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

• 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.

• 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

• 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

• 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.

•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.

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

• 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

• High sensitivity

• More Acurate

• Fast respon time

DISADVANTAGES

• Limited Temperature Range

(-40 C to +150 C)

• Fragility

• Non - linear response