Разработка ионно-плазменной технологической установки для нанесения функционального покрытия на крышки масляных фильтров, страница 11

4. On persistence distinguish three groups of thermoelements characterized by various values of a parameter of thermal inertia.

Parameter of thermal inertia is time necessary for entering of thermoelement in environment with constant temperature, a difference of environment temperature and any point of thermoelement has made 0,37 temperatures which environment during the moment of regular thermal mode  approach had. The parameter of thermal inertia of thermoelement depends on the physical phenomenon of its principle of action put in a basis and is defined at heat-transfer coefficient equal to infinity.

5.  On a way of application distinguish:

- Immersed,

- Superficial and

- Room thermoelements.

Immersed resistance thermoelements  are intended for measurement of temperature of gaseous or liquid chemically aggressive and nonaggressive environments which do not destroy protective armature of thermoelements.

Superficial thermoelements are intended for temperature measurement of a various products surface.

6. On conditions of use thermoelement can be:

- Stationary;

- Portable.

7. To constructive attributes thermoelement can differ:

- On a degree of security from influence of an environment (ordinary and explosion-proof performance);

- On tightness and in relation to the measured environment (ordinary and tight);

- On stability to mechanical influences (ordinary vibration-proof and shock-resistant);

- On conditions of operation (in normal tropical or in other conditions).

For measurement of temperature in test chambers and in various temperature control systems wide application find thermistors. Measurement of temperature in this case is based on dependence of conductors and semiconductors resistance on temperature.

Conductor thermistor are characterized:

- High stability of resistance temperature factor (RTF),

- Linear dependence of resistance on temperature,

- Good reproducibility of properties,

- Inertness to influences of an environment.

Conductor thermistor produce from copper, platinums, and also from tungsten and nickel.

Resistance of copper conductors in a range of temperatures - 50 … +180^оС is defined under the formula

,                                                    (2.6)

where α = 4,26*10^–ЗK; R₀ – resistance at 0^оС, T₁ – preset temperature value.

Resistance of platinum conductors in a range of temperatures 0 … +650^оС is defined under the formula

,                                         (2.7)

where R₀ – resistance at 0^оС, А=3,90784*10^–3 К^–1, В=5,7841*10^–7 К–², T₁ – preset temperature value.

Basis of conductor thermistor are thermometric sensitive elements. Elements of resistance produce from a platinum wire in diameter 0,05…0,07 mm  and from copper wire in diameter 0,07…0,08 mm.

Necessity of ultralow temperatures measurement leads to application of platinum of high cleanliness. Thermoelement using conductors from the specified material allow to measure temperatures up to - 260^оС.

There are exist industrial thermometers of resistance of two types and copper.

Accuracy of thermometers of resistance is characterized by the several classes estimated by the maximum deviation of resistance ΔR in relation to resistance at 0^оС.

In conformity with way of application for measurement of temperature of a surface carry out the "flat" thermoelement design of the superficial type representing a winding from a platinum wire in diameter of 0,05 mm covered by a viniflex varnish.

Are issued resistance thermoelement intended for measurement of very low temperatures. Find application of thermoelement from indium, germanium, graphite and coal, allowing taking temperature up to - 273^оС.

Semi-conductor thermistor (SCT) differs from conductor thermistor by following basic properties:

- Greater sensitivity change of temperature, RTF of semi-conductor thermistor  is negative and decreases in inverse proportion to a square of absolute temperature: α = В/Т² (RTF makes 3,6 % on 1^оС while for conductor thermometers RTF is equal, on the average, 0,4 %);