Temperature measurement

Temperature Measurement.

What is Temperature?

Temperature is defined as the degree of hotness or coldness measured on a definite scale. Hotness and coldness are the results of molecular activity.

If the movement of the molecules is fast it indicates that temperature is higher, if the movement of molecules is slow its mean temperature is low.

Absolute zero temperature

Absolute zero is the temperature at which the movement of molecules completely stops.

Definition of Heat?

Heat is a form of energy and heat is measured in calories.

Importance of temperature measurement

Temperature measurement is very important in daily life as well as in industrial processes. In daily life ambient temperature is measured in routine, as well as a body temperature of a person is directly related to health.

In the industrial system, most processes depend upon temperature, if no proper temperature process disturb and product quality affected. Temperature is also measured from the safety point of view of persons and industrial equipment. Uncontrolled temperatures can cause structural deterioration of pipelines and vessels, damage of bearings of motor and gears, and moving shafts.

Temperature measurement units

Centigrade

Fahrenheit

Kelvin

Rankin

Conversion of temperature units

Fornhite to centigrade:

Centigrade= 5/9(f-32)

= (f-32)/1.8

centigrade to kelvin

Kelvin=c+273

centigrade to forenhite

Fornhite = (1.8*c)+32

The temperature can't be cooler than 0K or -273 centigrade

Temperature Sensors

Different types of temperature sensors are used to measure temperature. Temperature measurement devices commonly used are:

1. Thermometers,

2. Resistance Temperature Detectors (RTDs)

3. Thermocouples,

4. Thermostats

Thermometers

In Temperature measurement devices, the basic device is a thermometer.

There are two types of the thermometer;

a) Filled Thermometers

b) Bimetallic Strips

Filled Thermometers

Thermometers filled with, liquid-like mercury, or an evaporating fluid are called filled thermometers.

Either the filled material is liquid or evaporating fluid, the sensor head, and the connecting tube are full of liquid or fluid.

When temperature increases, expansion or evaporation of the liquid takes place and the sensor is pressurized. This pressure is directly proportional to the temperature that we can read on a pressure gauge or sensor.

b) Bimetallic Strips

Two metals are connected to make a bimetallic strip, these two metals have different coefficients of thermal expansion.

When one end of the bimetallic strip is heated, the metal with a high-temperature coefficient of expansion expands more. so the metallic strip bends towards low thermal coefficient metal. This movement is used to measure the temperature change.

Bimetallic strip applications:

Bimetallic strips are used as a fast-acting thermostat to control air temperature. In electrical iron bimetallic strips are used to make it automatically on and off.

2. Resistance Temperature Detector

RTD is an abbreviation of Resistance Temperature Detector.

This temperature sensor is used to measure temperature by measuring the change in resistance of a specific material having a linear change in resistance with respect to temperature.

The material used for RTD

RTD is made up of different materials like

Platinum,

Nickel

Copper

Molybdenum

Any conductor can be used to construct an RTD, but a few Types of material are identified as having more described characteristics than others.

To choose a material for RTD construction following characteristics are considered

1. Stability of RTD:

Stability in the temperature range to be measured is a basic characteristic of material for RTD.

The material must not melt or change electrical characteristics when subjected to the environment in which it will operate.

2. High resistively of RTD:

The high resistive material is better to use because less quantity of the material is needed to manufacture an RTD, so the size of the RTD is small with a specific resistance.

3. Linearity of RTD:

Linearity is a basic characteristic of the material to be used as an RTD.

Resistance of a substance change with temperature change, it should be as liner as possible over the range of measurement.

Pt 100 RTD

Commercially RTD is available in a resistance range from 50 to 1000 ohms.

Platinum is used to construct the RTD because its characteristics are best for RTD construction.

Platinum is mostly used for industrial temperature measurement range from -150°C to 650 °C.

Platinum RTD known as Pt100, because it has 100 Ohm resistance at 0°C and increases resistance is 0.385 ohms for every °C of temperature rise. So roughly speaking 1-ohm resistance increase when 2.6°C

By measuring the resistance of the RTD, the temperature can be calculated by the following formula:

°C = ( Ohms reading – 100 ) / 0.385

Pt100 ohm resistance at 0-degree centigrade, resistance increases as temperature increases

(ratio of temperature and resistance is approximately 2.6 c/ohm)

Pt 1000

(It gives 1000 ohm resistance at zero degrees centigrade. Some more types of platinum RTD are Pt100, Pt200 Pt500 Pt1000, Pt3000 Pt6000 Pt9000

Types of RTDs

1. Film Type RTD (Less accurate but inexpensive)

2. Wire Type RTD (More accurate but expensive)

Metal Film RTD's

A new construction technique used to construct RTD is metal film RTD. Platinum or metal glass slurry film is deposited or screened onto a small flat ceramic substrate, then etched with a laser trimming system, and sealed.

The metal film RTD technique reduces assembly time size with high resistive. Due to its small size, it responds quickly to step temperature changes. Film RTDs are less stable, but popular because of their small size and cost.

TYPES OF RTD ACCORDING TO CONNECTION

RTDs are available in 2-wire, 3-wire, and 4-wire configurations

2-wire connection of RTD:

To detect the small variations of resistance of the RTD, a temperature transmitter in the form of a Wheatstone bridge is generally used.

The circuit compares the RTD value with three known and highly accurate resistors.

A problem arises when the RTD is installed some distance away from the transmitter. Since the connecting wires are long, the resistance of the wires changes as the ambient temperature varies. The variations in wire resistance would introduce an error in the temperature measurement at the transmitter. To eliminate this problem, a three-wire RTD is used.

3-wire RTD connection:

The three connecting wires are made of the same length and same resistance. The power supply is connected to one end of the RTD at the top of the Wheatstone bridge.

The resistance of the right leg cable of the RTD is equal to the left leg cable of the RTD.

Since resistances of the wires cancel and therefore the effect of the connecting wires is eliminated.

Advantages of RTD

Some advantages of RTD are illustrated here;

1. Linear Response

RTD output response is linear for a large range of temperature measurements.

2. Good stability

RTD remains very stable even at high in any industrial process temperatures measurement.

3. High Repeatability

RTD has high repeatability and Good precision.

4. Low Drift.

RTD is Most accurate and stable over time

Drawbacks of RTD

1. Low sensitivity

RTD has a low Sensitivity and response time is slow as compared to the thermocouple.

2. RTD is not capable of Point sensing.

3. Shock and vibrations affect the reading of the temperature when RTD is used as a temperature sensor.

4. Electronics Circuit for temperature measurement with RTD is a little complicated as it requires 3/4 wire so cost increase due to this factor especially when the cable length is long enough.

Thermocouple