QUESTIONS

The force exerted by the fluid per unit area of the wall of the container is called the absolute pressure.

It is the difference between the absolute and the local atmospheric pressure.

Vacuum pressure is the amount by which atmospheric pressure exceeds the absolute pressure.

U – tube manometer Well type manometer Inclined type manometer Ring balance manometer Micro manometer

The U-tube manometer is the simplest measuring instrument used for gauge pressure measurement by balancing the pressure against the weight of a column of liquid.

Bourdon tubes Bellows Diaphragms

Bourdon tube is a device that senses pressure and converts the pressure into displacement. Since the bourdon tube displacement is a function of the applied pressure it may be amplified and indicated by a pointer.

Diaphragms are used to measure pressure. The pressure to be measured is applied to the diaphragm, causing it to deflect; the deflection is proportional to the applied pressure. The movement of the diaphragm depends on its thickness and diameter. The diaphragm element is essentially a flexible disc which may be either flat or corrugated type.Two different arrangement are used for capsular elements, the convex and nested type

Flat type Corrugated type

The output of a parallel plate capacitor depends on the gap between its movable and fixed plates. Due to pressure, if the gap between the plates is altered, its capacitance also changes. This change in capacitance becomes a measure of pressure.

Advantages: Simple construction Useful for measurement of low, medium, high pressures. Used to measure absolute, gauge, differential pressures Disadvantages: Errors will occur

When a conductivity wire gets heated electric current flows through it. The rate at which heat is dissipated from this wire depends on the conductivity of the surrounding media.

Thermocouple type conductivity gauge Pirani gauge Ionization gauge

The hot filament of the hot cathode gauge emits electrons into the vacuum, where they collide with gas molecules to create ions. These positively charged ions are accelerated towards a collector where they create a current in a ion gauge detector circuit. The amount of current is proportional to the gas density.

Steam boiler Tanks and Container Manufacturing and chemical industries

It is a process of knocking off an electron from an atom and thus producing a free electron and positively charged ion.

When pressure is applied to the bellow element the bellow element will deflect and a displacement will be produced. That displacement will be proportional to the applied pressure.

C- type Spiral Helical

It consists of a oval tube which is made into an arc of 2500. Hence the name C- type. One end of the tube called tip is sealed and it has a link mechanism which moves over a pointer. Other end is attached to a socket where pressure to be measured is applied. When pressure is applied to bourdon tube, tube will change its position and hence the tip will undergo some displacement and which in turn proportional to pressure applied. Advantages: Low cost Simple construction Safety Disadvantages: Shocks and vibration will occur

It is made by winding several turns of the tube in the form of spiral. One end is connected to a pointer and other is attached to the socket where pressure can be measured is applied. Here there is no need of geared sector and pinion arrangement. When pressure is applied to bourdon tube, tube will change its position and hence the tip will undergo some displacement and which in turn proportional to pressure applied

It is made by winding several turns of the tube in the form of helical. One end is connected to a pointer and other is attached to the socket where pressure can be measured is applied. Here there is no need of geared sector and pinion arrangement. When pressure is applied to bourdon tube, tube will change its position and hence the tip will undergo some displacement and which in turn proportional to pressure applied

Junction formed by Faulty soldering technique Thermocouple may be used outside their applicable range Faulty reference junction compensation may be employed Installation faults Usage of wrong type of thermocouple.

Total radiation method and selective or partial radiation method

Technique for measuring temperature without contact. It depends on the relation between temperature of heat body and electromagnetic radiation emitted by the black body.

Used for very high temperature measurement 700 degc -3500 degc without the actual contact with the radiating body, Very high response.

Within the visible region a given wavelength has a fixed color and the energy of radiation is interpreted as intensity or brightness. Hence the brightness of the light of given color emitted by hot source, indicates the value of temperature.

Thermocouples are available in different combinations of metals, usually referred to by a letter, e.g. J, K etc. Each combination has a different temperature range and is therefore more suited to certain applications than others. Although it is worth noting that the maximum temperature varies with the diameter of the wire used in the thermocouple.

In some equipments a speed below a threshold value but close to zero is referred to as zero speed.

Cold junction compensation is done for temperature measurement using thermocouples. The t/c output is a function of temp. at hot junction as well as temp. at ref. junction. In order to measure hot junction temp. accurately, it is necessary to measure ref. junction temperature and compensate for its variation. This is done by modern digital instruments, by using a ref. junction temp. sensor to measure cold junction temp., and compensating for it using a look-up table corresponding to the t/c.

A pressure switch typically consists of a spring-loaded plunger actuated by process pressure to operate a micro switch. The pressure at which the micro switch operates is dependent upon spring force, which may be varied by an adjusting nut.

An offset is difference between a desired value and an actual value. The phenomenon of a dependent variable assuming a different set of values when the corresponding independent variable is increasing than when it is decreasing is known as hysteresis. Offset is fixed bias from desired value. Hysterisis is difference between set and reset.

The sizes of proximity switches are selected based on sensing distance and mounting requirements.

Control valve sizing refers to calculation of flow capacity of a valve for a given fluid, operating conditions and piping arrangement.

Type B thermocouples can be used up to 1600°C with short term excursions up to 1800°C. They have a low electrical output, therefore are rarely used below 600°C. In fact the output is virtually negligible up to 50°C, therefore cold junction compensation is not usually required with this type.

Type E thermocouples are often referred to as Chromel-Constantan thermocouples. They are regarded as more stable than Type K, therefore often used where a higher degree of accuracy is required. Constantan is Copper-Nickel.

Type J thermocouples degrade rapidly in oxidising atmospheres above 550°C. Their maximum continuous operating temperature is around 750°C though they can with stand short duration excursions to 1000°C. They are generally not used below ambient temperature due to condensation forming on the wires leading to rusting of the iron. Constantan is Copper-Nickel.

Type K are the most widely used thermocouples in the Oil & Gas, and refining industries due to their wide range and low cost. They are occasionally referred to as Chromel-Alumel thermocouples. Note that above about 750°C oxidation leads to drift and the need for recalibration.

Type N thermocouples can handle higher temperatures than type K, and offer better repeatability in the 300 to 500°C range. They offers many advantages over Type R & S at a tenth of the cost, therefore prove to be popular alternatives.

Type R thermocouples cover similar applications as Type S but offers improved stability and a marginal increase in range. Consequently, Type R tend to be used in preference to Type S.

Type S thermocouples can be continually at temperatures up to 1450°C. They can with stand short duration excursions up to 1650°C. They need protection from high temperature atmospheres to prevent metallic vapour ingress to the tip resulting in reduction of emf generated. Protection commonly offered is high purity recrystallised alumina sheath. For most industrial applications, thermocouples are housed in a thermowell.

Type T thermocouples are rarely used in industrial applications, and lend themselves more to use in laboratory situations.

The 4mA to 20mA signal is an industrial standard electrical signal that was designed to send information from one point to another. The 4-20mA signal can represent anything, and be scaled to any range of that quantity. What this means is that the 4-20mA signal can represent temperature, pressure, depth, humidity, or any number of things. It also means that the 4-20mA signal can represent any range of values for that quantity, such as 0% to 100%, or 20% to 80%. Exactly what a 4-20mA signal represents is defined by the transmitter (sometimes referred to as a transducer) that is controlling the signal.

The components that are typically part of a 4/20mA installation are: A power supply to provide the electricity for the loop Wire (usually a “twisted pair”) to carry the electrical current A transmitter to control the current through the circuit A sensor wired to the transmitter to detect the quantity being measured An indicator to show what the transmitter is measuring A controller to act on the measured signal Note: power supply, indicator, and controller are combined into one device. Sometimes the sensor and transmitter are combined into one device.

Sensors are used to measure something, like temperature, pressure, weight, humidity, etc. The signal that a sensor makes is very small. A transmitter consists of circuitry that amplifies this small signal and adjusts it to the 4-20mA output. When a sensor and a transmitter is designed and built as a single unit, it is frequently called a transducer.

In general, we come across two types of flow in liquid flow Measurement operations. Laminar flow: This type of flow occurs at very low velocities or high viscosities. In this, the liquid flows in smooth layers with the highest velocity at the center of the pipe and low velocities at the boundary (wall) of the pipe where the viscous forces hold it back. Turbulent flow: It takes place at high velocities or low viscosities. In this, the liquid flow breaks up into turbulent eddies which flow through the pipe with the identical average velocity. In this type of flow, fluid velocity is not much significant, and the velocity profile is a lot more uniform in shape.

Function of pneumatic valve is to control the pressure, flow rate and direction of air flow in pneumatic circuits.

Direction control valves Non – return valves Flow control valves Pressure control valves

Directional control valves start, stop or change the direction of flow in compressed air applications.

2 way valves are used for ON-OFF conditions. Three way DCV are used to control single acting cylinders. Four way DCV are used to control double acting cylinders

The check valves are used to allow free flow of compressed air in only one direction and it prevents the flow of compressed air in the opposite direction. Since check valves blocks the reverse flow, they are also known as Non-return valves.

Shuttle valves, also known as double check valves, are used when control is required from more than one power source. It can be used as OR gate fluid logic control valve.

Poppet (ball or disc), Spool, Piston, Diaphragm and Sliding plates.

The breakdown region of a p-n diode can be made very sharp and almost vertical diodes with almost vertical breakdown region are known as zener diodes. A zener diode operating in the breakdown region is equivalent to a battery. Because of this current through zener diode can change but the voltage remains constant. It is this constant voltage that has made the zener diode an important device in voltage regulation. Voltage regulator: The output remains constant despite changes in the input voltage due to zener effect.

A solenoid is electrically operated valve, It consists of solenoid coil in which magnetic plunger moves. This plunger is connected to the plug and tends to open or close the valve. There are two types of solenoid valves: Normally Open Normally closed

Variable area meters are special form of head meters. Where in the area of flow restrictor is varied. So as to hold the differential pressure constant. The rota meter consists of a vertical tapered tube through which the metered fluid flows in upward direction. As the flow varies the ‘float’ rises or falls to vary the area of the passages that the differential across it balances the gravitational force on the ‘float’. The differential pressure is maintained constant. The position of the ‘float’ is the measure of the rate of flow.

An electric potential is developed when a conductor is moved across the magnetic field. In most electrical machinery the conductor is a wire. The principle is equally applicable to a moving, electrically conductive liquid. The primary device of commercial magnetic meters consist of straight cylindrical electrically insulated tube with a pair of electrodes nearly flush with the tube walls and located at opposite end of a tube diameter. This device is limited to electrically conducting liquids. The magnetic meter is particularly suited to measurement of slurries and dirty fluids.

This is typically performed in modern programmable instruments by means of measuring actual reference junction temperature using a temperature sensor mounted close to the ref. junction and compensating for the same using appropriate look-up table stored within the instrument’s memory.

Three wire is a better alternative. Primary objective of 3-wire and 4-wire arrangements to eliminate effects of lead resistance on temp. measurement.

Density is measured here by measuring the resonant frequency of a vibrating U-tube.

OPEN LOOP: This is nothing but to sense the process signals from the field and to send it to the control room for operator observation. CLOSE LOOP: This is measuring the process signals for operator’s action. Means Tx sense the process and send it to the control room. Where the operators takes action i.e. control action and that given to the final control element as per the process requirement.

The stroboscope is a simple, portable manually operated device which may be used for measurement of periodic or rotary motions. A stroboscope having a scale that reads in flashes per minute or in revolutions per minute, the speed of a rotating device is measured by directing the stroboscopic lamp on the device, adjusting the flashing rate until the device appears to be stationary, then reading the speed directly on the scale of the instrument.

A load cell is a device that converts force into pressure. If the area on which the load is placed is known then the calibration of force can be done directly. Applications: Measurement of torque, weight, force, pressure...

Hydraulic Load Cell Pneumatic Load Cell Magneto – Elastic Load Cell Piezo Electric Load Cell Strain Gauge Load Cell

This type of load cell converts the load acting in them into electrical signal. The strain gauges are bonded onto the beam that deforms when weight is applied. This change in the dimension of the strain gauge causes its resistance to change. This change in resistance becomes a measure of the applied force.