Co2 Metro Thermometer
Sensors are the sensing element of a system that converts physical quantities such as pressure, heat, humidity, flow, etc. into continuous or discontinuous electrical quantities or even non-electrical quantities (such as changing the internal resistance of a sensor). These sensors are used in a variety of measuring devices and systems Analog control And digital are used like PLC. The performance of sensors and their ability to connect to various devices, including PLC, has made the sensor an integral part of the automatic control device. The sensors send various information about the status of the moving parts of the system to the control unit and change the performance status of the devices.
In this section, we will first explain the methods of measuring the four important quantities of heat, flow, level, and pressure, and finally we will talk about contactless switches.
Temperature measurement
Various detectors are used to measure temperature.
Which are classified into two general groups:
Detectors that are in contact with the fluid.
Detectors that are not in contact with the fluid.
1-1) Detectors that are in contact with the fluid
These detectors in which the method of fluid contact with the receiving element Temperatures Used include the following types:
1-1-1) Thermocouple
One of the most common temperature sensitive devices is the thermocouple. The story of the thermocouple goes back to the discovery of See Beck in 1821 about the existence of an electric current in a closed circuit of two heterogeneous metals while two connection points are at different temperatures. Such a thermocouple is shown in the figure below.
Here A and B are the two metals and T1 and T2 are the temperatures of their connection points. I represents the flow Thermoelectric Is in the current circuit. A is usually thermocouple-positive than B if the T1 connection is colder.
Factors influencing the choice of metal Thermocouple:
Different materials can be used for two thermocouple metals, each of which has its own characteristics. Factors that affect the choice of thermocouple material are:
A) Temperature restrictions
B) Linear relationships between temperature and EMF
C) The amount of EMF for each degree of temperature change
1) Limit of error and sensitivity
2) Retrievability
3) Accuracy
D) Physical resistance at high temperatures
E) Atmospheric effects
1) Oxidation
2) Reduction
Standardized thermocouples:
الف) (Copper – Constantan)CC
– Normal temperature range from -150 to +400 degrees Celsius
– Oxidation above 400 degrees Celsius
– Vulnerable to acid vapors
ب) (Iron – Constantan)IC
– Normal temperature range from zero to 800 degrees Celsius
– Vulnerable to sulfur, oxygen and moisture
– Low atmospheric effects (especially below 400 degrees Celsius
ج) (Chromel – Alumel)CA
– Normal temperature range from zero to 130 degrees Celsius
– Resistant to oxidizing atmospheres
– Vulnerable to sulfur
)(Platinum- Platinum 10%/13%Rhodium) Pt- PtRH
– Normal temperature range from zero to 1500 degrees Celsius
– Resistant to oxidizing atmospheres
Corruption (corrosion) above 1000 degrees Celsius
– Need a protective tube resistant to gas penetration
ه) (Chromel – Constantan)ChC
– Normal temperature range from zero to 800 degrees Celsius
– Resistant to oxidizing atmospheres
– Vulnerable to sulfur
Thermocouple protection :
To protection Thermocouple uses a device called Well. Using well has the following advantages:
– Use of thermocouples in corrosive and oxidized atmospheres
– High heat
Use Well with closed bottom
Use Well by passing a neutral gas through it
– Low temperature
– High pressures
– Ability to use against shocks, pressures and vibrations
– Protection against sudden temperature changes
– Acting for molten materials
– Thermal conductivity
– Prevent radiation errors
– Prevent errors due to fluid movement
The above are the reasons why we use Well for thermocouples. One drawback that a well can create for our measurement is having a time constant. Therefore, in places where there are no problems that cause the use of Well, less thermocouple without Well is used to achieve a fixed time.
1-1-2) Resistance thermometers
Resistance thermometer theory
thermometer Resistance is based on the inherent properties of metals to repair their electrical resistance to temperature changes. Although Resistance Thermometer Bulbs are usually made of copper, copper, or nickel, the effect of temperature on the resistance also applies to semiconductors, which may be due to the rapid recovery of the semiconductor material. Have more uses in resistance thermometers. Basically, a resistance thermometer is a device for measuring electrical resistance, but a resistance that is rated based on a unit of temperature instead of a unit of resistance. This makes the resistance thermometer unlike many other electrical converters Electrical signal Converts to one of the variable units of the process, such as temperature.
Specifications and advantages of mercury-filled spring thermometer:
– Linear response
– Stability
–Speed In reply
– Usability with compensator
Specifications and advantages of hydrocarbon-filled spring thermometer:
– Linear response
– Measure the range of small changes
– Having a small bulb
– Low temperature measurement
– Usability with compensator
Ambient temperature compensator
In this type of filled meters, due to the effect of ambient temperature on the hair follicles Fluid,An increase in volume occurs which causes a measurement error. To compensate for this error, special systems are installed Becomes.