K type thermocouple what are the advantages and disadvantages compared with n type thermocouple _ instrument
The working principle of thermocouple when there are two different conductors or semiconductor A and B to form A loop, its both ends are connected, as long as the temperature of the two nodes is different, one end of the temperature T, referred to as the work end or hot end, another end temperature T0, called free end (
Also known as the reference side)
Or the cold end, the circuit will generate an electromotive force, the direction and size of the electromotive force is related to the conductor material and the temperature of the two contact.
This phenomenon is called thermoelectric effect, two kinds of conductor circuit known as the 'thermocouple', composed of the two conductors referred to as the 'hot' electrode, produces electromotive force is known as the 'thermoelectric emfs.
Thermoelectric emfs is composed of two parts of electromotive force, part two conductor contact electromotive force, the other part is a single conductor of temperature difference electromotive force.
The size of the thermocouple loop thermoelectric emfs, only with the composition of the thermocouple conductor materials related to the temperature of the two contact, and has nothing to do with the shape size of the thermocouple.
After the thermocouple fixed two electrode materials, contact temperature t and thermoelectric emfs are two t0.
The function is poor.
This equation has been widely applied in the actual temperature measurement.
Because of the cold end t0 constant, produced by the thermocouple thermoelectric emfs only with the hot side (
Temperature changing, that is, a certain amount of thermoelectric emf corresponds to a certain temperature.
As long as we use the method of measuring the thermoelectric emfs can achieve the purpose of temperature measurement.
Thermocouple temperature measurement is the basic principle of two kinds of different ingredients of closed loop conductor material composition, when when there is a temperature gradient at both ends, loop will have an electric current passes through, the existed between the electromotive force on both ends -
Thermoelectric emf, this is the so-called seebeck effect (
Two different components of homogeneous conductor electrode as heat, the temperature is higher for work at the end of the end, one end of the low temperature as the free end, usually free end under a constant temperature.
According to the thermoelectric emf as a function of temperature, the thermocouple indexing table;
Indexing table is free end temperature at 0 ℃, under the condition of different thermocouples with different indexing table.
Access in the thermocouple loop when the third metal material, the two contacts at the same temperature as long as the material, produced by the thermocouple thermoelectric is set to remain the same, which is not affected by the third metal access in the loop.
Therefore, when the thermocouple temperature measurement, can be connected to the measuring instrument, measured after the thermoelectric emfs, can know the temperature of the measured medium.
Thermocouple measuring temperature to the cold end (
Measure the end for the hot end, by the end of the lead connected to the measurement circuit is called cold junction)
Temperature remains constant, the size of thermoelectric potential and measured temperature in certain proportion relationship.
When measuring, the cold end (
Temperature changes, will seriously affect the accuracy of measurement.
Take action at the cold end compensation due to the impact of the cold end temperature change is called the thermocouple cold junction compensation is normal.
Connected to the measuring instrument with special compensation conductor.
Thermocouple cold junction compensation calculation method: from millivolt to temperature: measure the cold end temperature and conversion for the corresponding millivolt values, millivolt values with thermocouple, the temperature conversion;
From the temperature to the millivolt: measure the actual temperature and the cold end temperature and conversion for millivolt values, respectively, after subtracting the millivolt values, quick temperature.
The advantages of n type thermocouple: 4 n1o * x6r -
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High temperature oxidation resistance is strong, long term stability.
K type thermocouple nickel chromium anode preferential oxidation caused by cr, si element in the alloy composition is uneven and thermoelectric drift, cr, si content in n type thermocouple, nickel-chromium alloy oxidation patterns by internal oxidation into external oxidation, the oxidation reaction on the surface only;
Short-term thermal cycling stability at low temperature, and inhibit the magnetic change;
Radiation resistant ability.
Easy disintegration of n type thermocouple is cancelled k type element mn, co, further strengthen ability to resist neutron irradiation;
, within the range of 400 ~ 1300 ℃, the thermoelectric properties of n type thermocouple linear than k type.
p / n !
N type thermocouple drawback: n type thermocouple material is harder than k type, the more difficult processing;
The price is relatively expensive.
The thermal expansion coefficient of n type thermocouple 15% lower than stainless steel, so the outer casing of n type thermocouple shall adopt the nicrsi/nisi alloy;
200 ~ 400 ℃ within the scope of nonlinear error is bigger.