Thermocouple temperature transmitter

1 basic introduction

The thermocouple temperature transmitter consists of circuit elements such as reference source, cold junction compensation, amplification unit, linearization processing, V/I conversion, burn-off processing, reverse connection protection, and current limiting protection. It is to compensate the thermoelectric potential generated by the thermocouple through the cold end compensation, and then the non-linear error of the thermoelectric potential and the temperature is eliminated by the linear circuit, and finally amplified and converted into a 4-20 mA current output signal. In order to prevent accidents caused by temperature control failure due to galvanic breakage in the thermocouple measurement, the transmitter also has a power-off protection circuit. When the thermocouple breaks or the connection is poor, the transmitter will output the maximum value (28mA) to turn off the power.

2 temperature transmitter principle

A temperature transmitter is a meter that converts a temperature variable into a standardized output signal that can be transmitted. Mainly used for measurement and control of industrial process temperature parameters.

A transmitter with a sensor usually consists of two parts: a sensor and a signal converter. The sensor is mainly a thermocouple or a thermal resistor; the signal converter is mainly composed of a measuring unit, a signal processing and a conversion unit (since the industrial thermal resistance and the thermocouple indexing table are standardized, the signal converter is also called as a stand-alone product). Transmitter), some transmitters have added display units, and some have fieldbus functionality. As shown on the right:

If the transmitter consists of two sensors for measuring the temperature difference, there is a given continuous function relationship between the output signal and the temperature difference. It is called a temperature transmitter.

There is a given continuous function relationship (usually a linear function) between the transmitter output signal and the temperature variable. The output signal of the earlier produced transmitter is linearly related to the resistance value (or voltage value) of the temperature sensor. .

The standardized output signals are mainly DC signals of 0 mA to 10 mA and 4 mA to 20 mA (or 1 V to 5 V). Other standardized output signals with special regulations are not excluded.

The temperature transmitter can be divided into two-wire system and four-wire system according to the power supply wiring mode.

The transmitter has a series of electric unit combination meters (DDZ-II type, DDZ-III type and DDZ-S type) and a miniaturized modular type, multi-functional intelligent type. The former does not have a sensor, and the latter two types of transmitters can easily form a transmitter with a sensor with a thermocouple or a thermal resistor.

3 technical data

input signal:

Thermocouples: k, e, j, b, s, t, n. Thermal resistance: pt100, cu50, cu100 three-wire system, four-wire system. The input signal of the intelligent temperature transmitter can be arbitrarily set by the handheld device and the PC;

output signal:

Outputs a 4-20ma DC signal over the range, linear with the input signal of the thermocouple or thermistor or linear with temperature. Intelligent temperature transmitter output 4-20ma DC signal simultaneously superimposed with hart standard protocol communication; isolated temperature transmitter: input and output isolation, isolation voltage 500v, increased anti-common mode interference capability, more suitable for computer connection Net use;

Basic error:

0.5% fs, 0.2% fs, intelligent 0.2% fs;

Wiring:

Two-wire system, three-wire system, four-wire system;

Display method:

Four lcd displays on-site temperature, intelligent four-digit lcd can be accessed by PC or handheld

Set to display any of the parameters of field temperature, sensor value, output current, and percentage;

Operating Voltage:

Ordinary model 12v-35v, intelligent 12v-45v, rated working voltage is 24v

Allowable load resistance:

500ω (24vdc power supply); ultimate load resistance r(max)=50vmin-12, for example, when the rated working voltage is 24v, the load resistance can be selected from 0-600ω.

working environment:

a: Ambient temperature -25-+80 °C (conventional type)

-25-+70°C (digital display type)

-25-+75°C (smart type)

b: relative humidity: 5%-95%

c: mechanical vibration f ≤ 50hz, amplitude ≤ 0.15mm

d: no corrosive gas or similar environment;

9. Environmental impact coefficient: δ≤0.05%/°C.

4 main features

Simple structure: no moving or elastic components, so the reliability is extremely high and the maintenance is minimal.

Easy to install: The built-in structure shows this feature in particular, without the need for any special tools.

Convenient adjustment: Zero and span two potentiometers can perform zero shift or range change arbitrarily within the effective range of temperature detection. The two adjustments do not affect each other.

Wide range of applications; suitable for liquid level measurement of high temperature, high pressure, strong corrosion and other media.

Main Specifications

Accuracy: 0.5, 1 and 1.5

Pressure range: negative pressure, normal pressure, high pressure (32MPa or less)

Applicable medium: acid, alkali, salt or any medium that is non-corrosive to PTFE

Output signal: 4-20mA, two-wire system

Power supply: load resistance 0-750Ω DC24V

Fixing method: Thread mounting M20×1.5, M27×2 Flange mounting DN15, DN25, DN50, DN80.

5 selection information

Z

Thermal resistance

M

Nickel-chromium-nickel-chromium

N

Nickel-chromium-nickel silicon

E

Nickel-chromium-copper-nickel

F

Iron-copper nickel

C

Copper-copper nickel

P

platinum

C

copper

B

Temperature Transmitter

1

No fixture

2

Fixed thread

3

Movable flange

4

Fixed flange

5

Active pipe joint

6

Fixed threaded cone

7

Straight pipe joint

8

Fixed threaded pipe joint

9

Movable threaded pipe joint

2

Blowout prevention

3

Waterproof

4

Explosion-proof

0

Ф16

1

Ф12

2

Ф16 high aluminum tube

3

Ф20 high aluminum tube

G

Variable cross section

M

Analog display

S

Digital Display

no

Normal installation

Z

Separate installation

W

R

N

B

2

4

0

G

S

Z

6 equipment maintenance

The instrument indicates an abnormal phenomenon, such as high, low, no change or no disorder, which itself contains two factors: First, the process factor, the thermal resistance temperature transmitter instrument truly reflects the abnormal process; The second is the instrument factor, because the fault of a certain part of the meter measurement system is wrong. The following inspection process is limited to instrument system failures. When it is determined that the source of the fault is not a process problem, it is checked according to the fault determination procedure to eliminate the instrument fault.

If the meter records a dead line (a line that has no change at all), or if the recorded curve is originally fluctuating, it suddenly becomes a direct thermal resistance temperature transmitter line; the fault is likely to be in the meter system. Because the recording instruments are mostly DCS computer systems, the sensitivity is very high, and the change of parameters can be very sensitive. At this point, you can artificially change the process parameters to see the curve changes. If it does not change, the basic conclusion is that the instrument system has a problem; if there is a normal change, the base thermal resistance temperature transmitter has determined that the instrument system has no major problems.