Temperature Measurement (RTD)

Resistance Temperature Detectors or RTDs for short, are wire wound and thin film devices that measure temperature because of the physical principle of the positive temperature coefficient of electrical resistance of metals. The hotter they become, the larger or higher the value of their electrical resistance.
They, in the case of Platinum known variously as PRTs and PRT100s, are the most popular RTD type, nearly linear over a wide range of temperatures and some small enough to have response times of a fraction of a second. They are among the most precise temperature sensors available with resolution and measurement uncertanties or ±0.1 °C or better possible in special desions.

Usually they are provided encapsulated in probes for temperature sensing and measurement with an external indicator, controller or transmitter, or enclosed inside other devices where they measure temperature as a part of the device's function, such as a temperature controller or precision thermostat.

Temperature Measurement

Temperature is measured in industries extensively. The most common primary sensing elements for measuring temperature are Thermocouples and RTDs. There are several other temperature sensors also but right now we will be discussing only these two types.

A Thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control[1] and can also be used to convert heat into electric power. They are inexpensive[2] and interchangeable, are supplied fitted with standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy: system errors of less than one kelvin (K) can be difficult to achieve. Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.

Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.

Type K (chromel–alumel) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C, chromel positive relative to alumel.[5] It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C range. Type K was specified at a time when metallurgy was less advanced than it is today, and consequently characteristics vary considerably between samples. One of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is that they undergo a step change in output when the magnetic material reaches its Curie point (around 354 °C for type K thermocouples).

Type J (iron–constantan) has a more restricted range than type K (−40 to +750 °C), but higher sensitivity of about 55 µV/°C.[2] The Curie point of the iron (770 °C) causes an abrupt change in the characteristic, which determines the upper temperature limit.

Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium (the positive or "+" wire) and a second wire of 100% platinum (the negative or "-" wire). Like type R, type S thermocouples are used up to 1600 °C. In particular, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).

Venturi Tube for Flow Measurement

Venturi Tube is also a very popular primary sensing element for flowmeasurement. It is mostly used for low pressure flow measurement like gas, air etc flow. If we compare it with the Orifice Plate, it works on the same principle but is a very precise form of it. In the same way Orifice Plate can be called the raw form of a venturi tube.
The basic working principle of both of them is exactly the same i.e, differential pressure.

Magnetic Flow Measurement

The most common flow meter apart from mechanical flow meters is the magnetic flow meter, commonly referred to as a "mag meter" or an "electromag". A magnetic field is applied to the metering tube, which results in a potential difference proportional to the flow velocity perpendicular to the flux lines. The physical principle at work is Faraday's law of electromagnetic induction. The magnetic flow meter requires a conducting fluid, e.g. water, and an electrical insulating pipe surface, e.g. a rubber lined nonmagnetic steel tube.

Vortex for Flow Measurement

Another method of flow measurement involves placing a bluff body (called a shedder bar) in the path of the fluid. As the fluid passes this bar, disturbances in the flow called vortices are created. The vortices trail behind the cylinder, alternatively from each side of the bluff body. This vortex trail is called the Von Kármán vortex street after von Kármán's 1912 mathematical description of the phenomenon. The frequency at which these vortices alternate sides is essentially proportional to the flow rate of the fluid. Inside, atop, or downstream of the shedder bar is a sensor for measuring the frequency of the vortex shedding. This sensor is often a piezoelectric crystal, which produces a small, but measurable, voltage pulse every time a vortex is created. Since the frequency of such a voltage pulse is also proportional to the fluid velocity, a volumetric flow rate is calculated using the cross sectional area of the flow meter. The frequency is measured and the flow rate is calculated by the flowmeter electronics using the equation f = SV / L where f is the frequency of the vortices, L the characteristic length of the bluff body, V is the velocity of the flow over the bluff body, and S is the Strouhal number, which is essentially a constant for a given body shape within its operating limits.

Orifice Plate for Flow Measurement

Orifice Plate is the most widely used primary sensing element used for flow measurement. It works on the principle of differential pressure. An orifice plate is a plate with a hole through it, placed in the flow; it constricts the flow, and measuring the pressure differential across the constriction gives the flow rate. It is basically a crude form of Venturi meter, but with higher energy losses. There are three type of orifice: concentric, eccentric, and segmental.


An orifice meter is a device used for measuring the rate of fluid flow. It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which says that there is a relationship between the pressure of the fluid and the velocity of the fluid. When the velocity increases, the pressure decreases and vice versa.

An orifice plate is basically a thin plate with a hole in the middle. It is usually placed in a pipe in which fluid flows. As fluid flows through the pipe, it has a certain velocity and a certain pressure. When the fluid reaches the orifice plate, with the hole in the middle, the fluid is forced to converge to go through the small hole; the point of maximum convergence actually occurs shortly downstream of the physical orifice, at the so-called vena contracta point (see drawing to the right). As it does so, the velocity and the pressure changes. Beyond the vena contracta, the fluid expands and the velocity and pressure change once again. By measuring the difference in fluid pressure between the normal pipe section and at the vena contracta, the volumetric and mass flow rates can be obtained from Bernoulli's equation.

Flow Measurement

There exist about (four) 04 flow measurement techniques. All these techniques are based on different working principles. They are

Rotary Piston
Electromagnetic
Ultrasonic
Differential Pressure
Vortex

Among these above mentioned techniques, differential pressure is the most widely used flow measurement technique in the industry. This technique is based on the differential pressure principle. In this technique two different pressures are created by placing a hindrence in the path of flow. As a result of this restriction pressure tends to change as it passes the ristriction. The differential of the pressures before and after the restriction gives us the amount of flow.

The task of an electromagnetic flow meter is to measure flow volume of electrically conducting liquids, including water, food, beverages, chemicals, slurry, pulp & paper and mining slurries with magnetic particles. Whether it's a standard flow meter with modular pulsed DC technology, a high-performance AC pulsed flow meter, or a battery-operated electromagnetic water meter, an electromagnetic flow meter from world`s leading manufacturers facilitates flow rate management.

Interface Level Measurement

Key Applications of Interface Level Measurement Transmitter are liquids, slurries, powders, granules, food and pharmaceuticals, chemicals, hazardous areas.
The main purpose of this type of transmitters are to measure the interface level between two different liquids like oil and water.
Its benifits are:
Easy installation with verification by built-in LED
Low maintenance with no moving parts
Sensitivity adjustment
Integrated cable or PBT enclosure versions available
Intrinsically Safe, Dust Ignition Proof and General Purpose options available

Continuous Level Measurement Instrument

Continuous Level Measurement

Continuous level measurement is used for monitoring dynamic processes. These measurements are transmitted as an analog signal or digital value. Nowadays modern instrument manufactueres offers a comprehensive range of continuous level measurement transmitters based on a variety of technologies; ultrasonic, radar, guided wave radar, capacitance, gravimetric and hydrostatic.

Continuous level detection techniques based on radar, ultrasonic and hydrostatic principle are more popular in industries.

Radar level measurement is non-contacting and low-maintenance. As well, radar level measurement devices do not require any form of carrier medium and, the process environment (steam, pressure, dust or extreme temperatures) has practically no influence on measurement. There are a broad range of radar level measurement devices availablein the market that are highly reliable and effecient.

Ultrasonic level measurement is a highly cost-effective solution for short- and long-range measurement, even under difficult environmental conditions such as vibrations and dust. Ultrasonic level measurement is a non-contacting technology used in numerous industrial areas to monitor and control the level of liquids, slurries and solids.

The hydrostatic level measurement is a low-cost process for direct mounting or use with remote seals on tanks or containers. Gauge pressure transmitters are suitable for measurements on open containers; for closed containers, it is preferable to use the differential pressure transmitters.

Point Level Measurement

Point Level Measurement is a technique that is used to detect a particular level of liquid in a chamber. For example it is used to detect the high and low level in a tank. The modern techniques that are used in the industry nowadays are of Ultrasonic, Rotary Paddle, Capacitance, Vibrating and Ball Float type. Ball float is the oldest and most widely used technique for point level detection, and it is also considered very reliable too.

Level Measurement

Level Measurement in a process industry can be categorized into three (03) main types. They are Point Level, Continuous Level and Interface Level Measurements. Each type of measurement is further classified into different techniques.

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Pressure Measurement

Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges.

A manometer could also be referring to a pressure measuring instrument, usually limited to measuring pressures near to atmospheric. The term manometer is often used to refer specifically to liquid column hydrostatic instruments.

A vacuum gauge is used to measure the pressure in a vacuum which is further divided into two subcategories: high and low vacuum (and sometimes ultra-high vacuum). The applicable pressure range of many of the techniques used to measure vacuums have an overlap. Hence, by combining several different types of gauge, it is possible to measure system pressure continuously from 10 mbar down to 10−11 mbar.

Although pressure is an absolute quantity, everyday pressure measurements, such as for tire pressure, are usually made relative to ambient air pressure. In other cases measurements are made relative to a vacuum or to some other ad hoc reference. When distinguishing between these zero references, the following terms are used:

Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.
Differential pressure is the difference in pressure between two points.
The zero reference in use is usually implied by context, and these words are only added when clarification is needed. Tire pressure and blood pressure are gauge pressures by convention, while atmospheric pressures, deep vacuum pressures, and altimeter pressures must be absolute. Differential pressures are commonly used in industrial process systems. Differential pressure gauges have two inlet ports, each connected to one of the volumes whose pressure is to be monitored. In effect, such a gauge performs the mathematical operation of subtraction through mechanical means, obviating the need for an operator or control system to watch two separate gauges and determine the difference in readings. Moderate vacuum pressures are often ambiguous, as they may represent absolute pressure or gauge pressure without a negative sign. Thus a vacuum of 26 inHg gauge is equivalent to an absolute pressure of 30 inHg (typical atmospheric pressure) − 26 inHg = 4 inHg.

Atmospheric pressure is typically about 100 kPa at sea level, but is variable with altitude and weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For example, when a car drives up a mountain, the tire pressure goes up. Some standard values of atmospheric pressure such as 101.325 kPa or 100 kPa have been defined, and some instruments use one of these standard values as a constant zero reference instead of the actual variable ambient air pressure. This impairs the accuracy of these instruments, especially when used at high altitudes.

Use of the atmosphere as reference is usually signified by a (g) after the pressure unit e.g. 30 psi g, which means that the pressure measured is the total pressure minus atmospheric pressure. There are two types of gauge reference pressure: vented gauge (vg) and sealed gauge (sg).

A vented gauge pressure transmitter for example allows the outside air pressure to be exposed to the negative side of the pressure sensing diaphragm, via a vented cable or a hole on the side of the device, so that it always measures the pressure referred to ambient barometric pressure. Thus a vented gauge reference pressure sensor should always read zero pressure when the process pressure connection is held open to the air.

A sealed gauge reference is very similar except that atmospheric pressure is sealed on the negative side of the diaphragm. This is usually adopted on high pressure ranges such as hydraulics where atmospheric pressure changes will have a negligible effect on the accuracy of the reading, so venting is not necessary. This also allows some manufacturers to provide secondary pressure containment as an extra precaution for pressure equipment safety if the burst pressure of the primary pressure sensing diaphragm is exceeded.

There is another way of creating a sealed gauge reference and this is to seal a high vacuum on the reverse side of the sensing diaphragm. Then the output signal is offset so the pressure sensor reads close to zero when measuring atmospheric pressure.

A sealed gauge reference pressure transducer will never read exactly zero because atmospheric pressure is always changing and the reference in this case is fixed at 1 bar.

An absolute pressure measurement is one that is referred to absolute vacuum. The best example of an absolute referenced pressure is atmospheric or barometric pressure.

To produce an absolute pressure sensor the manufacturer will seal a high vacuum behind the sensing diaphragm. If the process pressure connection of an absolute pressure transmitter is open to the air, it will read the actual barometric pressure.

process instrumentation

Measuring, positioning, recording and controlling are key parameters for all industrial processes. Thus, top priorities for process instruments are achieving high levels of precision and absolute reliability. Process instrumentation from worlds leading manufacturers satisfies these demands and provides an efficient means to increase plant efficiency and improve product.

Mainly there are four parametric measurements carried out in a process industry. They are

1. Pressure Measurement
2. Level Measurement
3. Temperature Measurement
4. Flow Measurement