resistance thermometer with wheat stone bridge

Temperature Measurement

Resistance Thermometer with wheat stone bridge USN :5WJ10MTZ05

Harinivas P

Resistance Thermometer with wheat Stone Bridge

Presenter:Harinivas PUSN No: 5WJ10MTZ05IV SEM (TPE ) / M.TechSIT, Tumkur.



Harinivas P

We are all familiar with modern electrical measuring instruments.

These instruments accurately measure ac and dc volts, ac and

dc currents, resistance and other electrical quantities to a high accuracy.

But, have you ever thought about how these instruments are calibrated?

How do we know, for example:

That when the instrument indicates 1.00V, there is actually 1.00V present?



Harinivas P

Before the arrival of the Digital Multimeter (DMM), galvanometers such as the moving coil meter, were the main way of measuring electrical quantities.

Here, a coil is suspended in a permanent magnet field and when a current is passed through that coil it generates its own magnetic field.

These fields then react with each other so causing a mechanical force to exist between them.

With the aid of a spring and pivots, the coil rotates with respect to the permanent magnet and a pointer attached to the coil also moves.



Harinivas P

Resistance Thermometer

Resistivity of metals is a function of temperature.

Resistance thermometers, also called resistance temperature detectors (RTD)

Sensors used to measure temperature by correlating the resistance of the RTD element with temperature.

Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic orglass core.

The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it.

The RTD element is made from a pure material whose resistance at various temperatures has been documented.

The material will have predictable change in resistance as the temperature changes; it is this predictable change that is used to determine temperature.



Harinivas P

Resistance Thermometer

As they are almost invariably made of platinum, they are often called platinum resistance thermometers (PRTs).

They are slowly replacing the use of thermocouples in many industrial applications below 600 °C, due to higher accuracy and repeatability.

Common RTD sensing elements constructed of platinum copper or nickel have a unique, and repeatable and predictable resistance versus temperature relationship (R vs T) and operating temperature range.

The Resistance vs Temperature relationship is defined as the amount of resistance change of the sensor per degree of temperature change



Harinivas P

Wheat stone Bridge :

• Figure shows a purely simplest form of resistive wheat stone bridge.

• The excitation voltage mat be AC or DC, we Consider Only DC supply.

• The Basic Principle May be of two ways.

• The Null Method

• Deflection Method

• Let us assume resistance are been adjusted to make the bridge balanced.

• ie R1 / RA = R2 / R3

• Now , if we change R1 Resistance, this will make the bridge unbalance and there will

be a voltage change appear across.

• The meter reading is an indication of the change in R1 and this actually can be utilized

to computer this change, this is known as deflection method.

Deflection Method



Harinivas P

Null Method

• In Null method, one of the resistors is adjustable manually.

• This if R1 changes, causing a meter deflection, R2 can be adjusted manually until

• Its effect just cancels that of R1 and the bridge returns to balanced conditions.

• The adjustment of that the meter returns to its null or zero position.

• In this case the numerical value of the changes in R1 required to effect the balance

is recorded.



Harinivas P

Wheat stone Bridge :

• Uses ratio of 3 known resistors.

• Measures fourth unknown resistance

• Balanced voltage between point 1 and battery’s negative, and between point 2 and

battery’s negative allows the measurement

• By changing resistors to adjusting variable resistors to balance the device, the

mathematical ratio is used to calculate the fourth (unknown) resistance.

• The Wheatstone Bridge is a very simple design, although there are more complex

versions of achieving the same outcome

• Can be adjusted easily, Fairly inexpensive.

• Also indirectly measures any variable that would change the resistance of a material.

Ex: Temperature, force, pressure

Early Wheatstone bridge designs.



Harinivas P

Wheat stone Bridge // Deflection Method

In the case of a balanced bridge, there is no voltage drop between, B and C, hence, Ig = 0.

We have 0RIRI 3311 0RIRI 4422

Furthermore,21 II & 43 II

&

Therefore, the following condition is established for a balanced bridge:

3

4

1

2

R

R

R

R

Any change in one arm of the bridge will imbalance in bridge. However, one can use the measured voltage across the bridge to calculate changes in one arm.

Suppose that a volt meter of infinite impedance (Ig = 0) is used to measure the voltage across points B and C. Because (Ig = 0),

Early Wheatstone bridge designs.



Harinivas P

Example:

A platinum RTD is connected as one arm of a Wheatstone bridge as shown

The fixed resistors, R2 and R3 are 25Ω. The RTD has a

resistance of 25Ω at 0oC, and the coefficient of resistance of the RTD is C/00395.0 o

Temperature measurement with Resistance thermometers :

A temperature measurement is made by placing the RTD in the measuring environment and balancing the bridge by adjusting R1 to a new value of 37.36 Ω.

Determine the temperature of the measuring environment..?

2

31 R

RRRRTD

Solution

At balanced condition, we have

)1(2536.37 TFrom

We can find that the temperature is 126o C.



Harinivas P

Potential Problems / Limitations

RTDs are more fragile than thermocouples.

Platinum often used since it can be used for a wide temperature range and has excellent stability. We can use Nickel or nickel alloys are used as well, but they aren’t as accurate.

An external current must be supplied to the RTD.

This current can heat the RTD, altering the results.

For situations with high heat transfer coefficients, this error is small since the heat is dissipated to air.

For small diameter thermocouples and still air this error is the largest.

Use the largest RTD possible and smallest external current possible to minimize this error.

Be careful about the way you set up your measurement device. Attaching it can change the voltage.

RTDs are more accurate but also larger and more expensive than thermocouples.



Harinivas P

Applications:

1. In Foundry to measure of temp in Furnace wall.

2. Fool proofing arrangement / Safety Devices.

3. Food processing Industry

4. Chemical Industry for Controlling process.

5. In Automobiles Radiators, to send signal about temp of cooling water.

1. More accurate Instruments.2. Compact instrument.3. Output can be connected to DIGATAL to have continuous readings.

Advantages :



Harinivas P

Thank you

resistance thermometer with wheat stone bridge

Documents

wheat stone bridge

wheatstone bridge

balanced bridge

measuring environment

deflection method

null method

batterys negative

temperature measurement