Thermal resistance is made by pure metal stuff mostly, application is at present most is platinum and copper, in addition, already began to use the material such as nickel, manganese and rhodium to create thermal resistance now. The sort of feeling lukewarm material with metallic commonly used thermal resistance is more, the most commonly used is platinic silk. Industry is measured outside dividing platinic silk with material of metallic thermal resistance, still nickel of copper, nickel, — of iron, iron waits.

 

Principle of thermal resistance job

 

Of thermal resistance measuring lukewarm principle is be based on conductor or change this one character will measure temperature and the parameter that concern with temperature. Thermal resistance is made by pure metal stuff mostly, application is at present most is platinum and copper, already began to use the material such as nickel, manganese and rhodium to create thermal resistance now. Thermal resistance needs to transmit resistor signal to computer control equipment through down-lead normally or other on 2 appearance.

 

Thermal resistance (graph 2)

 

The structure of sensor: Two lines make:

 

Value of sensor resistor change and the output that join lead resistance makes sensor jointly are worth, the add error that brings as a result of lead resistor makes on the high side of real measured value, use at measuring the situation with precision not high demand, and the length of lead has shoulded not be grow. 3 lines make:

 

The area of 3 lead cut with derivative requirement and length homogeneous phase are the same as, the circuit that measures platinic resistance is lopsided bridge commonly, platinic resistor regards the arm of a bridge of bridge as resistor, wire a power source that receives bridge is carried, on the bridge arm that the others receives platinic resistor twice to be in respectively and the bridge arm with its photograph adjacent, when bridge road is balanced, the change of lead resistor does not have any influences to measuring an outcome, eliminated what lead circuitry resistor brings to measure an error so, but must be congruent arm bridge, remove the effect of lead resistor completely impossibly otherwise. Use 3 lines to make the add error that can reduce lead resistor to bring greatly, 3 lines are used to make commonly on industry receive a standard. 4 lines make:

 

Should measure resistor numerical value very hour, the resistor that tests a line may introduce apparent error, 4 lines are measured with two add test line is offerred constant electric current, additional two voltage drop that check a line to measure sealed resistance, impedance is inputted in volmeter enough below tall condition, electric current spreads overvoltage list scarcely, press with respect to what can measure sealed resistor accurately to go up so fall, the signal that point to uses a few lines to define computation reachs a few lines make resistance.

 

Voltaic output change send implement output physical quantity changeover into 4~20mA electric current, the power source outside wanting to have necessarily is its power supply. The most typical is to change send implement need two power supply cord, add on two electric current to output a line, want to receive 4 lines in all, say make for 4 lines change send implement.

 

Of course, voltaic output is OK as public as power source a line (public VCC or GND) , can save a line, those who say make for 3 lines change send implement.

 

Actually everybody may notice, itself of 4-20mA electric current can send to change implement power supply, if pursue,1 is shown. Change send implement a special load is equivalent to in circuit, special place is sending at changing implement electric current of bad news report is outputted according to sensor between 4~20mA and change. Indication appearance needs to string together only in circuit can. This kind changes send implement 2 lines are received outside needing only, be called consequently two lines are made change send implement. Floor level of level of annulus of industrial electric current is 4mA, because this wants to be inside span limits only, change send implement have 4mA power supply at least. This makes the design that two lines make sensor becomes a possibility.

 

In industrial application, measurement point is in commonly the spot, and indication equipment perhaps controls equipment to be commonly on control room or control ark. Both span leaves hundreds meters tens of possibly coming. Be apart from computation by 100 meters, 2 signal transmission lead means leave out cost reduces nearly 100 yuan! Additionally 4 lines are made change send implement make with 3 lines change send implement must use costly screen line because of asymmetry of the electric current inside lead, and two lines are made change send implement can use dog-cheap lead of double skeining thread, because this is made in the two lines in application,change send implement it is first selection necessarily.

 

 

Two lines are made compensate without circuit resistor, layout is simple, but the add error that should take down-lead resistance. Because of the thermal resistance of precision of this not applicable production class A, and the down-lead when be being used and lead should not be too long.

 

3 lines make:

 

Circuit resistor is compensated, can remove the effect of down-lead resistor, measure precision prep above 2 lines are made. Regard a process as detecting element, its application is the widest.

 

4 lines make:

 

In the root ministry of thermal resistance two end join the means of two lead calls 4 lines to make each, among them two down-lead are offerred for thermal resistance constant voltaic I, change R into voltage signal U, repass is additional two down-lead bring U to PLC. Means of this kind of down-lead can remove the resistance effect of down-lead completely, but cost is higher, the temperature that basically is used at high accuracy detects. Change send implement, in waiting, sensor, flowmeter also has two the line is made and 4 lines are made

 

Two lines are made is signal and power source are together, present appearance uses this kind of means. 5r/r 6l-T6c7`$L8} of electric technology alliance) s4 line is made is power source and signal departure, if high-power changes,send implement, 220VAC gives change send implement enlarge board power supply, differential transformer changes small displacement signal into telegraphic date, enlarge board changes his into the constant current source of 0-10mA to send appearance or DCS the system, had fallen into disuse now.

 

Problem: In press difference / in force transducer and flowmeter performance data, output signal via often can appearing: Second line is made (4 ～ 20) MA, 3 lines are made (0 ～ 10) MA. The problem is, what is second line is made and 3 lines are made, what is their distinction? How wiring?

 

2 thready advantage is wiring simple, sensor with only applicable and general little power, be like: Pressure change, difference pressing changes, lukewarm change, capacitance type fluid plan, flowmeter of radio frequency admittance, electromagnetism, eddy street flowmeter. Sensor itself is made by second line with report in get, it is to affect its belt to carry ability surely.

 

Because be,4 lines are made part power source and power, so the power of this machine and signal are the correlation that goes up without power, apply to high-power sensor, be like ultrasonic (as a result of its to increase interference rejection capacity, the power that projects so will be very great, want when this product type selecting so as far as possible of 4 lines, the ability of average interference rejection of second line is weaker) , cannot consummate of 2 lines, can be 4 lines only, it is working power source respectively 2, output 2.

 

Problem: Why is the signal that two lines make instrument initiative is 0 o’clock 4mA is 0mA?

 

Because be two lines,transmit, use namely change the supply current that sends unit serves as signal, change the static job electric current with send unit need constant, so the floor level of signal cannot be chosen in 0mA, according to standard of international Electrotechnical Committee, the floor level of voltaic signal is 4mA, namely vivid ” of so called “ at 0 o’clock. Appearance compared 0mA safety for 4mA at 0 o’clock, facilitating divisional appearance is to cut off the power still is a directive the smallest. The account that uses voltaic signal is to not allow to suffer interference easily. And infinity of current source internal resistance, lead resistor series connection does not affect precision in loop, hundreds meters can be transmitted on common double skeining thread. Upper limit takes 20mA is the requirement because of explosion proof: The electric current of 20mA connects the scintilla energy that cause can’t ignition gas. The account that floor level did not take 0mA is to can detect line: When regular job won’t under 4mA, open circuit because of breakdown when transmission line, annulus road electric current falls for 0. Often take 2mA to serve as a line to call the police value.

 

Problem: Two lines make instrument is voltaic output signal mostly, is there what voltage outputs signal? How to take voltage?

 

The method of voltaic changeover voltage is very simple, in wanting to shed deferent loop in report only, string together receive resistor of standard of a 250 Europe (precision is being lost 0. 05% ) , in its two end are taken out 1- – 5V DC signal.

 

The signal join means of thermal resistance

 

Thermal resistance is a temperature component that metabolic changeover changes for resistance, at present the down-lead of thermal resistance basically has 3 kinds of kind:

 

4 lines make thermal resistance, 3 lines are made, two thready distinguish comparative analysis

 

1, second line makes: In the two end of thermal resistance each join a lead comes the means of derivative resistor signal calls second line to make: This kind brings a method very simple, but because join lead is put in down-lead resistor R necessarily, the material of R size and lead pledges the element with length is concerned, because means of this kind of down-lead is applied to only,measure the situation with inferior precision.

 

2, 3 lines make: Join in the one aspect of the matter of the root ministry of thermal resistance a down-lead, another end joins the means of two down-lead calls 3 lines to make, this kind of means is used with bridge form a complete set normally, can better elimination the influence of down-lead resistor, it is the most commonly used down-lead resistor in industrial procedure control.

 

3, 4 lines make: The root ministry in thermal resistance two end

 

Each join the means of two lead calls 4 lines to make, among them two down-lead are offerred for thermal resistance constant voltaic I, change R into voltage signal U, repass is additional two down-lead bring U to 2 appearance. It is thus clear that means of this kind of down-lead can remove the resistance effect of down-lead completely, the temperature that basically is used at high accuracy detects. The distinction of method of these 3 kinds of wiring:

 

Because the block value of thermal resistance is little, because this joins the resistor of lead and osculatory resistor can are opposite,its measure lukewarm precision to produce bigger effect, introduce so 3 lines are made or 4 lines are made even if should eliminate these influences and thermal resistance connective to detect equipment (lukewarm accuse input of watch, PLC to wait) have 4 terminal child. I+ , I- , V+ , V- .

 

Among them, end of I+ , I- is to provide constant electricity to thermal resistance, v+ , V- is the voltage variation that uses the thermal resistance that monitor, ordinal detect temperature changes. 4 lines are from thermal resistance two end are derivative 4 lines, with 4 terminal join. 3 lines are derivative 3 lines, this need detects square I-V- weak point receives facility. 2 lines make derivative 2 lines, this need detects equipment square I-V- , I+/V+ is short receive 2, different wiring kind is right the influence of precision:

 

2 lines, voltaic loop and voltage measure loop to close 2 for one, precision is poor.

 

(two thready error basically sheds loop to arise to be pressed certainly in cable in report fall cause measure an error) 3 lines, the reference of voltaic loop the reference that measures return with voltage for a line, precision is a bit good 4 lines, circuit loop and voltage measure loop to part independently, precision is tall, but cost line. Two lines make output wiring is current imitate quantity the most advanced output means in stringing together a mouth, have 6 big advantages:

 

(1) suffer parasitism thermocouple and resistor of edge electrical wiring not easily to press fall and Wen Piao’s influence, can use wire of dog-cheap finer double skeining thread;

 

(2) resistor is outputted in current source enough big when, induction of coupling of classics magnetic field arrives the voltage inside lead annulus road, won’t produce distinct effect, because disturb the voltaic minimum that the source causes, general situation uses double skeining thread to be able to reduce interference;

 

(3) capacitance sex interference can bring about receiver resistor to concern an error, to 4 ～ 20mA two lines make annulus way, receiver resistor is 250Ω normally (1 ～ of sampling Uout ＝ 5V) this resistance is little to can’t produce appreciable error, accordingly, the electrical wiring length that can allow is longer than voltage remote supervising system farther;

 

(4) undertake changing receiving between the different passage that the stage shows each sheet to read device or record device to be able to differ in electrical wiring length, not because of electrical wiring length differ the difference that creates precision;

 

(5) use 4mA Yu Ling n, make judgement carries line open a way or sensor attaint (0mA condition) very convenient;

 

(6) fortify is added easily in mouth of two lines output surge and the parts of an apparatus that prevent thunder, be helpful for the safe explosion proof that prevent thunder.