Blog posts of '2018' 'October'

How to properly install and use the thermocouple temperature sensor?

When installing and using the thermocouple temperature sensor, you should pay attention to the following matters to ensure the measurement results:

1. Error caused by improper installation

For example, the installation position and insertion depth of the thermocouple cannot reflect the real temperature of the furnace.In other words, the thermocouple should not be installed too close to the door and heating place, the insertion depth should be at least 8-10 times of the diameter of  the protective pipe;The gap between the protective sleeve and the wall of the thermocouple is not filled with adiabatic substances,so that the heat in the furnace overflows or the cold air invades.Therefore, the gap between the thermocouple protective pipe and the hole of the furnace wall should be blocked by adiabatic substances such as refractory mud or asbestos rope to avoid the convection of hot and cold air, which affects the accuracy of temperature measurement;The cold end of the thermocouple is too close to the furnace to make the temperature exceed 100 ℃; Thermocouples should be installed as far as possible to avoid strong magnetic fields and strong electric fields. Therefore, thermocouples and power cables should not be installed in the same conduit to avoid errors caused by interference; The thermocouple cannot be installed in the area where the measured medium rarely flows.When the temperature of the gas in the pipe is measured by a thermocouple, the thermocouple must be installed against the flow velocity direction and fully in contact with the gas.

2. Error caused by insulation deterioration

For example,the thermocouple is insulated, too much dirt or salt residue of the protective pipe and the drawing board may lead to poor insulation between the thermocouple and the furnace wall,and it is more serious at high temperature,which not only causes the loss of the thermoelectric potential but also introduces interference.  

3. Error caused by thermal inertia

Because of the thermal inertia of the thermocouple,the indication value of instrument is behind the change of the measured temperature, which is especially prominent in the rapid measurement.Therefore, thermocouples with thinner thermal electrodes and smaller diameters of protective pipes should be used as much as possible.When the temperature measurement environment permits, the protective pipe can even be removed.Because of the measurement lag, the amplitude of temperature fluctuation detected by thermocouple is smaller than that of furnace temperature fluctuation.The larger the measurement lag, the smaller the amplitude of the thermocouple fluctuation, and the greater the difference from the actual furnace temperature.When the temperature is measured or controlled by a thermocouple with a large time constant, although the temperature displayed by the meter fluctuates little, the fluctuation of the actual furnace temperature may be large.In order to accurately measure the temperature, a thermocouple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient, and is proportional to the diameter of the hot end of the thermocouple, the density of the material, and the specific heat, if the time constant is to be reduced, the most effective way is to reduce the size of the hot end as much as possible, in addition to increasing the heat transfer coefficient.In use, it is usually a material with good thermal conductivity, a protective pipe with a thin tube wall and a small inner diameter. In the more precise temperature measurement,a bare wire thermocouple without a protective sleeve is used, but the thermocouple is easily damaged and should be corrected and replaced in time.

4. Thermal resistance error

At high temperatures, if there is a layer of coal ash on the protective pipe, dust attached to it, the thermal resistance increases, hindering the conduction of heat, at this time the temperature indication is lower than the true value of the measured temperature.Therefore, the external cleaning of the protective pipe of thermocouple should be kept to reduce the error.

Main technical features of V-cone flow meter

V-cone flow meter is a new type of differential pressure flow meter.In addition to all the advantages of differential pressure flow meter,the V-cone flow meter has the lowest requirement for straight pipe section in all differential pressure flow meters.Here are the main features and technical parameters of the V-cone flow meter:

1. High accuracy and high repeatability

The measurement accuracy of V-cone flow meter can reach + 0.5% and repeatability is between ±0.1%.

2. Wide range

The range of the V-cone flow meter is beyond the traditional differential pressure flow meter, and the range ratio can reach 10:1.

3. No harsh requirements for straight pipe section

The V-cone can effectively improve the profile of the flow velocity distribution, making it more able to reduce the requirements for upstream straight pipe sections than other differential pressure instruments. The V-cone upstream straight pipe section 1-2D, downstream 3-5D.This greatly reduces the cost consumption and trouble of the user on the straight pipe section.

4. self-cleaning function

Acceleration of the flow velocity at the pipe wall makes the viscous impurities,condensate,and dust in the fluid can not be deposited or adhered to the pipe wall near the V cone and the cone,and the pressure guiding hole is not easily blocked.

5. Wear resistance, good long-term stability

The cone throttling element is designed to be obtuse. When the fluid passes through the cone, the boundary layer is formed,leaving the fluid out of the throttling element.The boundary layer effect prevents the dirty fluid from wearing the throttling element,while its self-cleaning function keeps the inner diameter of the pipe constant.Therefore, its β value remains unchanged for a long time,so there is no need to repeatedly calibrate,and has long-term stability.

6.Stable differential pressure signal

Traditional differential pressure flow meters have a "signal jump" phenomenon, which means that even if the fluid flow is stable,the signal pulsations still exist.The constant eddy current caused by the common orifice plate produces low-frequency, large-amplitude interference signals, which seriously affects the stability of differential pressure and the V-cone flow meter is the opposite. It produces a small vortex with high frequency and low amplitude, which has little effect on the stability of the differential pressure.Typical applications:coke oven gas, flue gas,  propylene gas,natural gas, hydrogen, nitrogen, organic gases, air, circulating water, multiphase liquid, slurry, liquefied petroleum gas, feedstock oil, ammonia steam, etc.

7.Small relative pressure loss

Because there is no sharp edge, the pressure loss caused by V-cone flow meter is constant and much smaller than that caused by orifice plate.

8. Wide range

It can measure liquid, gas, steam and gas-liquid two-phase medium. The flow conditions can be from low temperature to supercritical state, the working temperature is up to 800℃,the maximum pressure is 40 MPa,and the differential pressure signal generated is from the lowest  less than 0.1 KPa to the highest 10 KPa.

How to select the digital display meter?

1. Meter size. That is, the size of the meter, this is a very basic problem. The digital display meter should be installed on the cabinet, so it is necessary to consider the overall coordination. If it is too large, it may not be installed. If it is too small, the display number will not be seen clearly . In addition, the large-sized instrument generally has strong  functional extensibility, and the same function price may be expensive,and the small-sized instrument may have poor functional extensibility.At present, the international standard sizes of digital display meter plates are mainly as follows: 48*24mm; 48*48mm; 48*96mm; 72*72mm; 96*96mm; 96*48mm; 160*80mm.  

2. Display digits.This is directly related to the measurement accuracy of the digital display meter.Generally speaking, the higher the display digits, the more accurate the measurement,and the more expensive the price, mainly the following:two digits (99, special); three digits (999, very few); three and a half digits (1999, the general digital display meter is the mainstream); four digits (9999, intelligent digital display is the mainstream); four and a half digits (19999); four and three-quarters digits(39999);five digits and more than five digits (common in counters, accumulative meters and high-end instruments),users can select several digits digital display meters according to measurement accuracy requirements.

3. Input signal. Refers to the measurement signal directly input into the instrument.Some industrial signals are directly connected to the instrument to measure. Some signals are connected to the instrument after being converted,and the nature of the measurement signal must be clarified.Otherwise,the instrument can not be used, or even damage the instrument and the original equipment. To find out the type of signal: current or voltage, AC or DC, pulse signal or linear signal, etc., but also to understand the size of the signal. The name of the meter is not the same concept as the input signal. For example, the input signal is a 0-75mVdc ammeter (the name is an ammeter, but the input signal is a voltage signal because the current is passed through the shunt to obtain the voltage signal); the input signal is 0- 10Vdc tachometer (name is tachometer, input signal is voltmeter, because the inverter converts the speed signal into voltage signal).

4. Working power. All digital display meters require working power. The working power of the digital display meter mainly includes: 220Vac; 110/220Vac; 85-265VAC/DC switching power supply, 24Vdc (generally customized), 5Vdc (small panel meter).

5. Instrument function.Instrument functions are generally modular,optional, the price of the instrument will vary with the function, The digital display meter mainly has the following optional functions:alarm function and the number of groups of alarm output(i.e. relay action output), feed power output and output voltage size and power, transmitting output and transmitting output type(4-20mA) Or 0-10V, etc.), communication output and communication methods and protocols (RS485 or RS232, Modbus or other protocols), for the regulating control Instruments, optional functions are more,specifically, refer to the manufacturer's selection spectrum to select a standard model, and communicate with the manufacturer and confirm that it is correct before ordering.

6. Several important parameters should be paid attention to:  measurement accuracy (the smaller the value is more accurate),response speed (the smaller the value,the faster the response),the working environment, temperature coefficient (the smaller the value is less affected by temperature),overload capacity.

7. Special requirements. If the user has special requirements, it should be proposed to let the manufacturer confirm whether it can meet the requirements. Do not take it for granted. For example, IP protection level, high temperature working place, strong interference occasion, special signal occasion, special working mode, etc.

In fact, the selection of digital display meter is not complicated.For simple digital display meter,you can buy it to use.For those users who use it for the first time or choose a digital display meter with complex function, as long as you grasp the above points, you can also purchase the right product.