Device and principle of operation of thermocouples
Thermocouple ammeter is called the device for temperature measurement assembled from the thermocouple, the secondary device and bonding wires. Let's consider the thermocouple device.
Big requirements are imposed to materials for thermoelectrodes of thermocouples. The main requirement is creation rather big thermo-e. of page (together with other material), than is more thermo-e. of page, subjects less sensitive can be the secondary device.
As the comparative thermoelectrode (or as speak, the normal electrode) the thermoelectrode from platinum is accepted now. This results from the fact that the playotina has high temperature of melting (1779 °C), rather easily it turns out in chemically clean look and has constancy of thermoelectric properties.
Very important property for thermocouples is the vzaimozameyonyaemost. Thermocouples which in identical temperature conditions develop identical teryomo-e are considered as interchangeable. of page also, therefore, can work with the same izmeriyotelny device of this graduation. Two electrodes from the same alloy are not always interchangeable, on thermo - э. of page influence the slightest foreign impurity in alloy or neyopravilny annealing after the broach. If the thermocouple not the vzaimoyozamenyaema, then it is necessary to peregraduirovat the device that difficult and is undesirable.
Depending on material of electrodes of the thermocouple, received practical application, are separated into two main groups: thermocouples from precious metals and the thermocouple from neblagorodyony metals.
From serially being issued thermocouples two thermocouples belong to the first group: platinorodiya - the platinum Chamber of Commerce and Industry thermocouple (graduation designation in accordance with GOST 6616-61PP-1). In the name of thermocouples the positive electrode, and the second - negative usually is specified to the first. Platinorody represents alloy which part 90% of platinum (Pt) and 10% of rhodium (Rh) are.
The following thermocouples belong to the second group: the chromel - alumel, the chromel - the thaw and the thermocouple from HK-CA alloy. The chromel represents alloy from 89% of nickel (Ni), 9,8% of chrome (Sg), 1% of iron (Fe) and 0,2% of manganese (MT). Structure алюмеля: 94% of Ni and 6% of Al, Mn, Si. Structure копеля: 56% of Si and 44% of Ni.
The Chamber of Commerce and Industry thermocouple (platinorodiya - platinum) can be used to measurement of temperatures up to 1600 °C quickly and up to 1300 °C is long. Termo-e. to of page at the same time it is equal sootvetyostvenno 16,71 and 13,13 mV. The advantage of this thermocouple is stability of the thermoelectric characteristic (i.e. small deviations э. of page of the thermocouple from the face value established stanyodarty), interchangeability of thermoelectrodes and high resistance to chemical influences even at high temperatures.
Wire thickness in view of the big cost of electrodes (the platinorodiya and platinum) is rather small - 0,5 mm. Diyoametr of electrodes of thermocouples from ignoble metals hesitates within 0,5 - 5 mm.
Thermocouples of Chamber of Commerce and Industry as working thermocouples, control for check, and also are used to measurement of temperatures in responsible processes.
The USAn Labour Party thermocouple is used in the same cases, as thermocouples of Chamber of Commerce and Industry. Limits of temperature measurement make 300 - 1600 (1800 °C quickly).
The THL thermocouple (the chromel - alumel) at kratkovreyomenny application allows to measure the maximum temperayotura of 1300 °C (э. to of page at the same time it is equal to 52,43 mV). Working temperature depending on quality and the chemical composition of covers nakhoyoditsya within 900 - 1000 ° C.
The THK thermocouple (the chromel - the thaw) at kratkovremenyony application can take temperatures to 800 °C (э. to of page at the same time it is equal to 66,42 mV); working temperature is equal to 600 °C (thermo э. of page at the same time makes 49,02 mV).
Thermocouples the chromel - alumel and the chromel - the thaw have the widest circulation as operational devices in industrial furnaces.
The TNS thermocouple is interesting that thermo-e. of its page at temperatures from 0 to 200 °C is practically absent. At the maksiyomalny temperature (about 1000 °C) it develops э. of page of 13,39 mV. Characteristic of the TNS thermocouple is that the accuracy of its work is almost not influenced by temperature of the free end (cold junction).
Constructive performance of thermocouples is defined by the choice of material protective tru6y (armature) and isolation. The protective aryomatura has to protect the thermocouple from effect of the hot, chemically aggressive gases destroying it. Therefore the armature of the thermocouple of the dolyozhn to be gasproof, mekhanichesyok resistant, heatproof and at the same time it is good to carry out heat.
Steel pipes without seam (are applied to protection of thermocouples from ignoble metals at temperatures of measurement up to 600 °C) and from stainless steel (at measurement of the tempeyoratura up to 1100 °C). Quartz and porcelain pipes are applied to protection of thermocouples from precious metals.
The working ends of thermocouples connect soldering or welding, in other parts thermoelectrodes have to be isolated from each other. Thermoelectrodes are isolated by asbestos when the measurement limit no more than 300 °C, quartz pipes or the beads (at t to 1000 °C), porcelain tubes or the beads (prit up to 1300 - 1400 °C).
The thermocouple design made of ignoble metals is given in fig. 1.
At the normal way of turning on of the izyomeritelny device in the thermocouple chain the free ends of the thermocouple располагаются in its head. As to maintain temperature of the head constant and low in the zone usually high measured температур it is quite difficult, the free ends of the teryomopara transfer to the zone of continuous and low temperature. For this purpose priyomenyat so-called compensation wires. For thermocouples from ignoble materials compensation wires izyogotovlyatsya from the same materials, as the thermocouple. In thermocouples, vypolnenyony of precious metals, kompensayotsionny wires are selected from the materiyoal developing among themselves at identical temperatures thermo-e. of page of the same size, as well as main termoyopara. Besides, the taken-out cold junction is surrounded with thermal insulation with the high thermal lag. Apply also spetsiyoalny compensation boxes to automatic compensation of fluctuations of temperature of cold junction.
The special device which represents several resistance forming the mostoyovy scheme (fig. 2) is used to automatic compensation of temperature of the free ends of the thermocouple
Soprotivleniyar1r2r3 ir4 are connected to the unbalanced measuring bridge; soprotivleniyar1r2r3 IRD are executed from the manganinovy wire, and R4 resistance - from copper. Sizes of resistance are picked up in such a way that at the temyoperatura of the environment of 20 °C between points In and the potential difference is equal to zero. In this case the bridge does not influence size measured э. of page. At the izmeyoneniye of ambient temperature (the free ends of the termoyopara) thermo-e changes. of page of the thermocouple: increases at the poniyozheniye of temperature below 20 °C and decreases at increase in the tempeyoratura above 20 °C, size the soprotivleyoniya of R4 which decreases at fall of temperature below 20 °C and increases at increase in the temyoperatura above 20 °C at the same time changes. Sledovatelyyono, these deviations change the razyonost of potentials between points In and kompensiruyotsya diversely and almost mutually.
Added resistance of RD is established in the chain the pitayoniya of the bridge and for different materials of thermocouples has different size. Thanks to it bridge supply voltage, postupayuyoshchy on clips And yes the B, for different thermocouples builds up at the required size. Are included in the diagonal of VG consistently the thermocouple T, compensatory provodark, connecting proyovodarsi millivoltmetrmv.
Besides, joins in the chain of bonding wires podyogonochny soprotivleniyerp intended for adjustment of the soyoprotivleniye of the external line up to the size specified on the millivoltmeter scale.
The compensation box eats tension of postoyanyony current in 4 century. For this purpose she joins the power source of SP - to the device consisting of the reducing transformer, the selenic rectifier and subracing resistance.Top