1, the transmitter of the development process of nearly 50 years Pressure transmitter is currently the most important class of automated instrument products, its use is very wide, usually include: pressure transmitter, micro pressure transmitter, differential pressure transmitter Devices, micro differential pressure transmitters, etc. In 1969, Rosemount Corporation of the United States began production of the 1151 series of capacitive transmitters, becoming a brand name product that has enjoyed worldwide reputation for many years. In the early 80s of the last century, the United States Honeywell company launched the intelligent field instrument-ST3000 smart transmitter, then Rosemount, Japan Yokogawa, ABB, Foxboro, etc. Featured smart transmitter products. The working principle of the transmitter in addition to the capacitive type, there are inductive type, single crystal silicon resonant type, diffusion silicon type, ceramic type and so on. At present, the range of high-end products ranges from 0 to 0.025kPato0 to 60MPa, the accuracy index is 0.075% to 0.025%, the turndown ratio is 100:1 to 400:1, and the Rosemount 3051S transmitter is a typical representative of high-end products. There are domestic 3051 pressure transmitters.
2, the principle of range selection Pressure and differential pressure transmitter range selection is an important part of the transmitter selection, when the need to purchase the transmitter, you should first determine the manufacturer and model of the transmitter used, Then determine the transmitter's range according to the requirements. When choosing the range, follow the principles of adjustable upper and lower ranges, highest accuracy, and lowest price. The maximum pressure or differential pressure required by the process professional is the basis for range selection. The selected range is usually determined by 1.5 to 2.0 times the pressure or differential pressure. Do not go extreme when selecting a type. Do not set the transmitter's usage range to the minimum or maximum range. Because at the production site, changing the range often occurs, the range needs to be adjusted upwards or downwards. Considering that there is room for the range to be adjusted up and down, it is not difficult to meet the need to adjust the range. Of course, if the on-site conditions change may only be one-way, if you can only adjust downwards, you may choose the largest range. Highest Accuracy Principles Some mid to high-end transmitters on the market achieve 0.2%, 0.1%, 0.075%, 0.05% or even 0.04% accuracy. Since a high-precision transmitter has been selected, we certainly hope that the selected transmitter will achieve the highest accuracy in use. High-precision transmitters are often characterized by high range ratios. The accuracy and range ratio of the transmitter are two independent indicators. It seems that they are not related to each other. In fact, a large number of transmitters are only in a small range ratio range. Can achieve the high-precision indicators advertised by the product. Take the MV2000T range code C pressure transmitter as an example, the range is 0.4 to 40 kPa. According to the product sample, when it adjusts the range from 0 to 4 to 0 to 40 kPa, the accuracy index can reach the accuracy listed in this product sample. Index (0.075% or 0.05%), but when the range is adjusted from 0 to 0.4kPa to 0 to 4kPa, the accuracy index is not the high-precision index listed in this product sample, and it needs to follow the formula: ± (0.005 × maximum range/ Adjust the range -0.05)% to calculate the additional error. Therefore, in order to ensure the highest accuracy, our actual use range must be between 0 ~ 4kPa to 0 ~ 40kPa, that is, the range of the selected transmitter as close as possible to the principle of the transmitter's maximum range of the lowest price. The lowest price here refers to the principle that should be followed when determining the range after determining the manufacturer and the specific model.
As mentioned above, a certain range of actual use may be appropriate for both range codes. Generally, the maximum range-appropriate range code should be selected according to the principle of highest accuracy. However, in some cases, due to the large difference in price between these two range codes, it is also possible to consider this comprehensively: Sacrifice a little precision, and use a larger range of measuring codes. If MV2000T differential pressure transmitter is selected, if the transmitter's range of use is 0~800Pa, then two range codes A (50~1000Pa) and B (200~6000Pa) can be selected. These two range codes The range of actual usable range ratios is 100 to 1000 Pa and 600 to 6000 Pa, respectively. It is reasonable to say that both range codes are available. However, considering the highest accuracy principle, the A range code should be selected. However, considering the minimum price principle, the differential pressure range has a high price increase range, and the B range code should be selected. Which of the range codes you choose should be determined by combining these two factors. Another example is the measurement of micro-pressure or low pressure, there are pressure transmitter and differential pressure transmitter two options, according to the author's point of view, in the case of ensuring the accuracy of the pressure transmitter should try to use, because the pressure transmitter The price is usually lower than that of the differential pressure transmitter of the same range (since its structure is simple, only one pressure tap is taken), and the excess pressure taken by the differential pressure transmitter for pressure measurement is sometimes affected by the ambient air flow ( If the wind blows, it will bring additional errors.
Micro-pressure micro-differential pressure transmitter detects the micro pressure, micro-differential pressure, usually considered several kPa pressure, differential pressure is micro pressure, micro pressure, this article discusses is roughly a kPa pressure, Differential pressure. Due to the development of sensor technology and microprocessor technology, the accuracy of the transmitter has increased from 1% in the 1960s to 0.5% in the 1970s, 0.25% in the 1970s, 0.1% in the early 1990s, and the mid-to-late 1990s. 0.075%, 0.05%. However, for most transmitters, the accuracy mentioned above is the accuracy of the universal range, such as the medium pressure, high pressure, medium differential pressure, high differential pressure range accuracy, and the micro pressure differential pressure range is often the exception, The accuracy is one grade lower than the general range. The actual accuracy of the micro-pressure micro-differential pressure transmitter is often not as high as we imagine, and the difference in the indicators of different products is also very large. Many users also personally experience the fact that the stability and accuracy of the micro-pressure differential pressure transmitter are relatively poor. Based on the above reasons, the selection of micro-pressure differential pressure transmitters should be a special problem in the selection of transmitters.
From Table 1, it can be seen that the accuracy of the universal range of the transmitter is familiar to everyone, and even if you don't see the sample, you can remember it, and know that the accuracy of the micro-pressure differential pressure transmitter may be different from the universal range accuracy. Now. In Table 1, about half of the manufacturers of micro-pressure differential pressure transmitter accuracy is lower than the accuracy of the universal range, so in the micro-pressure differential pressure transmitter selection, first of all should pay attention to its accuracy is lower than the commonly used range.
4. Influence of Ambient Temperature on Measurement Accuracy The transmitter's index accuracy is the accuracy that can be achieved when the ambient temperature, static pressure, and power supply voltage are relatively stable. In the actual production process, the ambient temperature, static pressure, power supply voltage, etc. The parameters may change. Under such conditions, the accuracy index will also decrease. In particular, the impact of changes in the ambient temperature is even greater.
We know that the effect of ambient conditions on measurement accuracy is expressed in terms of the relative value of full scale. For example, when the ambient temperature changes by 28°C, the formula for calculating the L range of the EJA110A is: ±(0.08% range + 0.09% URL), M The calculation formula of the range is: ± (0.07% range + 0.02% URL), where: URL is the maximum range, and the L, M ranges are 10kPa and 100kPa, respectively. After calculation, comparisons of the measurement ranges for 1kPa, 1.58kPa, 2kPa, 3kPa, 4kPa, and 6kPa are shown in Table 2:
As can be seen from Table 2, although the same measurement range can be selected from any of the two ranges of L and M, it is clear that selecting the L range is more appropriate, and the influence is about twice as small.
5 Concluding remarks In summary, you may have learned that pressure transmitter selection is indeed a special problem. It takes more effort and comparison to select the best transmitter. When selecting the transmitter, the user should select the range of the transmitter according to the measurement range of the process requirements. At this time, the following three points should be achieved: The range of the selected transmitter is within the measurement range of the transmitter; The range of the selected transmitter is within the actual usable range ratio of the transmitter; the range of the selected transmitter is as close as possible to the maximum range of the transmitter. In general, the first point is very easy to do, because it is the most basic; the second point is to make a choice in several adjacent ranges, usually it may also be possible; the third point depends on the various manufacturers to send The ratio of the adjacent ranges of the device can sometimes be selected, and sometimes there is no choice. Only when the first point is reached, the measurement accuracy of the transmitter is difficult to guarantee; when the second point is reached, the measurement accuracy of the transmitter is basically guaranteed; when the third point is reached, the measurement accuracy of the transmitter may reach the optimum value. .
2, the principle of range selection Pressure and differential pressure transmitter range selection is an important part of the transmitter selection, when the need to purchase the transmitter, you should first determine the manufacturer and model of the transmitter used, Then determine the transmitter's range according to the requirements. When choosing the range, follow the principles of adjustable upper and lower ranges, highest accuracy, and lowest price. The maximum pressure or differential pressure required by the process professional is the basis for range selection. The selected range is usually determined by 1.5 to 2.0 times the pressure or differential pressure. Do not go extreme when selecting a type. Do not set the transmitter's usage range to the minimum or maximum range. Because at the production site, changing the range often occurs, the range needs to be adjusted upwards or downwards. Considering that there is room for the range to be adjusted up and down, it is not difficult to meet the need to adjust the range. Of course, if the on-site conditions change may only be one-way, if you can only adjust downwards, you may choose the largest range. Highest Accuracy Principles Some mid to high-end transmitters on the market achieve 0.2%, 0.1%, 0.075%, 0.05% or even 0.04% accuracy. Since a high-precision transmitter has been selected, we certainly hope that the selected transmitter will achieve the highest accuracy in use. High-precision transmitters are often characterized by high range ratios. The accuracy and range ratio of the transmitter are two independent indicators. It seems that they are not related to each other. In fact, a large number of transmitters are only in a small range ratio range. Can achieve the high-precision indicators advertised by the product. Take the MV2000T range code C pressure transmitter as an example, the range is 0.4 to 40 kPa. According to the product sample, when it adjusts the range from 0 to 4 to 0 to 40 kPa, the accuracy index can reach the accuracy listed in this product sample. Index (0.075% or 0.05%), but when the range is adjusted from 0 to 0.4kPa to 0 to 4kPa, the accuracy index is not the high-precision index listed in this product sample, and it needs to follow the formula: ± (0.005 × maximum range/ Adjust the range -0.05)% to calculate the additional error. Therefore, in order to ensure the highest accuracy, our actual use range must be between 0 ~ 4kPa to 0 ~ 40kPa, that is, the range of the selected transmitter as close as possible to the principle of the transmitter's maximum range of the lowest price. The lowest price here refers to the principle that should be followed when determining the range after determining the manufacturer and the specific model.
As mentioned above, a certain range of actual use may be appropriate for both range codes. Generally, the maximum range-appropriate range code should be selected according to the principle of highest accuracy. However, in some cases, due to the large difference in price between these two range codes, it is also possible to consider this comprehensively: Sacrifice a little precision, and use a larger range of measuring codes. If MV2000T differential pressure transmitter is selected, if the transmitter's range of use is 0~800Pa, then two range codes A (50~1000Pa) and B (200~6000Pa) can be selected. These two range codes The range of actual usable range ratios is 100 to 1000 Pa and 600 to 6000 Pa, respectively. It is reasonable to say that both range codes are available. However, considering the highest accuracy principle, the A range code should be selected. However, considering the minimum price principle, the differential pressure range has a high price increase range, and the B range code should be selected. Which of the range codes you choose should be determined by combining these two factors. Another example is the measurement of micro-pressure or low pressure, there are pressure transmitter and differential pressure transmitter two options, according to the author's point of view, in the case of ensuring the accuracy of the pressure transmitter should try to use, because the pressure transmitter The price is usually lower than that of the differential pressure transmitter of the same range (since its structure is simple, only one pressure tap is taken), and the excess pressure taken by the differential pressure transmitter for pressure measurement is sometimes affected by the ambient air flow ( If the wind blows, it will bring additional errors.
Micro-pressure micro-differential pressure transmitter detects the micro pressure, micro-differential pressure, usually considered several kPa pressure, differential pressure is micro pressure, micro pressure, this article discusses is roughly a kPa pressure, Differential pressure. Due to the development of sensor technology and microprocessor technology, the accuracy of the transmitter has increased from 1% in the 1960s to 0.5% in the 1970s, 0.25% in the 1970s, 0.1% in the early 1990s, and the mid-to-late 1990s. 0.075%, 0.05%. However, for most transmitters, the accuracy mentioned above is the accuracy of the universal range, such as the medium pressure, high pressure, medium differential pressure, high differential pressure range accuracy, and the micro pressure differential pressure range is often the exception, The accuracy is one grade lower than the general range. The actual accuracy of the micro-pressure micro-differential pressure transmitter is often not as high as we imagine, and the difference in the indicators of different products is also very large. Many users also personally experience the fact that the stability and accuracy of the micro-pressure differential pressure transmitter are relatively poor. Based on the above reasons, the selection of micro-pressure differential pressure transmitters should be a special problem in the selection of transmitters.
From Table 1, it can be seen that the accuracy of the universal range of the transmitter is familiar to everyone, and even if you don't see the sample, you can remember it, and know that the accuracy of the micro-pressure differential pressure transmitter may be different from the universal range accuracy. Now. In Table 1, about half of the manufacturers of micro-pressure differential pressure transmitter accuracy is lower than the accuracy of the universal range, so in the micro-pressure differential pressure transmitter selection, first of all should pay attention to its accuracy is lower than the commonly used range.
4. Influence of Ambient Temperature on Measurement Accuracy The transmitter's index accuracy is the accuracy that can be achieved when the ambient temperature, static pressure, and power supply voltage are relatively stable. In the actual production process, the ambient temperature, static pressure, power supply voltage, etc. The parameters may change. Under such conditions, the accuracy index will also decrease. In particular, the impact of changes in the ambient temperature is even greater.
We know that the effect of ambient conditions on measurement accuracy is expressed in terms of the relative value of full scale. For example, when the ambient temperature changes by 28°C, the formula for calculating the L range of the EJA110A is: ±(0.08% range + 0.09% URL), M The calculation formula of the range is: ± (0.07% range + 0.02% URL), where: URL is the maximum range, and the L, M ranges are 10kPa and 100kPa, respectively. After calculation, comparisons of the measurement ranges for 1kPa, 1.58kPa, 2kPa, 3kPa, 4kPa, and 6kPa are shown in Table 2:
As can be seen from Table 2, although the same measurement range can be selected from any of the two ranges of L and M, it is clear that selecting the L range is more appropriate, and the influence is about twice as small.
5 Concluding remarks In summary, you may have learned that pressure transmitter selection is indeed a special problem. It takes more effort and comparison to select the best transmitter. When selecting the transmitter, the user should select the range of the transmitter according to the measurement range of the process requirements. At this time, the following three points should be achieved: The range of the selected transmitter is within the measurement range of the transmitter; The range of the selected transmitter is within the actual usable range ratio of the transmitter; the range of the selected transmitter is as close as possible to the maximum range of the transmitter. In general, the first point is very easy to do, because it is the most basic; the second point is to make a choice in several adjacent ranges, usually it may also be possible; the third point depends on the various manufacturers to send The ratio of the adjacent ranges of the device can sometimes be selected, and sometimes there is no choice. Only when the first point is reached, the measurement accuracy of the transmitter is difficult to guarantee; when the second point is reached, the measurement accuracy of the transmitter is basically guaranteed; when the third point is reached, the measurement accuracy of the transmitter may reach the optimum value. .
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