Vapour-pressure Thermometer

Vapour-pressure Thermometer (filled-system thermometer)

-What is Vapour-pressure Thermometer  ??

a thermometer in which the variable saturated vapor pressure of a volatile liquid is used as a measure of the temperature and which thus has the advantage over some other types of thermometers of being free from errors due to bulb expansion

-Vapor pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. 

-The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid).

-A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The pressure exhibited by vapor present above a liquid surface is known as vapor pressure.

-As the temperature of a liquid increases, the kinetic energy of its molecules also increases.

-As the kinetic energy of the molecules increases, the number of molecules transitioning into a vapor also increases, thereby increasing the vapor pressure.

Temperature Ranges of Liquid used in Vapour-preessure System

Liquid	Critical temperature	Boiling point	Typical ranges available
Argon	-122◦c	-185.7◦c	Used to measure very low temperature down to -253◦c
Methyl chloride	143◦c	-23.7◦c	30-130◦K(0◦-50◦c)
Sulphur dioxide	157◦c	-10◦c	30-120◦c
Ethyl-alcohol	243◦c	78.5◦c	200-350◦c
Toluene	321◦c	110.5◦c	150-250◦c
Ethyl-dioxide	187◦c	12.2◦c	30-100◦c
Bulane (n)	154◦c	-0.6◦c	20-80◦c
Methyl bromide	-	4.6◦c	80-180◦K
Di-ethyl ether	194◦c	34.5◦c	60-160◦c
Water	375◦c	100◦c	120-220◦c

Working Principle 

-The thermometer consists of a steel bulb, a steel tube and Bourdon tube.

-The bulb is partly filled with a volatile liquid, evacuated and sealed off. 

-With rising temperature, the average velocity of the molecules in the liquid will increase. 

-As a result, more liquid molecules will acquire enough energy to escape from the surface of the liquid and saturate the evacuated area.

-The result will be an increase in vapour pressure in the capillary tube and Bourdon tube. 

-The Bourdon tube, sensitive to the changes in pressure, will record the temperature via the pointer that is linked to the moving end of the Bourdon tube.

Advantage: 

-rugged construction.
-require low maintenance.
-low in cost
-no need electrical power (self-sufficient).
                   

Disadvantages:

 -narrow temperature range, from -50 to 300 °C;
 - need a large bulb for the sake of accuracy.
 - slow response time
 - non-uniformity of the temperature scale
 -Limited maximum temperature.

Spiral type.

spiral type pressure gauge.

Spiral type Bourdon Tube

The spiral Bourdon tube makes a few windings in one plane around the fixed shaft of the pointer.

When the tube is being uncoiled by the process pressure, the free end will have a larger displacement compared to the C-shaped tube. The more windings, the larger the displacement will be.

A transmission mechanism is therefore no longer necessary. When the number of windings are correctly determined for a selected measuring range, a fixed connection between the free end of the Bourdon tube and the pointer is sufficient for a full deflection on the scale.

Using a fixed link avoids transmission losses due to friction or backlash in the transmission mechanism. This increases the accuracy and sensitivity of the pressure gauge. Also, recalibration is no longer necessary.

Backlash is mainly caused by wear on the teeth of the gears. Vibrations and pulsations are making it even worse. A pressure gauge with backlash on the gears will always indicate a pressure which is too low or too high. Subjected to pulsations, the backlash will cause the pointer to rotate further to the left and right than would normally be the case without backlash. Having a fixed link is making the spiral Bourdon tube pressure gauge very resistant to extreme vibration or pulsation.

For low pressure ranges, the spiral is made of a flat oval tube, while round tube is being used for the high pressure ranges.

The same low pressures can be measured with the spiral Bourdon tube as with the C-shaped Bourdon tube but the cost for a spiral pressure gauge is higher.

The use of spiral Bourdon tubes is consequently more likely in the high pressure ranges which cannot be measured with the C-shaped tube because the wall thickness of the tube would be too large so that the tip movement is too small to have sufficient accuracy. The suitable ranges up to 30,000 kPa.

WORKING:

When pressure is applied, the elliptical tube (Bourdon tube) tries to acquire a circular cross section; as a result, stress is developed and the tube tries to straighten up. Thus the free end of the tube moves up, depending on magnitude of pressure. A deflecting and indicating mechanism is attached to the free end that rotates the pointer and indicates the Pressure reading. The materials used are commonly Phosphor Bronze, Brass and Beryllium Copper. For a  2″ overall diameter of the C-tube the useful travel of the free end is approximately 1/8″ . Though the C-type tubes are most common, other shapes of tubes, such as helical, twisted or spiral tubes are also in use.

As the fluid pressure enters the bourdon tube, it tries to be reformed and because of a free tip available, this action causes the tip to travel in free space and the tube unwinds. The simultaneous actions of bending and tension due to the internal pressure make a non-linear movement of the free tip. This travel is suitable guided and amplified for the measurement of the internal pressure. But the main requirement of the device is that whenever the same pressure is applied, the movement of the tip should be the same and on withdrawal of the pressure the tip should return to the initial point.

ADVANTAGES:

*Construction is simple and cost is low. 

* It has excellent sensitivity. 

* It has high accuracy.

DISADVANTAGES:

1. They are subjected to hysteresis
2. They are sensitive to shocks and vibrations
3. Amplification is a must as the displacement of the free end of the bourdon tube is low
4. It cannot be used for precision measurement

Link for the videos :

ROTAMETER CONSTRUCTION AND WORKING

What is a rotameter?

Rotameter is variable area flowmeter used to measure fluid flow. It works on the principle of upthrust force exerted by fluid and force of gravity. The buoyant force exerted on an immersed object is equal to the weight of liquid displaced by the object. Under this principle, the rotameter works with float-tapered tube system.

Construction:

A rotameter is made up of a tapered tube and a float inside it. The glass tapered tube has a scale on the surface or a scale is placed adjacent to it, according to purpose.

Tapered tube:

The tapered tube is placed vertically in the flow channel with a conical shape inside. The quantity measured is defined by the height of float going up. Glass tubes are used for both liquid and gas measurement. Metallic tubes are used where the process fluid with high temperature and pressure.

Float:

Stainless steal floats are commonly used, there are different types of metals from lead to aluminium used as floats. A float material, shapes are also varied according to applications considering density.

Spherical shape floats are used for small flows.

Working :

Fluid enters from the bottom of the tapered tube, then some of the fluid strikes directly into the float bottom and others pass aside the float. Now the float experience two forces in opposite direction, darg force upward and gravitational force downward.

Fluid flow moves the float upward against gravity.At some point, the flowing area reaches a point where the pressure-induced force on the floating body exactly matches the weight of the float. The float will find equilibrium when the area around float generates enough drag equal to weight - buoyancy.

As the float weight and gravity are constant, the distance float displaced upward is proportional to the flow velocity of the fluid passing through the tapered tube.

Measuring Principles of Variable Area Flowmeters:

However, the difference in this application is that the value inside the radicand is constant since the pressure difference will remain constant and the fluid density will likely remain constant as well. Thus, k will change in proportion to Q. The only variable within k relevant to float position is the flowing area between the float and the tube walls.

Advantages:

• No external power needed
• Simple Reliable Design
• Can Measure Liquid or Gas Flows
• Scale is approximately linear
• It can measure flow rates of corrosive fluid
• Better rangeability
• Low cost and low pressure drop

Disadvantages:

• It should be mounted vertically
• And requires lining mounting
• Uncertainty of measurement
• Difficult to handle the glass type

Reference: https://automationforum.in/t/working-of-rotameter-variable-area-flowmeter/2978