TEMPERATURE MEASUREMENT

Temperature is one of the important parameter which measured in every industry and it is the

effect of hotness and coldness of material.

Units of measurement:

1) Degree centigrade (oC)

2) Kelvin(o K)

3) Fahrenheit(o F)

4) Rankine (o R)

Conversion:

oF = (oC*9)/5+32

oK =0C+273.15

oR = oF+459.67

Temperature indicators:

There are two types of temperature indicators (thermometers)

1) filled bulb temperature indicator

2) bi-metallic strip temperature indicator

Filled bulb temperature indicator:

The principle behind filled bulb temperature indicator is that when filled bulb exposed to

process temperature, due to expansion of the fluid it will rise up and it is connected to bourden

tube and it will deflect with respect to the pressure of fluid and intern temperature of fluid.

We know that as temperature increases, pressure increases with the constant volume hence

temperature and pressure are proportional. The figures are shown below,

They are classified based on the filling material used

1) liquid filled(class I)

2) vapor filled (class II)

3) gas filled (class III)

4) mercury filled(class v)

Class Temperature range Response Class- I -125 oF to 600 oF slower

Class- II -40 oF to 600 oF faster

Class- III -450 oF to 1400 oF fastest

Class- V -40 oF to 1200 oF fastest

Fig1. Class I liquid filled temperature indicator.

Fig2. Class II Vapor filled temperature indicator

Suppose if we want temperature gauge to be panel mounted, by using capillary extension we

can pass the fluid but limited to small distance. The fig. shown below,

Fig. armored capillary extension temperature gauge

Bimetallic strip temperature indicator

Bimetallic strip temperature indicator uses two different material of different coefficient of

expansion and which are attached each other.

Figure above shows has fixed support and metals (metal1 and metal2) attached each other

assume metal1 has high expansion coefficient than metal 2, if we heat at the bottom metal1 try

to expand more than metal2 so it moves upwards.

By using this property it can be used in temperature indicators, the fig shown below,

Usually we use helical form bimetallic strip in which one end is fixed and other end is free to

rotate.

Range: -70 oC to 50 oC minimum

100 oC to 550 oC maximum

Advantages:

Good response time compared to filled tube thermometer

Good linear response.

Disadvantages:

Hysteresis is more in bimetallic strip thermometer

Limited to direct mounting only because we can’t mount thermometer in panel

mounted as in filled bulb thermometer by using capillary extension.

Deformation of bimetallic strip will cause failure or error in measurement.

Temperature transmitters

Temperature transmitter are used for local indication, transmission, controlling and recording

of measured temperature variable.

There are mainly 3 types of temperature sensor are used

1) RTD (Resistive Temperature Detector)

2) Thermocouple

3) Thermistor

RTD (Resistance Temperature Detectors)

It works on the principle that material will change its resistance, when there is a change in

temperature. RTD’ are positive temperature coefficient (PTC), it means, as temperature

increases, resistance increases.

RTD’s are linear in nature. The relation between the R vs. T gives the sensitivity of RTD 0.385

ohm/°C (nominal for Pt-100).

Each resistance versus temperature relation for an RTD is qualified by a term known as “alpha”. “Alpha” is the slope of the resistance between 0°C and 100°C. This is also referred to as the temperature coefficient of resistance, with the most common being 0.00385 ohm/ohm/°C. While replacing the RTD make sure that the temperature coefficient of new RTD should be same as the old RTD. RTD’s are constructed of resistive material with leads placed in sheath Resistive material may be

1) Platinum

2) Nickel

3) Copper

Platinum is more linear and has maximum range compared to nickel and copper so platinum is

preferred more than copper and nickel.

Ex: Pt-100 where Pt represents Platinum and 100 denotes the 100 ohm resistance at 0°C.

The resistive material can be wire wounded in sheath or thin film type.

Sensor sheath mainly used to protect the resistive material usually we often use stainless steel

sheath but for higher temperature we go for Inconel or haste alloy.

The fig. shown below.

The sensing element (RTD) may be single or dual in the single sheath, it is better we use two or

more sensing element and it is beneficial due to following reasons

1) Backup sensing element(if one sensing element fails)

2) Any drift in measurement can be identified

3) We can send the input to the two different independent controllers

4) Average of two outputs will be more accurate than single sensing element

Wiring configurations

There are three types of wiring configurations

1) Two wire configuration

2) Three wire configuration

3) Four wire configuration

Two wire configuration

It is the simplest wire configuration used where the high accuracy of measurement is not

needed.

The accuracy is less because, lead wires add the resistance with the sensor output. It is used

only up to 100meter cable.

Three wire configuration

In order to minimize the effects of the lead resistances, a three-wire configuration can be used.

Using this method the two leads to the sensor are on connected to arms. There is a lead

resistance in each arm of the bridge so that the resistance is cancelled out, so long as the two

lead resistances are accurately the same. This is used up to 600 meters of cable.

Four wire configuration

The four-wire resistance thermometer configuration increases the accuracy and reliability of

the resistance being measured. The resistance error due to lead wire resistance is zero. In the

diagram above a standard two-terminal RTD is used with another pair of wires to form an

additional loop that cancels out the lead resistance. The above Wheatstone bridge method uses

a little more copper wire and is not a perfect solution.

Features:

Range: -200° to 1475°F

Interchangeability = ±0.25°C at 0°C

Sensitivity: 0.385 ohm/°C (nominal for Pt-100)

The current international standard which specifies tolerance, and the temperature-to-electrical

resistance relationship for platinum RTD is IEC 60751.

Accuracy of RTD’s are classified into Class A and Class B according to IEC-60751

Class A ±0.6 ohm at 0°C

Class B ±1.2 ohm at 0°C

Advantages:

RTD’s are more linear compared to Thermocouple and Thermistor.

Good accuracy.

Good repeatability.

Less drift compared to thermocouple.

Good stability at high temperature

Disadvantages:

Response time is high compared to thermocouple.

Less sensitivity compared to thermistor

Costlier (because of platinum resistive element)

Requires three or four-wire operation

Affected by shock and vibration

Thermocouple:

Thermocouple is a temperature-measuring device consisting of two dissimilar conductors that

contact each other at one spots. It produces a voltage when the temperature of one of the spots

differs from the reference temperature at other parts of the circuit. This is called Seeback effect.

The below figure shows that the reference temperature is maintained constant and other end is

exposed to target surface to measure the temperature. And output is taken at lead wires (cold or

reference junction). If lead wires are short copper wires are normally used.

As temperature increases the output voltage increases, the linearity between the temperature

and the voltage depends on the combination of metal used to make the thermocouple.

There are many types of thermocouple, Type B, E, J, K, N, R, S, and T.

Advantages:

Thermocouple can be used -450 oF to 4200 oF

It is very rugged

it is self-powered i.e. active transducer

there is no self-heat

Disadvantages:

Cold junction compensation is required.

nonlinear temperature vs. voltage relationship

Accuracy is less compared to RTD.

In conclusion selection of sensor is depends on the temperature range

We know that RTD can be used up to 850 oC above that thermocouple is the

choice. Even though thermocouple is less cost compared to RTD, it requires more

maintenance i.e. Frequent calibration, replacement and even it has lower

accuracy and this would lead to cost more than what initial savings instead of

using RTD.

Only reason to use thermocouple is that for temperature above 850 oC.

For applications where only trends required or several degrees of error is

acceptable than we go for thermocouple.

But for control applications where very high accuracy is needed we go for

properly designed and installed RTD sensor below 500 oC. Up to 850 oC we can use

but there is a drift after this range.

INSTALLATION:

All temperature measuring instruments either temperature indicators/thermometer (gauge) or

temperature transmitter requires special installation called thermowell.

Thermowell:

Thermowells are tubular fittings used to protect temperature sensors installed in industrial

processes.

The fig shown below,

The thermowell are either tubular or machined bar stock. Usually tubular Thermowells are

used for low pressure and low flow applications.

Immersion length is the total length from tip of the thermowell to the point of fluid immersed.

Insertion length is the total length from tip of the thermowell to bottom of end connection

either it may be flanged or threaded connection.

The temperature sensors are inserted into the bore of the thermowell and it should be tight fit

else there is a time lag in measurement, so that the materials which transfers temperature

more easily are filled in the gap.

Installing temperature elements in thermowells adds time lag and errors to the measurement.

However, by using a spring loaded fitting(stainless steel) so the element firmly contacts the

bottom of the thermowell and for temperatures over 288°C (550°F) Inconel 718® springs

should be provided rather than stainless steel.

Stem profiles:

There are three stem profiles available

1) Straight profile

2) Stepped profile

3) Tapered profile

Straight profile

The straight profile is shown in figure. And it has same diameter throughout the immersion

length i.e. U length.

Due to increase in tip diameter which has large drag force and slow response time compared to

other styles but good strength for high flow and high pressure.

Stepped profile

In stepped profile there is a change in the diameter to reduce the drag force, and due to small

tip diameter it will give faster response compared to straight profile.

Importantly this profile has higher natural frequency so it will avoid vibration effects as

compared to straight profile but one demerit is that it will erode easily due to smaller tip

diameter and even it has less strength for high flow and high pressure.

Tapered profile

We know that in stepped profile will provide less strength for high flow and high pressure but it

gives the faster response. But in straight profile will provide good strength for high flow and

high pressure but slower response.

To compromise between the strength and response time tapered profile is the perfect choice.

The optimum immersion length leads to good accuracy and response time and it is important

because Thermowells vibrates as their natural frequency is approached, this causes them to

eventually fracture and fail. So design of the optimum immersion length is very important.

We know that each thermowell has its natural frequency depends on the shape, length, and

MOC (Material of Construction) and the shedding frequency is generated around the

thermowell, when shedding frequency reaches the natural frequency of thermowell, vibration

of thermowell takes place and failure or breakage of thermowell will takes place. Hence to

avoid this failure, we should select a proper stem profile which as high natural frequency

(stepped profile) and proper immersion length.

Shedding frequency of thermowell should always less than equal to 80% of natural frequency is

the thumb rule for selection of thermowell. i.e. fs/fn ≤ 0.8

ASME Performance Test Code PTC 19.3 TW provides information and calculation procedures for

Thermowells.

The minimum recommendations for immersion length is Between 1/3 to 2/3 of pipe diameter

(Inside diameter) and if we come to response time of the sensor which depends on the tip

diameter and tip thickness.

End connections of thermowell:

there are mainly 3 types of connection

1) Threaded connection

2) Welded connection

3) Flanged connection

Threaded connection:

Threaded connection used for low pressure and low flow application.

Welded connection:

Welded connection used for high pressure and high flow application and it provides tight shut

shut off for hazardous fluid applications.

Flanged connection:

Due to corrosion effect on the welded connection, we go for flanged connection in which

thermowell flange should be one class higher than mating pipe flange(1 ½ “).