The variety of instruments is too complicated! A document that makes you an instrumentation expert

General principles for automated instrument selection

The general principles for the selection of instrumentation (components) and control valves are as follows:
1. Process conditions
The temperature, pressure, flow, viscosity, corrosivity, toxicity, pulsation and other factors of the process are the main conditions that determine the selection of the instrument. It is related to the rationality of the instrument selection, the service life of the instrument and the fire prevention, explosion protection, security, etc. of the workshop. problem.

2. The importance of operation
The importance of the parameters of each detection point is the basis for selecting the functions of the instrument, such as indication, recording, integration, alarm, control, and remote control. Generally speaking, variables that have little effect on the process, but need to be monitored frequently, can be selected; for important variables that need to be constantly understood, the record should be selected; and some that have a greater impact on the process, The variables to be monitored at any time shall be controlled; for variables that require measurement or economic accounting related to material accounting and power consumption, it is advisable to set up calculations; for some variables that may affect production or safety, alarms should be set.

3. Economic and unity
The selection of the instrument is also determined by the scale of the investment. The necessary economic accounting should be carried out under the premise of meeting the requirements of the process and self-control, and the appropriate performance/price ratio should be obtained.

In order to facilitate the maintenance and management of the instrument, the uniformity of the instrument should also be noted during the selection. Try to use the same series, the same specification and the same manufacturer's products.

4. Use and supply of instruments

The selected instrument should be a relatively mature product, which proves reliable performance through on-site use. At the same time, it should be noted that the selected instrument should be sufficient supply and will not affect the construction progress of the project.

Temperature instrument selection
<a> general principle
1. Unit and scale (scale)
The scale (scale) unit of the temperature meter is uniformly used in Celsius (°C).

2. Check out (measure) component insertion length
The selection of the insertion length should be based on the principle that the detection (measuring) component is inserted into the sensitive position of the temperature of the measured medium. However, in general, in order to facilitate the interchange, the entire device is generally selected to have a length of one to two gears.
When installing on flue, furnace and masonry equipment with thermal insulation materials, it should be selected according to actual needs.
The material of the detection (measuring) component cover should not be lower than the equipment or pipe material. If the shaped product cover is too thin or resistant to corrosion (such as armored thermocouples), a protective sleeve should be added.
Explosion-proof type should be used for temperature instruments, temperature switches, temperature detection (measuring) components and transmitters that are installed in flammable and explosive places.

<2> Selection of local temperature instrument
Accuracy level
General industrial thermometer: use grade 1.5 or grade 1.
Precision measuring and laboratory thermometers: 0.5 or 0.25 should be used.

2. Measurement range
The highest measured value is not more than 90% of the upper limit of the meter's measuring range, and the normal measured value is about 1/2 of the upper limit of the meter's measuring range.
The pressure thermometer should be measured between 1/2 and 3/4 of the upper limit of the meter's measurement range.

3. Bimetal thermometer
It should be preferred when meeting the requirements of measuring range, working pressure and accuracy.
The diameter of the case is generally φ100mm. In places with poor lighting conditions, high position and long viewing distance, φ150mm should be used.
The connection between the instrument case and the protection tube should generally be either universal or radial or radial according to the principle of convenient observation.

4. Pressure thermometer

It is suitable for on-site or on-site display with low temperature below -80 °C, inability to observe at close range, vibration and accuracy.

5. Glass thermometer
It is only used for special occasions with high precision, low vibration, no mechanical damage and convenient observation. However, due to mercury, it is not advisable to use a mercury thermometer.

6. Base instrument
For local or local instrumentation measurement and control (adjustment) instruments, base temperature instruments should be used.

7. Temperature switch
Suitable for occasions where temperature measurement requires contact signal output.

<3> Selection of centralized temperature instruments
1. Detecting (measuring) components
(1) According to the temperature measurement range, select the thermocouple, thermistor or the thermistor of the corresponding index number.

(2) Thermocouples are suitable for general use. Thermal resistance is suitable for vibration-free applications. Thermistors are suitable for applications where fast response times are required.

(3) According to the measurement object's response speed requirements, the following time constant detection (measurement) components can be selected:
Thermocouple: 600s, 100s and 20s;
Thermal resistance: 90 ~ 180s, 30 ~ 90s, 10 ~ 30s and <10s four;
Thermistor: <1s.

(4) According to the environmental conditions used, select the junction box according to the following principles:
Ordinary: a place with good conditions;
Splashproof, waterproof: wet or open space;
Explosion-proof type: flammable and explosive place;
Socket type: only for special occasions.

(5) In general, the thread connection method can be selected. For the following occasions, the flange connection method should be used:
Installation on equipment, lining pipes and non-ferrous metal pipes;
Crystallization, crusting, clogging and highly corrosive media:
Flammable, explosive and highly toxic medium.

(6) Thermocouples and thermal resistors used in special occasions:
For the case of a reducing gas, an inert gas and a vacuum having a temperature higher than 870 ° C and a hydrogen content of more than 5%, a tungsten-rhenium thermocouple or a blowing thermocouple is used;
Surface temperature of equipment, pipe outer wall and rotating body, surface or armored thermocouple, thermal resistance;
Hard solid particle medium, wear-resistant thermocouple;
In the same detection (measuring) component protection sleeve, when multiple temperature measurement is required, a multi-point (branch) thermocouple is selected;
An armored thermocouple can be used to save special protection tube material (such as 钽), to improve response speed, or to require the detection (measuring) component to be bent.

2. Transmitter
The transmitter is selected for measurement or control systems that are compatible with standard signal display instruments.
In the case of meeting the design requirements, it is recommended to use a transmitter that integrates measurement and transmission.

3. Display instrument
(1) The general indicator should be selected for single point display, and the digital indicator should be used for multi-point display. For historical data, it is advisable to use general recorder.
(2) Signal alarm system, it is advisable to use a pointer or recorder with contact signal output.
(3) A multi-point record should use a medium-sized recorder (such as a 30-point recorder).

4. Selection of ancillary equipment
(1) When multiple display devices are shared by multiple points, a reliable quality switch should be used.

(2) Using a thermocouple to measure the temperature below 1600 °C, when the temperature of the cold junction changes the measurement system can not meet the accuracy requirements, and the supporting display instrument has no cold junction temperature automatic compensation function, the cold end temperature automatic compensator should be used.

(3) Compensation wire
a. According to the number of thermocouples, the index number and the environmental conditions of use, the compensation wire or compensation cable that meets the requirements should be selected.

b. Select different levels of compensation wire or compensation cable according to the ambient temperature:
-20～+100°C select ordinary grade;
-40～+250°C selects heat resistant grade.
c. Where there is intermittent electric heating or strong electric or magnetic field, shield compensation wire or shield compensation cable should be used.

d. The cross-sectional area of the compensation wire shall be determined according to the value of the reciprocating resistance of the laying length and the external display resistance of the instrument, transmitter or computer interface.