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Dec 21, 2018

# Technical characteristics of pressure transmitter

With the development of science and technology, people have higher and higher requirements on the transmitter and more and more detailed requirements on its structural performance.Now the production of intelligent transmitter, a variety of technical indicators up to dozens of items.However, for users, it is not possible or necessary to verify the technical indicators of the transmitter in the field, and some indicators will not change.However, understanding and mastering these performance, for the use and maintenance of the transmitter is good, for this pressure transmitter technical characteristics you know how much, huaheng instrument arrangement let you know more.

## 1. Measurement range, upper and lower limit and range

Each transmitter used for measurement has a measuring range, which is the range of the measured variable measured by the instrument according to the specified accuracy.The minimum and maximum values of the measurement range are respectively called the lower limit of measurement (LRV) and the upper limit of measurement (URV), referred to as the lower limit and upper limit.

The range of the transmitter can be used to represent the size of its measurement range, which is the algebraic difference between the upper limit and the lower limit of its measurement, namely:

Range = upper limit of measurement - lower limit of measurement

The lower limit and upper limit can be used to fully represent the measuring range of the transmitter and to determine its range.Such as a lower limit temperature transmitter is 20 ℃, the upper limit is 180 ℃, and its measuring range can be expressed as - 20 ~ 180 ℃, the range of 200 ℃.Thus, it can be seen that the upper and lower limits and measuring ranges of the transmitter can be known if the measuring range of the transmitter is given, while the upper and lower limits and measuring ranges cannot be determined if only the measuring range of the transmitter is given.

Another way to express the measuring range of the transmitter is to give the zero point of the transmitter (i.e. the lower limit of the measurement) and the measuring range.According to the previous analysis, as long as the zero point and range of the transmitter are determined, its measurement range is also determined.Therefore, this is a more commonly used way to represent the measuring range of the transmitter.

## 2. Zero point migration and range adjustment

In actual use, due to the change of measurement requirements or measurement conditions, the zero point or range of the transmitter needs to be changed, so the transmitter can be carried out zero point migration and range adjustment.The purpose of range adjustment is to make the upper limit of output signal of transmitter correspond to the upper limit of measurement range.Figure 2.1 shows the input and output characteristics before and after the range adjustment of the transmitter.

As can be seen from the figure, the range adjustment is equivalent to changing the slope of the input and output characteristics of the transmitter. The range adjustment from feature 1 to feature 2 is the range increase adjustment.On the contrary, the adjustment from property 2 to property 1 is the range reduction adjustment.

In actual measurement, in order to correctly select the range size of the transmitter and improve the measurement accuracy, it is often necessary to transfer the starting point of measurement to a certain value (positive or negative), which is called zero point transfer.In the absence of migration, the starting point of measurement is zero.When the starting point of measurement changes from zero to a positive value, it is called positive migration.Conversely, when the starting point of measurement changes from zero to a certain negative value, it is called negative transfer.The purpose of zero adjustment and zero migration is to make the lower limit of transmitter output signal correspond to the lower limit of measurement signal.When, is the zero adjustment;When, is the zero point transfer

Figure 2.2 shows the input and output characteristics before and after the zero point transfer of the transmitter.As can be seen from the figure, after the zero point migration, the input-output characteristic of the transmitter shifted some distance to the right or left along the x coordinate, and its slope did not change, that is, the range of the transmitter did not change.The accuracy and sensitivity of the instrument can be improved by zero point migration and range compression.

FIG. 2.2 zero point transfer of transmitter

Zero positive and negative transfer refers to the adjustable range of transmitter zero, but it is different from zero adjustment.Zero adjustment is the adjustment when the input signal of the transmitter is zero and the output is not zero (lower limit);When the input of the transmitter is not zero, the output is adjusted to zero (lower limit).If the low pressure inlet of the differential pressure transmitter has input pressure and the high pressure inlet does not, the adjustment when the output is set to zero (lower limit) is called negative migration.If the high pressure inlet of the differential pressure transmitter has input pressure and the low pressure inlet does not, the adjustment of the output to zero (lower limit) is called positive migration.Since the migration is a zero adjustment when the transmitter has input, the migration is expressed in terms of how many input signals can be migrated, or in terms of what percentage of the measurement range.

As a result of same transmitter, its use range has big have small, so migratory amount also became have big have small.

Most manufacturers produce transmitters, the amount of migration is expressed as a percentage of the maximum range.For example, some transmitter zero point positive and negative transfer for the maximum range of plus or minus 100%, that is to say, if the transmitter measurement range is 0 ~ 31.1kPa to O ~ 186.8kPa, when the transmitter high or low pressure inlet through the O ~ 186.8kPa range of any pressure, the zero point can be moved to 4mA.However, the pressure of 186.8kPa at the high pressure inlet is the upper limit of the measurement range, and the re-pass is the overpressure. It is not impossible to change the zero point to 4mA DC, but it is meaningless. Therefore, it is generally added that the sum of the zero point migration and the use range cannot exceed the limit of the measurement range.namely

Where: is the migration quantity; Is the use range;Is the maximum range.Thus, if the range is 186.8kPa, the zero point positive transfer is

I can't move.

However, if the range is 62.3kpa, then the zero point positive migration is

For negative migration, there is no such limit, because it is the negative pressure inlet pressure, so no matter how much pressure is in the range of 0 ~ 186.8kPa, the zero point migration plus the use of differential pressure will not exceed the limit of the measurement range

## 3. Range than

Range ratio refers to the ratio between the maximum measurement range and the minimum measurement range of the transmitter, which is also a very important indicator.The measuring range and operating conditions used by the transmitter are constantly changing. If the range ratio of the transmitter is large, it has a large room for adjustment.According to the needs of the process, the scope of use can be changed at any time, which obviously brings users a lot of convenience.They do not require instrument changes, disassembly or reinstallation.Just change the range.For smart meters, just reset it on the handheld terminal.In this way, the number of spare parts in the warehouse can be greatly reduced and the work of planning and management will be much simpler.

From the simplest displacement differential pressure gauge to the current intelligent transmitter, the range ratio is constantly increasing, which indicates the progress of technology.However, it should be noted that when the range ratio reaches a certain value (for example, 10), its other technical indicators such as accuracy, static pressure, one-way performance will be bad, to a certain value (for example, 40), although it can be used, but its performance is very poor.In general, the greater the range ratio, the lower the measurement accuracy.

## 4. Four-wire system and two-wire system

Transmitters are mostly installed in the field, and their output signals are sent to the control room, where power is supplied.Transmitter signal transmission and power supply are usually two ways:

(1) four-wire system

The power supply and output signals are transmitted by two wires respectively, and the wiring mode is shown in figure 2.3.Such transmitters are called four-wire transmitters.DDZ - Ⅱ series instrument transducer adopts this kind of connection form.Since power and signal are transmitted separately, there is no strict requirement for zero point of current signal and power consumption of components.The power supply can be ac (220V) or dc (24V), and the output signal can be dead zero (0 ~ 10mA) or live zero (4 ~ 20mA).

Figure 2.3 four-wire transmission

(2) two-wire system

For the two-wire transmitter, there are only two wires connected to the transmitter, which transmit the power supply and output signals at the same time, as shown in figure 2.4.As can be seen, the power supply, transmitter and load resistance are in series.The two-wire transmitter is equivalent to a variable resistor whose resistance value is controlled by the measured parameters.When the measured parameters change, the equivalent resistance of the transmitter changes accordingly, so does the current flowing through the load.

Figure 2.4 two-wire transmission

The two-wire transmitter must meet the following conditions:

(1) the transmitter's normal working current, must be equal to or less than the minimum signal current, that is

Since the power line and signal line are common, the power supply to the transmitter is provided by the signal current.When the output current of the transmitter is the lower limit, the semiconductor device inside the transmitter should still work normally.Therefore, the lower limit of signal current cannot be too low.Because at the lower limit of the transmitter output current, the semiconductor device must have a normal static operating point, which needs to be supplied by the power supply to work normally, so the signal current must have a live zero.The international unified current signal adopts 4-20madc, which creates conditions for the manufacture of two-wire transmitter.

(2) the transmitter can work normally under the voltage condition

Where: is the output terminal voltage of the transmitter;Is the minimum power supply voltage;Is the upper limit of the output current, usually 20mA;Is the maximum load resistance value of the transmitter;Is the resistance value of the connecting wire.

The two-wire transmitter must be supplied with a single dc power supply.The so-called single power supply refers to the zero potential as the starting point of the power supply, rather than with zero voltage symmetry of the positive and negative power supply.The output voltage U of the transmitter is equal to the difference between the supply voltage and the output current in RL and the resistance r of the transmission wire.To ensure the normal operation of the transmitter, the output voltage value can only change within a limited range.If the load resistance increases, the supply voltage must increase.Conversely, the power supply voltage can be reduced;If the supply voltage decreases, the load resistance must decrease.Conversely, the load resistance can be increased.

(3) the transmitter can work normally the minimum effective power

Because the power supply of the two-wire system transmitter is very small, and the load voltage varies greatly with the output current and load resistance, the working voltage of each part of the line varies greatly.Therefore, when making the two-wire transmitter, it is required to use the low-power integrated operational amplifier and set up the link of voltage and current stabilization with good performance.

The advantages of two-wire transmitter are many, can greatly reduce the installation cost of the device, is conducive to safety and explosion-proof.Therefore, at present, most countries in the world use two - wire transmitter system.

The load characteristic is refers to the transmitter output load capacity, usually only the electric transmitter has this technical index.The load characteristics of all different types of two-wire transmitters are similar.Figure 2.5 shows the load characteristics of 1151 analog transmitter.

Figure 2.5 load characteristics of 1151

As can be seen from the figure, the lowest terminal voltage is stipulated to ensure the normal operation of the transmitter (i.e., 20mA current). It is recommended to use the case of E and J, whose minimum voltage is 12V, otherwise it cannot work normally.In the work zone in the transmitter, load resistance (Ω) and based on the relationship between the power supply voltage (V)

Where, o. 02 is the maximum output current, A.

Because the electric transmitter has constant current performance, so the output short circuit, the instrument will not be damaged.

## 6. Power supply mode

All electric instruments need power supply, and power supply mode is also an important problem in electric instruments.There are two kinds of power supply for electric meters: ac power supply and dc centralized power supply.

(1) ac power supply.The power frequency 220V ac voltage is introduced into each instrument, and the transformer is used to reduce the voltage. Then rectification, filtering and voltage stabilization are carried out as the respective power supply. This power supply mode was used in the early electric instrument system.The disadvantages are: this kind of power supply mode requires the addition of power transformer, rectifier and voltage regulator circuit in each table, so as to increase the volume and weight of the instrument;The heat of the transformer increases the temperature rise of the instrument;220V ac is directly introduced into the instrument, reducing the safety of the instrument.

(2) centralized dc power supply.Dc centralized power supply is the unified supply of each meter by dc low-voltage power box.Power frequency 220V ac voltage is supplied to each meter power supply after voltage transformer, rectifier, filter and voltage stabilization are carried out in the power box.There are many advantages to centralized power supply:

(1) each table saves the power transformer, rectification and voltage control part, thus reducing the volume of the instrument, reduce the weight of the instrument, and reduce the heat element parts, so that the temperature rise of the instrument;

(2) because of the use of dc low-voltage centralized power supply, can take anti-power failure measures, so when the industrial use of 220V ac power failure, can be directly into the dc low-voltage (such as 24V) backup power supply, so as to constitute no power failure device;

(3) there is no industrial use of 220V ac into the meter, for the instrument explosion-proof provides favorable conditions.

## 7. Damping characteristics

Differential pressure transmitter is commonly used to work with throttling device to measure fluid flow, but also according to the principle of static pressure measurement container medium liquid level, flow, liquid level these two kinds of physical parameters are sometimes very easy to fluctuate, resulting in a very large record curve, can not see clearly, for this transmitter generally have damping (filtering) device.

Damping characteristics are expressed as the transmitter transmission time constant, which refers to the output from 0 to the maximum value of 63.2% of the time constant.The greater the damping, the longer the time constant.

The transmission time of the transmitter is divided into two parts, one part is the time constant of each link of the instrument, this part is not adjustable, the electric transmitter is about a few tenths of a second;The other part is the time constant of the damping circuit, which can be adjusted from a few seconds to a dozen seconds.

## 8. Liquid temperature and ambient temperature

Liquid temperature is the transmitter detection components contact the measured medium temperature, environmental temperature is the transmitter amplifier, circuit board can withstand the temperature, the two are not the same, the former range is large, the latter range is small.Such as rosemount transmitter for liquid temperature for 3051-45 ~ + 120 ℃, environment temperature of 40 ~ + 80 ℃.Therefore, in the use of attention, do not put the transmitter in the environment temperature is mistaken for the liquid temperature.

Temperature effect refers to the output of the transmitter, along with the change of environmental temperature changes are typically changes every temperature 10 ℃, 28 ℃ and 55 ℃ to characterization of the output of the change.The temperature influence of the transmitter is related to the range of the instrument. The larger the range of the instrument, the smaller the influence of the environmental temperature change will be.

## 9. Static pressure and unidirectional overpressure characteristics

(1) static pressure characteristics.

Static pressure is the working pressure of thedifferential pressure transmitter, usually much larger than the differential pressure input signal.Theoretically speaking, the output of the differential pressure transmitter is only related to the input differential pressure, and the working pressure of the transmitter is not related, but because of the design, processing, assembly and many other factors, the transmitter zero and range is changing with the static pressure.The transmitter static pressure index is the allowable range of this change.Here are two points to note:

(1) different range of transmitter, the output is affected by static pressure is not the same, range range

Large, less affected by static pressure change;On the contrary, it has a great impact.Manufacturers in order to make their own production of the instrument has a high technical indicators, so no matter how much users use in the measurement range, static pressure index is always in the maximum range, the zero and the range of the change of how much.

(2) transmitter static pressure can be positive pressure, can also be negative pressure.There is a limit to the positive pressure

16 mpa, 40 mpa;There is also a limit to negative pressure, for example, minus one mpa, but not an absolute vacuum.When we talk about the static pressure of the transmitter, we usually just talk about the upper limit pressure, and the lower limit pressure seems to be undefined, but that's not true.Transmitter in the absolute vacuum, the case of silicon oil will vaporize, will damage the instrument, so there are provisions.

(2) unidirectional overpressure characteristics.

Unidirectional overpressure is unidirectional overload namely, it is the one side that points to difference pressure transmitter is pressed, another side is not pressed.In the transmitter and throttle device supporting the use of the process, due to careless operation, sometimes a side of the pressure pipe valve open, and the other side is closed, so the transmitter static pressure is how much pressure, one-way overpressure is how much pressure.

For general instruments, the signal pressure can only be a little bit larger than the rated pressure, such as 30%, 50%, but for the differential pressure transmitter, one-way overload pressure is not a little bit larger than the signal pressure, but several times, dozens of times, a hundred times.In this case, the transmitter should not be affected, and its zero drift must be within the allowable range, which is the unique unidirectional characteristics of the differential pressure transmitter.

The earliest differential pressure gauge is not resistant to one-way overpressure, but now the transmitter one-way overpressure index set very high, one-way on the performance of the instrument basically has no impact.For example, yokogawa EJA series differential pressure transmitter can be used without a balance valve.One-way overpressure time is not specified, but from the point of view of use, no balance valve is not convenient.

## 10. Stability

Stability is another important technical index of transmitter. In a sense, it is more important than the accuracy of transmitter.Stability error refers to the ability of the output to remain constant for a specified period of time while the input remains constant under specified working conditions.URV / 6 months indicates that: within 6 months, the zero point change of the instrument does not exceed + / 0.1% of the upper limit of the measurement range.Note that this is the upper limit of the measurement range, not the range of use.For example, the measurement range of a transmitter is 0 ~ 2kPa to O ~ 100kPa. If it is used in O ~ 10kPa, its stability is not plus or minus 0.1%, but plus or minus 1%.So when you look at the error of the meter, you have to look at what range it's in.