(1) the choice of temperature measurement point
Thermocouple installation position, that is, the choice of temperature measurement point is the most important. The location of the temperature measurement point, for the production process, must be typical, representative, otherwise the meaning of measurement and control will be lost.
(2)Insertion depth
Thermocouple inserted into the place to be measured, along the length of the sensor will produce heat flow. When the ambient temperature is low there will be heat loss, resulting in the thermocouple and the measured object temperature inconsistency and temperature measurement error. In short, the error caused by heat conduction is related to the insertion depth. And insertion depth and protection tube material. Metal protection tube because of its good thermal conductivity, the depth of insertion should be deeper (about 15 to 20 times the diameter), ceramic materials with good adiabatic properties, can be inserted into some shallow (about 10 to 15 times the diameter). For engineering temperature measurement, the depth of its insertion is also related to the measurement object is static or flow state, such as flowing liquid or high-speed air temperature measurement, will not be subject to the above limitations, the insertion can be shallow, the specific value should be determined by the experiment.
2. The impact of the response time
The basic principle of contact temperature measurement is that the temperature measurement element to reach thermal equilibrium with the object to be measured. Therefore, the temperature measurement needs to be maintained for a certain period of time, in order to make the two reach thermal equilibrium. The length of time, with the thermal response time of the temperature measurement element. The thermal response time depends on the structure of the sensor and the measurement conditions, which vary greatly. For gas media, especially static gas, should be kept at least 30min or more to reach equilibrium; for liquid, the fastest should be more than 5min.
For the temperature is constantly changing the measured place, especially the instantaneous change process, the whole process is only 1s, then the sensor response time is required in milliseconds. Therefore, the ordinary temperature sensor not only can not keep up with the measured object of the temperature change rate of hysteresis, but also because of the thermal equilibrium can not reach the measurement error. It is best to choose a fast response sensor. For thermocouples, in addition to the protection of the tube, thermocouple diameter of the measuring end is also the main factor, that is, the finer the wire, the smaller the diameter of the measuring end, the shorter its thermal response time. Temperature measurement element thermal response error can be determined by the following formula.
Δθ=Δθ0exp(-t/t) (1)
In the formula
Δθ - at the moment of t, the error caused by the temperature measuring element, K or ℃
Δθ0 - the "t = 0". Error caused by the temperature measuring element at the moment "t=0", K or ℃
t - measurement time, s
τ - time constant, s
< p>ε - the base of the natural logarithm (2.718)Therefore, when t = τ, then Δθ = Δθ0/e
that is, 0.368,
If when t = 2τ, then Δθ = Δθ0/e2
that is, 0.135.
When the temperature of the measured object rises or falls with a certain speed α (k/s or ℃/s), after a sufficient time, the resulting response error can be expressed by the following formula:
Δθ∞=-ατ (2)
In the formula
Δθ∞-after a sufficient time, the error caused by the temperature measuring element
From the formula (2) It can be seen that the response error is proportional to the time constant (τ). In order to improve the calibration efficiency many companies use automatic calibration device, the incoming thermocouple calibration, however, the device is not very perfect. FAW gearbox plant heat treatment workshop found that if the 400 ℃ point of the constant temperature time is not enough to reach thermal equilibrium, it is prone to misjudgment.
3. The effect of thermal radiation
The thermocouple inserted into the furnace for temperature measurement will be heated by the thermal radiation emitted by the hot object. Assuming that the gas inside the furnace is transparent, and, when the temperature difference between the thermocouple and the furnace wall is large, there will be temperature measurement errors due to energy exchange.
In unit time, the two exchanged radiant energy for P, can be expressed in the following formula:
P=σε(Tw4-Tt4) (3)
In the formula
σ - Stefan Boltz constant
ε -- emissivity
Tt -- temperature of the thermocouple, K
Tw -- temperature of the furnace wall, K
In a unit of time, the Thermocouple with the surrounding gas (temperature T), through convection and heat conduction will also occur in the exchange of heat energy for P'
P'=αA(T-Tt) (4)
The formula α - thermal conductivity
A - the surface area of the thermocouple
A - the surface area of the furnace wall
Tw - the temperature of the furnace wall, K
In a unit of time. - surface area of the thermocouple
In the normal state, P=P', the error is:
Tt-T=σε(Tt4-Tw4)/ αA (5)
For the unit area the error is
Tt-T=σε(Tt4-Tw4)/ α (6)
Therefore. In order to reduce the thermal radiation error, should increase the heat conduction, and make the furnace wall temperature Tw as close as possible to the thermocouple temperature Tt. In addition, in the installation should also pay attention to: thermocouple mounting position should be avoided as far as possible from the solid issued by the thermal radiation, so that it can not be radiated to the thermocouple surface; thermocouple is best to have a thermal radiation shielding sleeve.
4. Thermal impedance increases the impact
Thermocouple used at high temperatures, if the measured medium is gaseous, then the protective tube surface deposits of dust, etc. will be burned fused to the surface, so that the protective tube of the thermal impedance increases; if the measured medium is molten, in the process of using the slag will be deposited, not only increases the response time of the thermocouple, and also make the indication of the temperature is low. Therefore, in addition to regular calibration, in order to reduce the error, often sampling is also necessary. For example, imported copper melting furnace, not only installed continuous temperature measurement thermocouple, but also equipped with consumable thermocouple temperature measurement device for timely calibration of continuous temperature measurement with the accuracy of the thermocouple.
Three thermocouple temperature measurement should pay attention to matters
1. Thermocouple wire inhomogeneous effect
(1) thermocouple material itself inhomogeneous
Thermocouple calibration in the metrology room, according to the requirements of the procedure, inserted into the calibration furnace depth of only 300mm. so each thermocouple calibration results, exactly only reflect or mainly reflect from the measurement end Start 300mm long wire thermoelectric behavior, however, when the thermocouple is longer, most of the wire is in the high temperature region, if the thermocouple wire is homogeneous, then according to the homogeneous circuit rule, the measurement results have nothing to do with the length. However, the thermocouple wire is not homogeneous, especially the cheap metal thermocouple wire its homogeneity is poor, but also in the temperature gradient of the occasion, then its localization will produce thermal electromotive force, the electromotive force is called parasitic potential. The error caused by the parasitic potential is called inhomogeneous error.
In the existing precious metal, cheap metal thermocouple calibration procedures, the unevenness of the thermocouple has not yet made provisions, only in the thermocouple wire material standards, the unevenness of the thermocouple wire has certain requirements. On the cheap metal thermocouple using the first and last test method to find out the inhomogeneous thermoelectric potential. Formal thermocouple wire production plant, are required by national standards, the production of uneven thermal electromotive force meets the requirements of the product.
(2) the use of thermocouple wire produced by the uneven
For the new thermocouple, even if the uneven thermal potential can meet the requirements, but, repeated processing, bending thermocouple processing aberration, will lose the homogeneity; and the use of thermocouples in the long term at high temperatures will be due to the deterioration of the filament caused by the change in thermal potential, such as inserted into an industrial furnace in the thermocouple, will be Along the length of the wire deterioration, and with the temperature increases, deterioration enhancement, when the deterioration of the part of the place with a temperature gradient, will also produce parasitic electric potential superimposed on the total thermal electric potential and measurement error.
The authors in practice found that there are thermocouples by the metrological department qualified products (mostly cheap metal thermocouples) to the field use is not qualified, and then returned to the metrological department test is still qualified, which is mainly due to the main reason is that the dipole wire is not homogeneous caused by the quality. Production of thermocouple technicians are keenly aware that the failure rate of thermocouples also increases with the increase in its length, are affected by the uneven quality of the thermocouple wire. In short, by the inhomogeneity of the parasitic electric potential caused by the error, depending on the degree of inhomogeneity of the thermocouple wire itself and the size of the temperature gradient, it is extremely difficult to quantify.
2. armored thermocouple shunt error
(1) shunt error
Waxshaft Group carburizing furnace armored thermocouple, only one week on the use of inadmissible. In order to explore the reasons, the author has been to the site visit, did not find abnormalities, and from the furnace down by the measurement room calibration results qualified. So what is the problem? Finally, according to the branch of the thermocouple site installation characteristics, the study found that the above problem is armored thermocouple shunt error caused.
The so-called shunt error that the armored thermocouple measuring furnace temperature, when the middle part of the thermocouple temperature distribution of more than 800 ℃ exists, because of its insulation resistance decreased, the thermocouple value appears abnormal phenomenon. According to the principle of homogeneous circuit, with thermocouple temperature measurement only with the measurement end and reference end of the two ends of the temperature, and the middle of the temperature distribution has nothing to do. But because the armored thermocouple insulation is powdered MgO, the temperature rises every 100 ℃, its insulation resistance drops an order of magnitude, when the middle part of the temperature is high, there must be a leakage current, so that in the thermocouple output potential in the shunt error occurs.
(2)The conditions of shunt error
The armored thermocouple is inserted horizontally into the furnace, and its specifications and experimental conditions are as follows: the diameter Φ4.8mm, the length of 25m, the length of the heat tape in the middle part is 20m, and the temperature is 1000℃. In this experiment, the temperature difference between the measuring end and the middle part of the thermocouple is 200℃. If the temperature of the measuring end is higher than that of the middle part, a negative error is generated; on the contrary, a positive error is generated. If the temperature difference between the two is 200℃, then the shunt error is about 100℃. This is absolutely can not be ignored, shunt error conditions and armored thermocouple type and diameter and other factors, see table.
3. Shunt error factors and countermeasures
The phenomenon of high temperature armored thermocouple shunt error, is attracting attention, so it is necessary to understand the shunt error of the factors affecting the shunt error, and to take appropriate countermeasures to reduce or eliminate the impact of the shunt error.
(1) armored thermocouple diameter
For the length of 9m K-type armored thermocouple (MgO insulation), only the middle part of the thermocouple heating. Experimental results show that: the size of the shunt error is inversely proportional to the square root of its diameter (diameter is too fine, does not comply with this law), that is, the finer the diameter, the greater the shunt error.
When the temperature of the middle part is higher than 800℃, for Φ3.2mm armored thermocouple will produce shunt error. But for Φ6.4mm and Φ8mm armored thermocouple, when the temperature of the middle part is 900℃, still no shunt error is found. For Φ6.4mm (thermal electrode wire diameter Φ1.4mm) and Φ8mm (thermal electrode wire diameter Φ2.0mm) armored thermocouples, when the temperature of the intermediate part is 1100℃, the shunt error generated by the armored thermocouple with diameter Φ8mm is only half of that of Φ6.4mm. This value (50%) is approximated by the square ratio of the diameter of the electrode wire of the two armored thermocouples (1.42/2.02), and the square ratio of the diameter of the electrode wire, that is, the resistance ratio of the electrode wire. Therefore, in order to reduce the shunt error, should be selected as far as possible coarse diameter armored thermocouple.
(2) the temperature of the intermediate parts
such as the temperature of the intermediate parts of more than 800 ℃, it is possible to produce shunt error, the size of which will increase exponentially with the increase in temperature. Therefore, in addition to the measuring end, other parts should be avoided as much as possible to exceed 800 ℃.
When the middle part of the heating band temperature is higher than 800 ℃, the longer the length of the heating band, the farther away from the measuring end, the greater the shunt error. Therefore, the length of the heating tape should be shortened as much as possible, and do not heat away from the measuring end to reduce the shunt error.
(3) the resistance of the thermocouple wire
When the armored thermocouple diameter is the same, the shunt error will increase with the resistance of the thermocouple wire. Therefore, the use of small resistance thermocouple wire is better. For example: the diameter of the same S-type armored thermocouple with K-type thermocouple compared to the shunt error is reduced by 40%. Therefore, the S-type thermocouple can be used to measure the temperature field distribution in the furnace, although the cost is high, but more accurate.
(4) insulation resistance
High-temperature oxide resistivity will be exponentially lower with increasing temperature, the shunt error size depends mainly on the high temperature part of the insulation properties, insulation resistance is lower, the more likely to produce shunt error. When the insulation resistance increases by 10 times or decreases to 1/10, the shunt error also decreases to 1/10 or increases by 10 times. In order to reduce the shunt error, should be used as far as possible diameter thick armored thermocouple, increase the thickness of the insulation layer. If the above measures are ineffective, have to use the assembly thermocouple.
4. Short-range ordered structure changes (K state) of the impact
K-type thermocouple in the temperature range of 250 ~ 600 ℃ when used, due to changes in its microstructure, the formation of short-range ordered structure, so will affect the thermopotential value of the error, which is called the K state. It is Ni-Cr alloy unique lattice changes, when the Cr content in the range of 5 to 30% of the existence of the atomic lattice of the ordered → disordered transformation of the resulting error, due to the Cr content and temperature varies. K-type thermocouple heated from 300 ℃ to 800 ℃, every 50 ℃ to take a point, measure the point potential. The deviation is up to 4°C at 450°C, and is positive in the range of 350 to 600°C. The K state makes the K-type thermocouple more stable than the K-type thermocouple. Due to the existence of K state, so that the K-type thermocouple in the temperature or cooling calibration results are inconsistent, so in the cheap metal thermocouple calibration procedures in the explicit provisions of the calibration order: from low temperature to high temperature point by point temperature calibration; and in the 400 ℃ calibration point, not only the effect of heat transfer is not good, difficult to reach thermal equilibrium, but also in the state of the K error is exactly in the maximum range. Therefore, the point should be very careful when determining qualified or not.
Ni-Cr alloy short-range structural changes in the phenomenon, not only exists in the K-type, but also in the E-type thermocouple in the positive pole of the phenomenon, but as a change in the amount of E-type thermocouple is only K-type 2 / 3. In short, the K state of the temperature, time, when the temperature distribution or thermocouple position changes, the deviation will be very large changes, so it is difficult to accurately evaluate the size of the deviation.
5. The effect of the atmosphere
(1) selective oxidation
For Fe-containing Ni-Cr alloy, such as the oxygen partial pressure is lower than a specific value, the O2 affinity with the Cr will be selectively oxidized, which is unique to the grain boundary oxidation of Ni-Cr alloy. If the outer surface of the oxide layer with a microscope, you can see the green precipitates, this phenomenon is usually called "green corrosion". Especially when the temperature in the range of 800 ~ 1050 ℃, the system contains CO, H2 and other reducing gases, K-type thermocouple positive electrode is more likely to selective oxidation. This low thermal potential caused by the reduction of Cr content has become a limiting factor for the long-term use of type K thermocouples in the heat treatment industry.
If the gas used is very pure, and the system does not contain oxygen, can extend the service life of the thermocouple; can be such as the thermocouple wire surface has an oxidized layer, but still can be for the selective oxidation of Cr to provide enough oxygen. Therefore, when used in a non-oxidizing atmosphere, clean, polished wire should be used. At the same time, should be avoided as far as possible in the inert gas with trace oxygen or oxygen partial pressure is very low in the air. When the length and diameter of the protection tube is large (i.e., the protection tube is very thin), the residual small amount of oxygen can still provide conditions for selective oxidation of Cr due to poor air circulation and the formation of anoxic conditions.
(2) selective oxidation countermeasures
In order to prevent or slow down the K-type thermocouple due to selective oxidation caused by deterioration, in addition to improve the material, but also in the structure of the thermocouple should also take the corresponding countermeasures: (a) choose the oxygen affinity of Cr stronger metal as an absorbent, sealed into the protective tube, to prevent selective oxidation of Cr, but also can be used to increase the diameter of the protective tube or blowing method to increase the oxygen content. or blowing method to increase the oxygen content. (b) assembly thermocouple solid. The author developed a patented product - entity-type carburizing furnace thermocouple, that is, the development of a sealed structure of the assembly thermocouple, can prevent selective oxidation of Cr, by the Waxaxaxial Group, FAW, FAW, Epson Industrial Furnace, Shen Heavy, Shen tooth, Qianjiang motorcycle and so on more than a dozen enterprises for many years to use to prove that this program is effective. The service life is more than 12 months, the user is very satisfied.
(3) the use of the influence of the atmosphere
Thermocouple stability, due to the use of temperature, atmosphere is different, for the same kind of sensors, such as K-type thermocouple of the highest temperature of the use of the diameter varies, the diameter of the same K-type thermocouple is also due to the structure of the different, and the stability of its also has a great deal of difference. In the selection of thermocouples, must be considered for the use of conditions: the common temperature and the highest use temperature; redox and other use of the atmosphere; anti-vibration performance.
For assembled thermocouple, the influence of the atmosphere, first of all depends on the protection tube material and thermocouple structure, therefore, familiar with, master the physical and chemical properties of various protection tube material is very necessary. For example: in the powder metallurgy industry, commonly used molybdenum tube as thermocouple protection tube, in the 1600 ℃ H2 atmosphere, the use of good results. However, molybdenum tube in the oxidizing atmosphere, a very short time due to oxidation and etching. Secondly, should be used according to the atmosphere, select the appropriate thermocouple, in the oxidizing atmosphere above 1300 ℃, choose platinum rhodium thermocouple, in the reductive, vacuum conditions using tungsten rhenium thermocouple is better.
For K-type thermocouple, suitable for work in the air, O2 and other atmospheres, but when used in the H2 atmosphere, its surface is reduced by H2, short time no effect, such as long time exposure to H2, in accelerating the reduction at the same time, the wire will make the grain growth and break the line; in the CO or gas and other reducing atmospheres, its deterioration will be significantly accelerated and super poor.
For armored thermocouple, hydrogen atom radius is very small, easy through the coat into its internal, will also accelerate the deterioration, resulting in a significant reduction in the value of the thermal potential.
(4) the effect of insulation resistance
Thermocouple insulator, at high temperatures, its insulation resistance with the temperature and the sharp decrease, therefore, there will be leakage current, the current through the insulation resistance has decreased the insulator into the instrument, so that the instrument indication is unstable or produce measurement error, may also occur in the phenomenon of recorder chaotic hit point.
Four thermocouple deterioration and service life
1. Thermocouple deterioration
Thermocouple service life and its deterioration, the so-called thermocouple deterioration, that is, thermocouple after the use of the aging deterioration of the phenomenon. By the metal or alloy composition of the thermocouple, in the high temperature of its internal grain to gradually grow. At the same time, the alloy contains a small number of impurities, its position or shape will also change, and, to the surrounding environment in the reduction or oxidizing gases should also be reacted. Along with these changes, the thermocouple's thermoelectric potential will also be extremely sensitive to change. Therefore, the deterioration of the thermocouple is inevitable.
2. Thermocouple life
Thermocouple deterioration is a quantitative process, it is very difficult to quantify it, will vary with the type of thermocouple, diameter, use temperature, atmosphere, time. The service life of the thermocouple is the deterioration of the thermocouple develops to exceed the permissible error, or even break the line can not be used time.
(1) the life of the assembly thermocouple
Our standards only the stability of the thermocouple requirements. That is, the provisions of a certain temperature by 200h, before and after the use of the change in thermoelectric potential. However, has not been found on the service life of the provisions. Japan about the service life of the thermocouple requirements, is based on the Japanese JIS (C-1602-1995) standard for continuous use of thermocouples. For B, R, S type thermocouple for 2000h, K, E, J, T type thermocouple for 10,000h.
In practice, assembled thermocouples usually have a protective tube, only in special circumstances to bare wire use. Therefore, in most cases, the life of the protection tube determines the thermocouple life. The judgment of the actual service life of the thermocouple, must be through the long-term collection, accumulation of data in the actual state of use, it is possible to give more accurate results.
(2) armored thermocouple life
As the armored thermocouple casing protection and isolation from the external environment, so the casing material has a great impact on the armored thermocouple life, according to the use of thermocouple wire and metal casing selection. When the material is selected, the life of the armored thermocouple with the increase in diameter and increase. Armored thermocouple with assembly thermocouple, although there are many advantages, but the service life is often lower than the assembly thermocouple.