

• Traditional: Cold junction in ice bath (0°C).

• Modern: Electronic compensation simulates reference temperature.

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Characteristics:

•Platinum RTD (Pt100) is the most commonly used type.

•Temperature range:

•Platinum: -100°C to 650°C

•Nickel: -180°C to 300°C

•Temperature coefficient (α):

•Platinum: 0.00385/°C

•Nickel: 0.005/°C

Construction:

•The RTD element usually consists of a thin wire coiled around a ceramic or glass core, and is placed inside a protective probe.

Advantages:

•High accuracy and long-term stability

•Easy to recalibrate

•Linear response to temperature changes

Disadvantages:

•Higher initial cost compared to thermocouples

•Narrower operating temperature range

•More fragile due to delicate construction

Applications:

•RTDs are increasingly replacing thermocouples in industrial applications below 600°C due to better accuracy and repeatability.

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Definition:

RTDs are temperature sensors that measure temperature based on the change in metal resistance as temperature changes. Made from metals like platinum (Pt) or nickel (Ni).

Working Principle:

•The resistance of metals increases as temperature increases.

• The formula expresses the relationship between resistance and temperature:

R_T = R_0(1 + \alpha T)

•R_T: resistance at temperature T

•R_0: resistance at 0°C

•\alpha: temperature coefficient of the metal

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CRITERIA FOR SELECTING A TEMPERATURE-MEASURING INSTRUMENT

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CRITERIA FOR SELECTING A TEMPERATURE-MEASURING INSTRUMENT

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1. Circuit boards :

-Monitor and control temperature.

2. Computers :

-Control CPU temperature

3. Telecommunication :

- Cell phone & PDAs

4. Industrial immersion applications

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Thermistors are temperature sensitive resistor

• Made from semiconductor

• Change in temperature cause change

in resistance, which can be measured to determine the temperature

• Type of thermistors

-Negative Temperature Coefficient

-Positive Temperature Coefficient

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ADVANTAGES

• High sensitivity

• More Acurate

• Fast respon time

DISADVANTAGES

• Limited Temperature Range

(-40 C to +150 C)

• Fragility

• Non - linear response