Classification of conventional instruments
I. Pressure Instruments
II. Temperature Instruments
III. Flow meters
IV. Liquid Level Instrument
V. Analytical Instruments
Classified by function:
Indicating instruments include pressure gauges, temperature gauges, level gauges, flow meters, etc., which display information locally.
Common types of switch instruments include temperature switches, pressure switches, flow switches, and level switches.
Transmitter instruments include temperature transmitters, pressure transmitters, flow transmitters, level transmitters, etc.
definition
Testing instruments – The term "testing instruments" encompasses all testing elements, transmitters, and display devices.
Primary instruments are generally instruments used to convert the measured quantity into a easily measurable physical quantity; they are also known as sensing elements. Primary measuring instruments are in direct contact with the medium and are installed locally.
Secondary instruments are instruments that convert measured signals into measurable standard electrical signals and display them. They include both transmitters and display devices.
A transmitter is a converter that transforms the output signal of a sensor into a signal that can be recognized by a controller (or converts a non-electrical quantity input from a sensor into an electrical signal, and amplifies it for use as a signal source for remote measurement and control, typically a 4-20mA current).
A transmitter typically consists of two parts: a sensor and a signal converter. The sensor receives input information and converts it into an output variable of the same or different nature according to a certain rule. The converter receives a signal in one form and converts it into another form of output according to a certain rule.
Transmitters mainly include temperature transmitters, pressure transmitters, flow transmitters, and level transmitters.
I. Pressure Instruments
Pressure gauge: The pressure is displayed by the elastic deformation of the sensitive element inside the gauge, which is then transmitted to the pointer by the conversion mechanism inside the gauge, causing the pointer to rotate.
pressure switch
A pressure switch is a simple pressure control device that can issue an alarm or control signal when the measured pressure reaches the rated value.
The working principle of a pressure switch is as follows: when the pressure being measured exceeds the rated value, the free end of the elastic element is displaced, which directly or after comparison pushes the switching element to change the on/off state of the switching element, thereby achieving the purpose of controlling the pressure being measured.
II. Temperature Instruments
Temperature instruments are mainly classified into glass tube thermometers, bimetallic thermometers, pressure thermometers, resistance thermometers, thermocouples, temperature transmitters, temperature switches, and non-contact thermometers.
Classified by working principle:
They are divided into five categories: expansion thermometers, pressure thermometers, thermocouple thermometers, resistance thermometers, and radiation pyrometers.
According to measurement method:
They are divided into two main categories: contact and non-contact. In the former, the temperature sensing element is in direct contact with the medium being measured, allowing for sufficient heat exchange between the medium and the sensing element to achieve the purpose of temperature measurement; in the latter, the temperature sensing element is not in contact with the medium being measured, and heat exchange is achieved through radiation or convection to achieve the purpose of temperature measurement.
Glass tube thermometer:
This type of thermometer is very simple and common.
Currently, high-precision instruments are often used in instrument calibration laboratories.
Due to its low price, it is still being used in factories.
Bimetallic thermometers utilize the principle that different metals have different coefficients of thermal expansion. The bimetallic strip will bend to different degrees at different temperatures, and this degree of bending indicates the temperature. Due to the different coefficients of thermal expansion, the two sides of the bimetallic strip expand and contract at different rates when the temperature changes, thus the degree of bending changes at different temperatures.
Pressure gauge thermometer: A pressure gauge thermometer is made based on the principle that liquids, gases, or low-boiling-point liquids and saturated vapors in a closed container expand in volume or change in pressure when heated, and uses pressure to measure this change, thereby measuring the temperature.
Pressure gauge thermometers mainly consist of the following three parts:
Temperature sensor – A temperature sensor is a component that comes into direct contact with the medium being measured to sense temperature changes. Therefore, it is required to have high strength, a small coefficient of thermal expansion, high thermal conductivity, and corrosion resistance. Depending on the working medium and the medium being measured, the temperature sensor can be made of copper alloy, steel, or stainless steel.
2. Capillary tube – It is a seamless round tube made of materials such as copper or steel, which is used to transmit changes in pressure.
3. Bourdon tube – This is the elastic element used in general pressure gauges.
Resistance temperature detectors (RTDs) are the most commonly used temperature detectors in the medium and low temperature range. Their main characteristics are high measurement accuracy and stable performance. Platinum resistance temperature detectors (PTDs) offer the highest measurement accuracy and are widely used not only in industrial temperature measurement but also as standard reference instruments.
The principle of resistance temperature measurement (RTD): RTD temperature measurement is based on the property that the resistance of a metallic conductor increases with increasing temperature. Most RTDs are made of pure metal materials, with platinum and copper being the most commonly used.
Thermocouples are the most commonly used temperature sensing element in the medium- and high-temperature range. Their main characteristics are high measurement accuracy and stable performance.
The working principle of a thermocouple:
A thermocouple thermometer consists of three parts: a thermocouple (temperature sensing element); an electrical measuring part (moving coil instrument or potentiometer); and wires (compensating wires) connecting the thermocouple and the measuring instrument.
A thermocouple is made by welding or twisting together two different conductors or semiconductors (such as A and B in the diagram above). The welded end is called the hot junction (measuring or working end), and the end connected to the wire is called the cold junction (free end). The two conductors or semiconductors that make up the thermocouple are called thermoelectrodes. When the hot junction of the thermocouple is inserted into the production equipment where temperature measurement is needed, and the cold junction is placed outside the equipment, if the temperatures at the two ends are different (for example, the hot junction temperature is t, and the cold junction temperature is to), a thermoelectric potential E will be generated in the thermocouple circuit. This thermoelectric potential E is related to both the temperatures t and to at the thermocouple ends. If t is kept constant, the thermoelectric potential E is only a function of the measured temperature t. After measuring the value of E with an electrical measuring instrument, the magnitude of the measured temperature t is known.
Temperature switch
Traditional temperature switches are mostly mechanical, and they are divided into: vapor pressure thermostats, liquid expansion thermostats, gas adsorption thermostats, and metal expansion thermostats.
Non-contact thermometers sense and compare temperatures using methods such as infrared radiation, brightness, and color difference to determine the temperature of the object being measured. Their advantages include remote measurement capability, a large measuring range, and the ability to measure extremely high temperatures. Examples include infrared thermometers and brightness thermometers. The disadvantage is that their accuracy is generally not high. However, they are indispensable auxiliary temperature measuring components in factories.
III. Flow meters
Turbine flow meter
Orifice plate flow meters (including integrated orifice plate flow meters)
mass flow meter
Rotor flow meter
Vortex flow meter
Ultrasonic flow meter
Electromagnetic flowmeter
Target flow meter
According to principle:
Mechanics: differential pressure type, float type, target type, vortex street type, etc.;
Acoustics: Ultrasound
Electrical: Electromagnetic flowmeter, etc.
Differential pressure flow meter
A differential pressure flow meter consists of three parts: a throttling device (including a throttling element and a pressure tapping device) that converts the measured flow rate into a differential pressure signal, a pressure-conducting tube, and a differential pressure gauge or differential pressure transmitter and its display instrument. In the unit combination instrument, the differential pressure signal generated by the throttling device is converted into a corresponding electrical or gas signal by the differential pressure transmitter for display and regulation.
Since differential pressure and flow rate have a square root relationship, flow display instruments are equipped with a square root device to linearize the flow scale.
Fluids flowing in pipes possess kinetic and potential energy. Under certain conditions, these two types of energy can be interconverted, but the total energy involved in the conversion remains constant. Throttling elements measure flow rate based on this principle. Among throttling devices, orifice plates, nozzles, and Venturi tubes are the most commonly used.
Orifice plate flowmeter: When fluids fill a pipe and flow through a throttling device inside the pipe, the flow stream will form a local contraction at the throttling element of the throttling device, thereby increasing the flow velocity and lowering the static pressure. As a result, a pressure drop, or pressure difference, is generated before and after the throttling element. The greater the flow rate of the medium, the greater the pressure difference generated before and after the throttling element. Therefore, the orifice plate flowmeter can measure the flow rate of the fluid by measuring the pressure difference.
The three standard throttling elements in instruments:
a: Orifice plate
b: Nozzle
c: Venturi tube
A rotor flowmeter, also known as an area flowmeter or constant pressure drop flowmeter, is a flow measurement instrument based on the throttling principle of liquid flow. However, the accuracy of a rotor flowmeter is affected by the temperature, density, and viscosity of the measured medium, and the instrument must be installed vertically. Principle: A rotor flowmeter consists of an upwardly expanding conical tube and a rotor with a density greater than that of the measured medium that floats up and down with the flow rate of the measured medium.
As the liquid flows upwards, the rotor moves upwards due to the impact of the liquid. As the rotor rises, the annular flow area between the rotor and the conical tube increases, the liquid velocity decreases, and the impact weakens until the upward thrust of the liquid on the rotor balances the rotor's weight in the fluid. At this point, the rotor rests at a certain height within the conical tube. If the liquid flow rate increases further, the rotor will be at an even higher position at equilibrium; conversely, the opposite will occur. Therefore, the liquid flow rate can be determined by the height at which the rotor is suspended.
Turbine flow meter: When the fluid being measured flows through the turbine flow meter sensor, the impeller rotates under the action of the fluid. Its rotational speed is proportional to the average flow velocity in the pipe. The rotation of the impeller periodically changes the magnetoresistive value of the magneto-electric converter. The magnetic flux in the detection coil changes periodically accordingly, generating a periodic induced electromotive force, i.e., an electrical pulse signal. After being amplified by an amplifier, it is sent to the display instrument for display.
Doppler ultrasonic flow meters operate based on the Doppler effect. As shown in the attached diagram, the transmission frequency is linearly altered by reflecting the signals from particles and bubbles in the fluid. The end result is a conversion between the transmitter and receiver frequencies, a value directly related to the flow velocity.
The working principle of an electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. In an electromagnetic flowmeter, the conductive medium inside the measuring tube is equivalent to the conductive metal rod in Faraday's experiment, and two electromagnetic coils at its upper and lower ends generate a constant magnetic field. When a conductive medium flows through, an induced voltage is generated. Two electrodes inside the pipe measure this induced voltage. The measuring pipe is electromagnetically isolated from the fluid and the measuring electrodes by a non-conductive lining (rubber, Teflon, etc.).
IV. Liquid Level and Material Level Instruments
Differential pressure level gauge
float level gauge
Magnetic float level gauge
Glass plate level gauge
Steel strip level gauge
Radar level gauge
Ultrasonic level gauge
Depth level gauge
Nuclear radiation level gauge
The sensing element of a float-type level gauge is a float immersed in the liquid. As the liquid level changes, the float generates a change in buoyancy, which in turn drives a pneumatic or electric actuator to send a signal to the display instrument, indicating the measured liquid level.
The diagram illustrates the principle of a float-type liquid level transmitter. When the liquid level changes, the unbalanced force between the weight of the float 1 (also known as the sinker) and the buoyancy it experiences is transmitted to the torsion tube 3 via the lever 2. The torsion tube undergoes angular elastic deformation, which is transmitted out through the spindle 4, then through the push plate 5 to the Hall plate 6, where it is converted into a Hall potential. After power amplification, it is converted into a unified standard electrical signal output for remote transmission to the display instrument.
Differential pressure level gauges work by utilizing the principle that when the liquid level inside a container changes, the static pressure generated by the liquid column also changes accordingly.
The diagram shows the principle of a differential pressure level gauge. When one end of the differential pressure gauge is connected to the liquid phase and the other end to the gas phase, according to the principle of hydrostatics, we have:
Pb = Pa + ρgH
In the formula, H represents the liquid level height.
ρ------Density of the measured medium
g--------The gravitational acceleration at the measured local area
Therefore, we have: ΔP = Pb - Pa = ρgH
Under normal circumstances, the density and gravitational acceleration of the measured medium are known. Therefore, the differential pressure measured by the differential pressure gauge is proportional to the height H of the liquid level, thus transforming the problem of measuring the liquid level height into the problem of measuring the differential pressure.
The flappers of the level gauge are made of thin, magnetically conductive sheet metal. They are arranged vertically and each can rotate around a small axis on its frame (as shown in the figure). One side of the flapper is painted red, and the other side is painted silver-gray. During operation, the level gauge's connecting pipe is connected to the container via a flange, forming a communicating vessel. A float in the center of the communicating vessel changes position with the liquid level. A magnet is located in the center of the float, its position perfectly aligned with the liquid surface. When the liquid level rises, the magnet attracts the flappers, causing them to flip one by one, with the red side facing outwards; when it falls, they flip back, with the silver-gray side facing outwards. This color-coded indicator of liquid level is highly visible.
Glass plate level gauge: Based on the principle of communicating vessels, the liquid medium inside the container is led to the external glass plate level gauge, and the actual height of the liquid level inside the container is directly displayed through the transparent glass.
Steel strip level gauge: Contains a float and guide rope inside the tank. There are two common types of level indicators: one is a magnetic pendulum with a magnetic flap indicator; the other is a steel strip with numbers that directly displays the level.
V. Analytical Instruments
Combustible gas detector
Online analytical instruments
Online chromatographic analysis
Online dew point analysis
Online CO2 Analysis
A combustible gas detector is a detector that responds to the concentration of one or more combustible gases. Combustible gas detectors come in two types: catalytic and infrared optical.
Catalytic combustible gas detectors determine the concentration of combustible gases by measuring the change in resistance of a heated platinum wire. When combustible gas enters the detector, it causes an oxidation reaction (flameless combustion) on the surface of the platinum wire. The heat generated raises the temperature of the platinum wire, causing a change in its resistivity.
The principle of an infrared gas detector is as follows: when infrared light is absorbed by combustible gas, the receiving detector on the other side receives the weakened infrared light. By comparing it with a reference infrared receiver that does not pass through, the actual concentration can be measured.
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