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Common knowledge points of Instrument
Nov 26, 2018

Common knowledge points of Instrument


  There is no such thing as a thousand miles,no accumulation of small streams can not become a river.Learn more every week and you will know more.


  1 、High-precision instrument,it is true to measure in the field to obtain high-precision measurement results.Is this true?


  Meter accuracy is not only related to absolute error,but also to the meter's measurement range.The absolute error is large,the relative percentage error is large,and the accuracy of the meter is low.If two instruments with the same absolute error have different measurement ranges,the relative percentage error of the meter with a large measurement range is small,and the accuracy of the meter is high.Accuracy is a quality indicator that is important for the instrument.It is commonly used to standardize and represent accuracy levels.The accuracy level is the maximum relative percentage error minus the sign and%.China's classification of instrument accuracy is:2.5,1.6,1,1,0.5,0.2,0.1,the higher the value,the lower the accuracy level.


  If there are thermometers of the same range of the same range,their range is 0-1000°C,for the 2.5-level thermometer,the allowable error is 2.5 per thousand,that is,the detection error can reach 2.5°C in the full range,Similarly,a thermometer with a precision of 0.1 is allowed to detect only 0.1°C over its full scale range.Therefore,the smaller the accuracy value,the higher the accuracy.


  Conclusion:High-precision instruments can be measured in the field to obtain high-precision measurement results,which is false.


  2 、Some people say that there is no need to consider the transmission distance of the 4~20mA.DC signal.Is this true?


  Some people think that the power supply range of the transmitter is 12~40V.When the 4mA is guaranteed,the power supply of the transmitter is less than 40V.When the voltage is 20mA,it is greater than 12V.It is not necessary to consider the transmission distance of the 4~20mA.DC current signal.


  The transmitter's working power range is very wide.It is a fact that all transmitters in the field are powered by 24V.DC power supply box,that is,the transmitter's power supply voltage is fixed;but every production site It is impossible for the transmitter to be the same distance from the control room.It is unrealistic and impossible to change the supply voltage for a transmitter with a long transmission distance.It is also possible to change one or two units,but it is unrealistic to change the supply voltage one by one for dozens of transmitters.To change the power supply voltage of the transmitter,it is necessary to use a lot of adjustable power supply,so that one is not conducive to maintenance,the second is unable to concentrate power supply,and the third is to increase investment.


  Therefore,as long as the transmitter is used at the production site,it is necessary to consider the transmission distance of the 4~20mA.DC current signal,which is also a measurement requirement.


  Conclusion:There is no need to consider the transmission distance of the 4~20mA.DC signal.It is false.

Common knowledge points of Instrument

  3、 The proper application of the transmitter migration can improve the accuracy of the measurement.Is this true?


  Measuring the liquid level with a differential pressure transmitter is the most versatile method,and the migration function of the transmitter is used in the measurement.


  for example:


  Measure the water level of the high water tank,relevant parameters:water level measurement range h=1200mm,the vertical distance between the lowest water level and the center line of the positive pressure chamber of the transmitter is H=16000 mm,and the density of water is approximatedρ=1g/cm3,because there is no high water tank.Pressure,so initially according to the liquid level measurement scheme of the open container,the calculation is as follows:


  Measurement range:ΔP=hρg,approximately equal to 12 kPa.


  Positive migration:A=Hρg,approximately equal to 160 kPa.


  The measurement range after migration is:


  A~A+△P=160~172kPa.


  A differential pressure transmitter with a measuring range of 0~180kPa can be selected.However,the positive migration of 160 kPa is compared with the range of 12 kPa,the problem comes out,the output current of the transmitter will change little,and the measurement and control accuracy requirements can not be met.In order to ensure measurement accuracy,you can consider reducing the amount of positive migration as much as possible.The options are:


  Solution 1:Install the transmitter near the high water tank to shorten the distance of H to reduce the amount of positive migration.However,the design of the water tank does not have an inspection platform.This idea does not work.


  Option 2:Try to offset the static pressure of the water column with the low water level and the vertical distance H of the positive pressure chamber of the transmitter.That is,at the upper pressure port of the glass water level gauge,add a single-chamber balance container and pass the pressure guiding tube.Connected to the negative pressure chamber of the transmitter and filled with water in the single chamber balancing container and the pressure guiding tube,calculated as follows:


  Measurement range:ΔP=hρ,approximately equal to 12 kPa.


  Positive migration:-B=Hρ,approximately equal to-12 kPa.


  The measurement range after migration is:


  -B~-B+△P=-12kPa~0.


  The differential pressure transmitter with measuring range of 0~36kPa can be selected.Since the static pressure of the H section of the positive and negative pressure guiding tubes of the transmitter cancel each other out,it is only necessary to measure the water level change of h=1200mm and put the transmitter into the transmitter.The negative range of the range is-12 kPa~0.

Common knowledge points of Instrument

  It can be seen from the above that the range of the negative migration scheme transmitter is only 0~12kPa,compared with the transmitter range of 0~180kPa of the positive migration scheme,the range of the transmitter has been greatly reduced,under the same accuracy level.The resolution and measurement accuracy of small scales are definitely better than those of large scales,which meets the requirements of measurement and control.It can be seen that the measurement method and the migration function of the transmitter are applied properly,which can make the transmitter play a greater role and achieve satisfactory results.


  Therefore,proper migration of the transmitter can improve the accuracy of the measurement.


  Conclusion:It is true that the proper migration of the transmitter can improve the accuracy of the measurement.

    

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