temperature measurement

Robert Gordon UniversityTemperature Measurement System

COURSEWORK

ON

TEMPERATURE MEASUREMENT

SYSTEMBY

SUSHIL GOSWAMI

0903143

3RD YEAR BEng MECHANICAL & OFFSHORE ENGINEERING

Sushil Goswami BEng. Mechanical & Offshore Engineering Page 1


We are required to measure temperatures between 0C and 120C with a high accuracy. The system uses 10V voltage source producing an output ranging from 0mV to 120mV.

The following table lifted from lecture notes makes it easy to choose the correct temperature sensor:

It is quite evident from the above table that the choice is between RTD and Thermistor. We would be discussing RTD as the temperature sensor used for this particular system application.

What is an RTD?

Resistance temperature detectors (RTDs) are temperature sensors that contain a resistor that changes resistance values as its temperature changes. They have been used for many years to measure temperature in laboratory and industrial processes, and have developed a reputation for accuracy, repeatability and stability.

Why use an RTD instead of a thermocouple or Thermistor sensor?

Each type of temperature sensor has a particular set of conditions for which it is best suited. RTDs offer several advantages:• A wide temperature range (approximately-200 to 850°C)• Good accuracy (better than thermocouples)• Good interchange ability• Long-term stability



With a temperature range up to 850°C, RTDs can be used in all but the highest-temperature industrial processes. When made using metals such as platinum, they are very stable and are not affected by corrosion or oxidation.Other materials such as nickel, copper, and nickel-iron alloy have also been used for RTDs. However, these materials are not commonly used since they have lower temperature capabilities and are not as stable or repeatable as platinum.(The above has been adapted from: www.omega.com/TEMPERATURE/pdf/ RTD _Gen_Specs_Ref.pdf )

We would be using the platinum RTD for this coursework calculation. This RTD has a resistance (R0) of 100Ω and a temperature coefficient of resistance (α) of 4x10-3C-1.This platinum RTD is to be used as a sensor in a bridge circuit with other resistances R2, R3, and R4. This bridge circuit is supplied a voltage of 10V.The following figure shows the bridge circuit with R1 as the resistance of the temperature sensor:


http://www.omega.com/TEMPERATURE/pdf/RTD_Gen_Specs_Ref.pdf


We are given that:Temperature range: 0C-1 to 120C-1.Output range: 0mV to 120mV.At minimum conditions:T=0C-1, hence the resistance of the sensor, R1=R0(1+ αT)=100(1+0x0.004)=100ΩV0(minimum)= 0mVSubstituting this into the standard bridge equation:

V 0=E ( R1R1+R4

−R2

R2+R3 )

→ 0=10( 100100+R4

−R2

R2+R3)

→ R4=100R3R2

…… (1)

At maximum conditions:

T=120C, hence the resistance of the sensor, R1=R0(1+ αT)=100(1+0.004x120)=148ΩV0(maximum)= 120mVSubstituting this into the standard bridge equation:

V 0=E ( R1R1+R4

−R2

R2+R3 )

→ 120x10-3=10( 148148+R4

−R2

R2+R3)

→ 0.012= (1

1+R4148

− 1

1+R3R2

¿

→ 0.012= (1

1+ 100148

xR3R2

− 1

1+R3R2

¿

Solving the above equation, we get two real roots.

i.e. R3R2

≅ 37.6 and R3R2

≅ 0.039



As we know that for a linear output bridge circuit, R3 ≫R2, we neglect the value 0.039.

Hence, R3R2

≅ 37.6

Hence, from equation (1),

→ R4≅ 100 x 37.6

→ R4≅ 3760Ω

From balance conditions,

R4=R3=3760Ω

R1=R2=100Ω

The standard bridge equation: V 0=E ( R1R1+R4

−R2

R2+R3 ), demonstrates a non-

linear relationship between the output reading and the measured quantity, and this does not conform with the normal requirement for a linear input-output relationship.

In some applications the bridge non-linearity may be acceptable, but there are various methods available to linearize bridges.A simple linear relationship for a RTD quarter-bridge can be obtained from the bridge output voltage. Firstly the bridge must be balanced at a single temperature, i.e. 0C. This makes R1 = R2 (i.e. at zero degrees, R2 is equal to the nominal value of R1) and R3 = R4, giving:

V 0=E [R1 R3−R0 R3

(R1+R3 ) (R0+R3 )]

To achieve the best linearity R3 must be much greater than R2, giving the following output voltage:

V 0=E ¿= ER3

(R1−R0)

If R1 the temperature sensor, and exhibits a change in resistance such that R1 = R+δR, the output voltage at other temperatures becomes:

V0=ER3δR= E

R3R0αT



Hence, by varying the resistance values, we can linearize the bridge circuit.(The above has been adapted from the lecture notes.)

Current through the temperature sensor:I 1=

ER1+R4

= 10100+3760

= 103860

=2.59mA

Now, the last part of the coursework question requires us to feed the output of the sensor into an A/D converter. It is being given that the sensor is situated in a remote location, which means that the output signal from the sensor, might have some noise signal embedded in it. For this reason, we choose Difference Amplifier as it removes the common mode noise making it a god choice for a remote sensor.A difference amplifier amplifies the difference between the two input signals. The following diagram shows a typical Difference Amplifier:

Where Va, Vb are the two input signals while V0 is the output voltage. The differential voltage gain for the above operational amplifier is given by

V 0V b−V a

=R2R1

The input range of the above operational amplifier is given as from 0V to 10V. Let’s say Vb=10V and Va=0V, then

V 0=10R2R1

By altering the ratio R2R1

, we can alter the forward gain of the amplifier.



Let’s have a look at one of the practical application of Platinum RTDs in industry. Platinum RTDs have a very useful application in roaster control for coffee beans. Good roasting techniques can produce quite acceptable results from relatively low-grade beans; bad roasting can produce quite dreadful results from the best high-grade beans. Green coffee beans are placed into a drum or chamber, where they are agitated or tumbled as they are roasted by hot air. The critical variable is the temperature of the beans. Other variables can help to improve the control of the critical variable. The heated air is easier to measure, but it can differ from the bean temperature by as much as 200 degrees F, so it is not a sufficient indicator by itself.A thermal probe measuring the temperature within the coffee beans provides the most important feedback. The probe must withstand the pounding as beans are tumbled. Because of its rugged sheath, this kind of probe will not respond quickly, but the temperature profile will not change very fast either, and a reasonable balance is maintained.Platinum RTD probe is well suited for this application because it is reliable, accurate, stable, easy to use, and well suited for an operating temperature range of 100 to 500 degrees F.(The above has been adapted from: http://www.mstarlabs.com/sensors/rtd-example.html)


http://www.mstarlabs.com/sensors/rtd-example.html

http://www.mstarlabs.com/sensors/rtd-example.html













Documents

temperature measurement