How to choose the suitable temperature transmitter?
Temperature transmitter is widely used in industry, agriculture, petrochemical and other automation control fields, and the production technology is quite mature. Compared to the temperature sensor, the temperature transmitter will be directly measured by the temperature sensor signal into a current or voltage signal standard, and then transferred to the two meters, the site of PC or PLC, is easy to use, easy to install, convenient for transmission, strong anti-interference ability, especially the extensive application and popularization of PLC more the majority of engineers and technicians in favor of using temperature transmitter.
Although the temperature transmitter has been widely used, but not part of engineers and technicians in design and selection of design procurement standards or requirements (here does not refer to the related technical parameters of temperature transmitter), the procurement staff in the procurement process, in the face of numerous manufacturers, or blindly parity, focus or care for this and lose that. Or at a loss, the final purchase of temperature transmitter or price is not good, or low price is not good, or high priced purchase application occasions with low requirements, big horse car, do not even scolded by his boss. Then, how to choose the suitable temperature transmitter? From the manufacturer's point of view, I will explain the following as follows.
1, according to the temperature transmitter
Temperature transmitter from the structure is divided into integration, split, rail, miniature and other styles, different occasions using different styles. Therefore, the choice of suitable temperature transmitter, we should first consider the environmental factors. Environmental factors include on-site live, installation methods, electromagnetic interference, temperature and humidity, etc..
(a) a lot of temperature measurement, very dense distribution. In this case, first consider the selection of temperature transmitter RS485 output type, if the transmitter a lot of transverse or longitudinal section of the measuring point can choose the output of RS485, which can save the wiring, easy installation, convenient power supply.
If an integrated current type temperature transmitter is selected, it is better to use a low power temperature transmitter, so that the wiring of the field can be reduced as a result of the reduction of the power supply line.
B) the temperature is very far. If the distance of the measuring point is within tens of meters, the split type temperature transmitter can be selected and combined with the four wire platinum resistance temperature sensor, so that the transmission accuracy can be guaranteed and the cost can be reduced. The split type temperature transmitter can be selected by rail mounted in the control box or in a suitable position installed in the box. If the distance is more than 100 meters, the current type temperature transmitter can be selected. If the site is not convenient to supply power, a low-power temperature transmitter can be selected. If the field environment is &ldquo, clean &rdquo, and no electromagnetic interference, you can choose the wireless transmission of the temperature transmitter.
C) space is small. Especially the machinery, pipelines and other small space temperature transmitter installation, the best selection of micro temperature transmitter, the minimum is only equivalent to the size of the screw, the installation is very convenient, if you consider that the replacement is convenient, can use micro temperature transmitter with connectors.
(2) installation of
If installed on the pipe, insert thread or flange shall be inserted. If the bearing bushing can be selected by micro temperature transmitter, the temperature transmitter can be directly screwed into a screw. If the vibration is great, choose a temperature transmitter with shockproof design. Besides the shock absorber, spring and other parts, the internal structure of the temperature transmitter must have the design function of shock absorption.
Electromagnetic interference is an important issue that must be considered in choosing temperature transmitter. Engineers must analyze whether the environment itself has interference factors and the intensity of the interference, such as whether there are large motors, motors, inverters and high voltage power lines nearby the circuit. If the same power, although the power supply and the environment is very clean, “ ” other electric equipment is frequent start, start process is not a surge, whether the impact of large charge, will result in cumulative peak voltage and superposition. In the face of electromagnetic interference, it is necessary to select a temperature transmitter with isolation function, including input isolation, isolation between input and output, and isolation between the power supply and the input and output. In some cases, can not simply choose isolation type temperature transmitter for transmission lines corresponding to shield, two meters, PLC must be well grounded and isolated, and even with electrolytic capacitor filter and high frequency.
The conventional temperature transmitter using ambient temperature is basically -10~50 degrees centigrade, and the ordinary transmitter can be basically satisfied. But to be used in low temperature environment, the temperature transmitter is not common, such as in the northeast of Heilongjiang, Liaoning, Jilin, Inner Mongolia, and Xinjiang, Tibet, and Russia and other -40 C environment, we must choose the normal work under -40 DEG C temperature transmitter. For example, in a high temperature 60~80 degrees Celsius environment, the same temperature transmitter can work properly. In this extreme case, although the temperature transmitter will be biased, but must ensure the stability and consistency is better, this is the temperature transmitter bias can be revised in the two meters, PLC. It can work well under these two extreme temperatures, and it also has good consistency and stabilityThe temperature transmitter circuit design, component selection have high requirements, this is a better quality of the temperature transmitter. The ambient humidity of an ordinary integrated temperature transmitter is typically 20%~90%RH, roughly equivalent to the IP65 rating. If used in more than 98%RH humidity environment, or even directly used in water, you must use more than IP68 waterproof level. This temperature transmitter must be customized to meet the requirements.
2, according to the choice of precision temperature transmitter
The accuracy of the ordinary temperature transmitter and temperature instrument is mostly 0.2%FSGeneral analog circuit temperature transmitters are in this category.
The precision temperature transmitter is generally accurate at 0.1%FS, which belongs to the digital temperature transmitter or the intelligent temperature transmitter. The temperature transmitter has good stability, wide supply voltage range (9~30VDC), adjustable range (minimum range difference can reach 50 DEG C), the input resistance is generally two or three, four wire PT100 of arbitrary choice, according to customer requirements can also be connected to the PT1000. Unlike the analog temperature transmitter circuit with only three wire PT100, encountered tens of meters farther transmission distance, the lead wire resistance transmitter cannot overcome, can only be modified on the host computer, and the integrated temperature transmitter accuracy has not reached the technical parameters of 0.2%FS, the basic difference in the minimum range of more than 100 DEG C.
Scientific research institutes, colleges and universities, military enterprises and other units generally choose high-precision temperature transmitter. The precision of this kind of temperature transmitter is high, generally at 0.05%FS& and 0.1 degrees centigrade. It is mainly used in high precision test and precise measurement.
3, according to the two meter or PLC type temperature transmitter
The two instruments on the spot generally require the temperature transmitter to output the current signal of 4~20mADC. The output of the individual request is the RS485 signal, and the general acquisition card is the voltage signal of 0~5VDC more. PLC analog signals directly accessed by the CPU are typically 0~10VDC, with few 4~20mA signals and analog input modules of the 4~20mA. The 4~20mA analog input module, general wiring line was three, but in the procurement of temperature transmitter when in fact two-wire 4~20mA can, as long as the connection to PLC temperature transmitter and the ground terminal is connected to, some people find the world 4~20mA signal three wire temperature transmitter. There is no need to.
4, according to the performance of temperature transmitter
This is a mixed proposition. Products purchased at lower prices may only be realized at the initial budget, with lower cost, higher prices, and the final cost of the product may not be high, of course, including maintenance, after-sales, service and a series of problems. Therefore, consider the cost performance rather than just consider the price.