Commonly used in the industry temperature detection instrument is divided into two categories: non-contact temperature measurement instruments (such as: radiation, infrared). Contact thermometers (such as: expansion, pressure, thermocouple, thermal resistance).
Since industrial thermal resistors and industrial thermocouples are used widely and are mostly used in automatic interlocking control systems, the following describes common fault handling methods for temperature detection instruments in use.
First, industrial thermal resistance
Industrial thermal resistance is based on the characteristic that the resistance value of the metal increases as the temperature increases. The most commonly used are made of platinum and copper, the graduation number is Pt100, Pt10, Pt50 (temperature range -200 ~ 850 °C), Cu50, Cu100 (temperature range -50 ~ 150 °C).
Industrial thermal resistance temperature measurement system is generally composed of industrial thermal resistance, connecting wires and display instruments. The index number of the industrial thermal resistance and the display instrument must be the same. To eliminate the influence of the resistance change of the connecting wire on the temperature measurement, a three-wire connection must be used.
Common faults of industrial thermal resistors and their handling methods:
The common fault of industrial thermal resistance is the open circuit and short circuit of the industrial thermal resistance. General disconnection is more common because of the finer thermal resistance wires. Open circuit and short circuit are easy to judge, you can use the multimeter's "×1Ω" file. If the measured resistance is less than R0, there may be a short circuit; if the multimeter indicates infinity, you can determine that the resistor is disconnected. Resistor short-circuit is generally easier to handle, as long as it does not affect the length and thickness of the resistance wire, find the short-circuited place to dry and strengthen the insulation. Resistor body circuit repair must change the length of the resistance wire and affect the resistance value. For this reason, it is better to replace the new resistor body. If soldering repair is used, it must be verified after passing the welding. Thermal resistance temperature measurement system in the operation of common faults and treatment methods are as follows:
Fault phenomenon
Possible Causes
Approach
Display instrument indicator is lower than actual value or unstable
The protection tube has metal chips, dust, dirt between the terminals and a short circuit of the thermal resistance (water accumulation, etc.)
Remove metal chips, clean dust, water droplets, etc., find short-circuit points, strengthen insulation, etc.
Display meter indicates infinity
Industrial thermal resistance or lead wire breaks and terminal looses
Replace resistor body, or weld and tighten terminal screws, etc.
Display meter indicates negative value
Display instrument and thermal resistance wiring is wrong, or thermal resistance has short circuit
Correct the wiring, or find the short circuit, strengthen the insulation
Changes in resistance and temperature
Thermal resistance wire material is subject to corrosion deterioration
Replace the resistor (thermal resistance)
First, industrial thermocouple
Industrial thermocouples weld conductors or semiconductors A and B of two different materials together to form a closed loop. When there is a temperature difference between two welding points 1 and 2 of conductors A and B, a thermoelectric potential is generated between the two. Therefore, a certain amount of current is formed in the loop, and this phenomenon is called thermoelectric effect. Industrial thermocouples work by using this principle.
Industrial thermocouple common faults and treatment methods
Fault phenomenon
Possible Causes
Approach
Thermal potential is smaller than the actual value (indicating that the indicator value of the meter is low)
Hot electrode short circuit
Find the cause of the short circuit, such as due to moisture, you need to dry; as a result of damage to the insulator, you need to replace the insulator; cleaning ash;
Compensate the short circuit between wires: Find the short-circuit point, strengthen the insulation or replace the compensation wire
Thermoelectric potential is smaller than actual value (display instrument indicator is low
Industrial thermocouple hot electrode spoilage
If the length permits, cut the deteriorating section and re-weld, or replace the new thermocouple
Thermoelectric potential is smaller than actual value (display instrument indicator is low
Compensation wire and industrial thermocouple polarity reversed
Reconnect correctly
Thermoelectric potential is smaller than actual value (display instrument indicator is low
Compensation wire and industrial thermocouple do not match
Replace the matching compensation wire
Thermal potential is less than actual value (display instrument indicator is low
Industrial thermocouple improper installation position or insertion depth does not meet the requirements
Reinstall according to regulations
Thermal potential is less than actual value (display instrument indicator is low
Industrial thermocouple cold junction temperature compensation does not meet the requirements
Adjust cold junction compensator
Thermal potential is greater than actual value (indicating that the indicated value of the meter is high)
Industrial thermocouple and display instrument are not matched
Industrial thermocouple or display instrument matching
Thermal potential is greater than actual value (indicating that the indicated value of the meter is high)
Compensation wire and industrial thermocouple do not match
Replace the matching compensation wire
Thermal potential is greater than actual value (indicating that the indicated value of the meter is high)
A DC interference signal enters
Exclude dc interference
Thermal potential is greater than actual value (indicating that the indicated value of the meter is high)
Thermoelectric output is not stable
Poor contact between the industrial thermocouple terminal and the hot electrode tightens the terminal screw
Thermal potential is greater than actual value (indicating that the indicated value of the meter is high)
Industrial thermocouples measure broken insulation, causing intermittent short circuits or grounding
Finding fault points, repairing insulation
Thermal potential is greater than actual value (indicating that the indicated value of the meter is high)
Industrial thermocouple not mounted or external vibration
Fasten industrial thermocouples, eliminate vibration or take shock absorbers
Thermal potential is greater than actual value (indicating that the indicated value of the meter is high)
The hot electrode will break
Repair or replace industrial thermocouples
Thermal potential is greater than actual value (indicating that the indicated value of the meter is high)
External interference (AC leakage, electromagnetic induction, etc.
Detect interference sources and take shielding measures
Large thermal error
Hot electrode deterioration
Replace hot electrode
Large thermal error
Industrial thermocouples are not properly installed
Change the installation location
Large thermal error
Protection tube surface area gray
Clear dust
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