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Flowmeter selection considerations

1) Flowmeter accuracy and function
At present, there are many models of flow calculators produced at home and abroad, and the difference is not small. The difference in accuracy is especially concerned by users. A calculator matched with the analog input signal, the error is generally 0.2%, 1.5%, and the error with the pulse output flow transmitter is generally O.1%~0.5%. The accuracy level often affects the range of the system, especially the differential pressure flowmeter. If the accuracy is too low, a large range cannot be obtained.
The basic functions of the calculator are quite similar. The simplest calculator has no compensation function, and the input signal often has only one way (analog input or pulse input). The calculator with compensation function generally has three inputs of flow, temperature and pressure. Many of the functions described in Section 4 above are available as options for the user to select, such as communication functions, printing functions, power-down recording functions, quantitative control functions, and so on. The function of the calculator to meet the system requirements is the primary issue to consider when choosing. If you don't need the function, you can keep it if you don't increase your expenses. Otherwise, you can choose it.
The versatility of the calculator is also considered. If the versatility is strong, it can often reduce the variety of instruments, compress the total amount of preparations, and bring convenience to maintenance.
(2) Signal input channel matches transmitter and sensor
The input channels of the calculator can accept a variety of input signals. When selecting, pay attention to the signal type, index number, input resistance, signal range and so on.
I) Pulse flow input channel
a. The frequency input range of the pulse range input channel of the frequency range generally starts from 1 Hz. However, the volumetric flowmeter produced in the early stage has a lower output frequency lower limit. The instrument manufacturer should be contacted to customize the calculator with a low frequency lower limit.
b+ pulse amplitude Different types of flow sensing 嚣 output pulse amplitude varies. The same type of flow sensor may also differ in manufacturing design and must be matched to the selection.
At present, the power supply of the vortex flow sensor on the market is 24V (DC). Generally, the pulse low level is ≤2V, the pulse high level is ?22V; the power supply is 12V (DC), and the low level is ≤1V, high level. ≥5V. Some preamplifier-less sensors have lower output amplitudes and high levels (or positive pull-wave peaks) of only 2 - 3V. In this case, a low-response calculator should be used.
c:. 4-20mA current pulse signal When the flow sensor outputs 4~20rrLA current pulse signal, Generally, it is a two-wire system. On the one hand, the calculator should provide a suitable power supply for the sensor. On the other hand, the resistor should be converted into a suitable voltage resistor in the current loop to convert the current pulse signal into a voltage pulse signal that the calculator can accept. Figure 15.6 shows a typical line connection diagram.
Sensor connected to the calculator
d. When the passive contact signal calculator is used with the passive contact input signal, the calculator needs to provide a DC power supply. In the line connection, it is necessary to string the appropriate current limiting resistor to avoid the current flowing through the contact and the loop current. Large, burn out the contacts and overload the power supply.
e. Pulse input signal with open collector Some volumetric flowmeters are equipped with a non-contact proximity switch. The rotation of the detection component is converted into an electrical pulse signal output through the switch. Some switches are three-wire, and when the circuit is connected with the calculator, some of the line connections are two-wire. You can use the connection method shown in Figure 15, 7(b). The matching of the calculator with this contactless switch requires two points to be considered. One is that the DC power supply Vs provided by the calculator needs to meet the needs of the switch, and the other is the switch. After the two states are converted into current, the high level and low level obtained at the input of the calculator frequency signal must be within the response range of the calculator. However, since the equivalent resistance of the switch in the two states is far from the ideal switch, the level obtained by the input signal of the calculator is often too high, so that it falls outside the response range of the calculator. The solution to this problem is to change the connection method shown in Figure 15.7 (c). And properly configure the load resistor RL.
Calculator
Figure 15.7 Contactless Switch and Calculation 嚣 Connection
2) Analog flow input channel If the input channel is an analog input signal, it usually has 4-20mA (DC), 0-10OmA (DC), 1~5v (DC) and other signals. If you choose a voltage input, you also need to pay attention to its input resistance value. Some manufacturers produce calculators with input resistance of 250fl at l5v (DC) input. If the voltage signal is taken from a distributor (such as DFP-2100), you should pay attention to the signal input calculator. distortion.
3) The temperature input signal of the temperature input channel calculator is used more often by Ptl00 indexing number (platinum thermal resistance), K indexing number (thermocouple) and E indexing number (thermocouple). Go to temperature transmitters and distributors. but
~ 20mA input and l-5V input are also used.
4) The pressure input signal of the pressure input channel calculator is the most used 4-20mA. Sometimes also use 1. 5v or O-lOmA input.
In order to save investment, some users use a resistance remote pressure gauge to measure fluid pressure. At this time, the calculator needs to be made into a 0-400f2 resistance input. Use the “Pressure Start Value” and “Pressure Fullness Value” menus in the calculator to adjust the scale. However, the accuracy of the electric remote pressure gauge is low, generally only 1.5, so the compensation accuracy is worse than the pressure of the transmitter. Usually only used in places where the requirements are not too high.
The flowmeter is installed centrally in a steam distributor such as a boiler room, a gas distribution station, etc. Since the fluid pressures in front of each flowmeter are approximately equal, a pressure transmitter is commonly used for multiple calculators. At this time, the current signal representing the vapor pressure can be converted into a voltage signal by a precision resistor (generally O.Ol% accuracy) and input to the calculator. The pressure input channel of the calculator is a voltage signal and is made into a high input resistance. After the input resistance is as large as 1 M.O, even if there are several shutdowns in multiple calculators, it will not have a significant impact on the still running calculator.

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