Blog posts of '2018' 'November'

Remote type electromagnetic flow meter

Remote type electromagnetic flow meter adopts the principle of time difference measurement: one probe transmits a signal through the pipe wall, the medium, and the other side of the pipe wall, and is received by another probe.At the same time, the second probe also transmits the signal and is received by the first probe.Due to the influence of the flow velocity of the medium, there is a time difference Δt between the two.According to the calculation, the conversion relationship V=(C2/2L)×Δt between the flow velocity V and the time difference Δt can be obtained, and the flow rate value Q can be obtained.

Remote type electromagnetic flow meter has many advantages: high precision; large measuring range; measurement results are not affected by conductivity, pressure, temperature and viscosity; it is not in contact with medium, especially suitable for measurement of corrosive media; simple installation and low cost ;can be installed on existing pipelines without cutting off process pipelines;no spoiler, no need to reduce the diameter, no leakage, low cost; at the same time, the probe of remote type electromagnetic flow meter produced in China can cover the pipe diameter of DN6 ~ DN6500 range,which has the advantages of high cost performance and timely service of domestic products.

Application direction of remote type electromagnetic flow meter: the current industrial flow measurement generally has the problem of large diameter and large flow measurement difficulty. This is because the increase of the measurement pipe diameter will bring difficulties in manufacturing and transportation, and the cost will increase and the energy loss will increase.and the remote type electromagnetic flow meter can avoid these shortcomings. Because all kinds of electromagnetic flow meters can be installed outside the pipe, non-contact flow measurement,the cost of the instrument is basically independent of the diameter of the pipeline under test, while other types of products with the increase in caliber, the cost is greatly increased. Compared with the functional price of other types of flow meters with the same function, the larger the diameter, the better, and it is considered to be a better large-diameter flow measuring instrument. Remote type electromagnetic flow meter adopts advanced DSP technology and double UP high-speed processor, which increases the sampling frequency to 70ms and can be used to measure the change of fluid flow in a very short time.By erecting metal guided wave plate between pipeline and probe, the temperature of medium is reduced from 450℃ to below 200 ℃,which makes measurement possible, and has unique applications in the petrochemical industry, the nuclear power industry, and the heat transfer oil field.And the energy calculation function is adopted, and the temperature signal can be introduced while measuring the flow rate to obtain the energy parameter.  

485 communication knowledge

1.What kind of communication line should be used for the 485 bus?

Internationally accepted shielded twisted pair must be used. The recommended type of shielded twisted pair is RVSP2*0.5 (two-core shielded twisted pair, each consisting of 16 strands of 0.2mm wire). The use of shielded twisted pairs helps to reduce and eliminate the distributed capacitance generated between the two 485 communication lines and the common mode interference generated around the communication lines.

Most of the engineering contractors are accustomed to using 5 kinds of network lines or over 5 kinds of network lines as 485 communication lines, which is wrong. This is because:

(1) The common network cable has no shielding layer and cannot prevent common mode interference.

(2) The network cable is only 0.2mm square, and the wire diameter is too thin, which will result in a reduction in transmission distance and a reduction in the number of devices that can be attached.

(3)The network cable is a single-strand copper wire that is easily broken compared to a multi-core wire.

2.Why is it grounded?

The 485 transceiver will operate normally when the specified common mode voltage is between -7V and +12V. Exceeding this range will affect communication and severely damage the communication interface. Common mode interference increases the above common mode voltage. One of the effective means to eliminate common mode interference is to use the shielding layer of the 485 communication line as the ground line, connect the devices in the network such as implements and computers, and reliably connect to the earth from one point.

3.How should the 485 communication line be connected?

Keep the communication lines away from high-voltage wires as much as possible, not in parallel with the power cables, or bundle them together.

4. Why should the 485 bus adopt a hand-in-hand structure instead of a star structure?

The star structure produces a reflected signal that affects 485 communications. The length of the branch line from the bus to each terminal device should be as short as possible, generally not exceeding 5 meters. If the branch line is not connected to the terminal, there will be a reflected signal, which will cause strong interference to the communication and should be removed.

5.Can there be contacts between devices on the 485 bus?

Use the same cable in the same network system to minimize contact in the line.Make sure the joint is well welded and tightly packed to avoid loosening and oxidation.A single and continuous signal channel is guaranteed as a bus.

6. What is common mode interference and differential mode interference? How to eliminate interference on the communication line?

The 485 communication line consists of two twisted pairs of wires that transmit signals through the voltage difference between the two communication lines, so it is called differential voltage transmission.

Differential mode interference is transmitted between two signal lines ,which belongs to symmetric interference. The method of eliminating differential mode interference is to add a bias resistor in the circuit and use a twisted pair.

Common mode interference is transmitted between the signal line and ground,which belongs to the asymmetric interference. Methods to eliminate common mode interference include:

(1) Use shielded twisted pair and effectively ground.

(2) The galvanized tube shield should also be considered in the strong electric field.

(3) Keep away from the high voltage line during wiring, and do not bundle the high voltage power line and signal line together.

(4) Do not share the same power supply with the electronic lock.

(5) Adopt linear stabilized voltage supply or high-quality switching power supply  (ripple interference is less than 50mV).

7. Under what circumstances should the terminal resistance be added to bus 485?

Under normal circumstances, it is not necessary to add the terminal resistance. Only when the 485 communication distance exceeds 100 meters,the terminal resistance should be added at the beginning and end of 485 communication.

8. How to extend the communication distance of 485?

One of the specifications of the 485 network is 1.2 km in length and the number of 32 nodes.If this limit is exceeded, a 485 repeater or 485 hub must be used to extend the network distance or number of nodes.

With a 485 repeater or 485 hub, a large 485 network can be divided into several network segments. A 485 repeater or 485 hub is like a "bridge" between the 485 network segments.Of course, each network segment still follows the above 485 specification, which is 1.2 kilometers in length and the number of 32 nodes.

The 485 hub is used to construct the star 485 network.The 485 hub is a broad extension of the 485 repeater concept.It not only solves the problem of multi-forking, but also solves the problem of isolation between network segments, that is, there is a problem in one network segment (such as short circuit, etc.), which does not affect other network segments, thereby greatly improving the security and stability of large networks.

We can understand the benefits of the star-shaped cabling network from the development of the LAN from the bus type to the star type. Similarly, the star 485 network composed of 485 concentrator will also be a direction for the development of the 485 network.

Principle and application scope of sewage open channel flow meter

Industrial wastewater discharge includes pressure piping and open channels, pressureless pipelines and other forms of non-pressure discharge.The sewage flow meter discharged in the form of a pressure pipe is convenient to measure, and can be measured by a pipe flow meter such as an electromagnetic type, a laser type, or a venturi differential pressure type. At present, the sewage discharge in China mainly adopts the form of discharge without pressure open channel, and the sewage open channel flow meter is designed to solve the problem of sewage measurement in this form of discharge.

1. Working principle and composition of open channel flow meter

Pasteur trough and thin-walled weir are commonly used as measurement methods for open channel sewage flow.Its principle belongs to the throttling flow measurement technology. The direct object of measurement is the water level difference between upstream and downstream of the throttling device. According to the water level difference, the instantaneous flow on the cross section can be calculated.

The open channel sewage flow meter is composed of a measuring weir (flow measuring flume), liquid level sensor and secondary instrument. The work of forming the head (standard flow), measuring the height of water level and calculating the flow rate are completed respectively.

2. Types and scope of weirs and troughs

Water-measuring weir

A weir is a device that intercepts the flow of water and allows it to overflow from the top. There are many kinds of weirs: According to the thickness, it can be divided into thin-walled weirs (whose thickness is less than 0.67 times the geometric head), suitable weirs and wide-crested weirs, etc.According to the cross-section shape, it can be divided into triangular weir, trapezoidal weir, equal-width weir, etc. According to the connection form of jet and downstream water level, it can be divided into submerged weir and non-submerged weir.Thin wall triangular weir, thin walled trapezoidal weir and thin wall rectangular weir are commonly used.Under certain conditions, for a certain thin-walled weir, the flow rate has a certain function relationship with the geometric head height, and the sewage flow can be measured accordingly. When measuring the flow with water weir, the loss of water head is large, which is suitable for measuring the sewage with little suspended substance and non-fibrous substance.

Flow-measuring flume

Wide-crested weir with lateral contraction is called flume when the height of weir bar is zero. It is a special deformation of weir. It can be divided into short throat flume (Pasteur flume), no throat flume (Sun flume) and long throat flume.Commonly used in open channel sewage flow metering is Pasteur trough.Under certain conditions, for a certain Pasteur trough,there is a certain functional relationship between the flow and the upstream water level height,according to which the sewage flow can be measured. When measuring flow with flow measuring trough, head loss is small and water quality conditions are wide. When measuring sewage, aquatic organisms on the trough wall should be cleaned up in time to keep the trough wall smooth.

3. Liquid level sensor

There are mainly ultrasonic liquid level sensors, pressure type liquid level sensors, capacitive liquid level sensors, float type liquid level sensors and so on.These level sensors have their own characteristics  and different application ranges.

Ultrasonic sensor is a specific application of ultrasonic ranging method. Its characteristics are to achieve non contact flow and wide application range, but the required liquid is reflected well by ultrasonic wave. If there is abnormal reflection of foam, debris and biological mud, it will bring greater error. In addition, ultrasound is also affected by weather factors such as temperature, air pressure and wind speed. Although these factors can be compensated, the structure of the instrument is complex and the cost is high.

The characteristic of pressure type liquid level sensor is simple installation. However, the specific gravity of the tested liquid is basically constant, so its application in sewage with large variation of specific gravity is limited. In addition, attention should be paid to the smooth opening of the pressure measuring hole when used in sewage.

The characteristic of capacitive hydraulic sensor is that there are no moving parts, but the mechanical strength and dielectric properties of insulating medium are difficult to take into account, and the change of capacitance caused by the influence of temperature reduces the measurement accuracy. In addition, the electrode surface should be cleaned up in time when used in sewage.

The float liquid level sensor has the characteristics of low cost, not affected by water quality and wide application range, but it requires that the specific gravity of water body is basically constant, and attention should be paid to the effect of surface adhesives on the specific gravity of float.

4.Secondary instrument

Secondary instrument is an automatic device which can receive the voltage (current) signal of liquid level sensor and accumulate sewage flow based on it. It can not only give instantaneous flow, but also get cumulative flow. It can connect the recorder to observe the change of flow rate and the computer to realize the automatic flow monitoring.

Alarm function attached to the meter

The alarm function attached to the meter is significantly different from the insurance function, because the circuit of the additional alarm function part has many commonalities with the display of the meter, the power supply and the sensor, etc. It is necessary to emphasize that the failure of a common place may directly lead to the loss of alarm function.Therefore, in the case of a system that will cause significant consequences in the event of loss of control, an independent protective device must be provided as required.

If you want to achieve a more reliable, insurance-type alarm function, you should use bimetal or pressure instruments for protection, or use a complete set of electronic alarms from sensors to actuators.The instrument dedicated to electronic alarms, due to its simple function and structure, is relatively less reliable than the more complex display adjustment instruments.Most display adjustment instruments can be equipped with an alarm function.

Additional alarm functions are mainly implemented in the following ways:

1. Unsettable upper limit follow-up alarm function

This function means “to make an alarm action when the input value reaches the set value + full-scale direction—fixed value”. Take the XMTA 2311 0~300℃ meter as an example.The standard stipulates that the input value reaches the set value + (4% ~ 6%)×300, the alarm action should be made.If the meter setting value is 200℃, then the meter should make an alarm action when the input value is +12℃ to +18℃+ the instrument allows the setting error (212 to 218) ± 3℃.The default alarm action is the normally open contact closure of the alarm output relay.

2. Unsettable lower limit follow-up alarm function

This function means “to make an alarm action when the input value reaches the set value—starting range direction—fixed value”. Take the XMTA 2311 0~300℃ meter as an example.The standard stipulates that the input value reaches the set value—(4%~6%)×300, the alarm action should be made. If the meter setting value is 200℃, then the meter should make an alarm action when the input value is -12℃~ -18℃+ the instrument allows the setting error (182 ~ 188)±3℃. The default alarm action is the normally open contact closure of the alarm output relay.

3. Alarm function that can set alarm value

This function refers to the alarm action when the input value reaches the value that people intentionally set on the surface board, regardless of the upper limit, upper limit, lower limit and lower alarm. The default alarm action is the normally open contact closure of the alarm output relay (eg smart meter).

4. The instrument can also be used as a sound alarm output (special order)

In order to improve the anti-interference ability as much as possible, the position difference of the alarm bit is selected to be larger. Generally it is about 1% to 5% of the full scale.

5. Three-bit meter for alarm

Under normal circumstances, it is not advisable to use one bit of the three-bit meters for an alarm.If the upper limit control of the three-bit meter is used as the upper limit alarm, the action of the alarm is that the normally closed contact is closed again, that is,the meter is in an alarm state immediately after power up,and the default alarm action will be reversed.This is different from a meter specifically designed for alarms. If the upper limit alarm of the three-bit meter is used as the lower limit alarm or the lower limit alarm is used as the upper limit alarm, it is possible to make an alarm action when the instrument is powered on.

Which industries are ultrasonic flow meters applied to?

The ultrasonic flow meter adopts high-precision and ultra-stable double-balanced signal differential transmission and differential receiving digital detection technology to measure the sound wave transmission time in the forward and reverse flow directions, calculate the flow velocity according to the time difference, and has the characteristics of good stability, small zero drift, high measurement accuracy, wide range ratio and high anti-interference. At present, ultrasonic flow meters are mainly used in the following industries:

1. Petroleum industry:Mainly used for measuring medium such as high pressure water injection,circulating water, oil-water mixture, domestic water and sewage.

2. Metallurgical industry: Mainly used for measuring medium such as soft water for continuous casting, secondary cooling water for continuous casting, water for steelmaking equipment, oxygen lance water for converter, dust removal water for converter, vaporized soft water, semi-purified water, circulating water, domestic water and sewage.

3. Petrochemical industry: Mainly used for measuring medium such as production water, domestic water, circulating water, fire water and sewage.

4. Electric power industry: thermal power generation, hydroelectric power generation, nuclear power, mainly used for industrial water, raw water, intermediate water, circulating water, cooling water, boiler water, ash water, domestic sewage, chemical water and other media measurement.

5. Chemical Industry: Mainly used for the measurement of circulating water, chemical water, chemical slurry, drinking water, sewage and other media.

6. Coal mine/mine: Mainly used for groundwater, coal washing water, circulating water, sewage, etc.

7. Paper making industry: Mainly used for the measurement of circulating water, industrial wastewater, domestic water, domestic sewage and other media.

8. Water treatment industry:Mainly used for the measurement of domestic water and sewage.

9. Building construction: Mainly used for the measurement of circulating water in air conditioning.

10. Food and medicine: Mainly used for measuring small pipe runoff of ultrapure water, circulating water, drinking water, domestic sewage,industrial wastewater and other media.

11. Other industries: There are also a large number of applications in ultrasonic energy flow meters in energy-saving testing centers, water resources management, water supply and drainage associations, environmental protection departments, cigarette factories, fire protection, measurement and testing, on-site counterparts or departments.