As we move into the automation field we find the need for equipment that can transmit data remotely at high speeds, devices that can be installed in the process whose information can be monitored remotely from a control room. In this way, operators avoid being exposed to risks in the plants. In the places where I have worked there are high levels of both electrical and pneumatic energy, these equipment or instruments are installed during the construction phase of the plant (when there are no pressures, voltages or temperatures present) and require little maintenance, from now on we will call transmitters to any equipment or electronic device capable of receiving a signal from a sensor element in raw form and through a transducer convert it into an electronic signal that complies with some standard communication protocol and then send it to large distances either wired or wireless.
There are many manufacturers of these devices, the most commonly used being those of the ROSEMOUNT brand, are those that can be seen in the photograph in which the left is a temperature transmitter mounted on a resistance whose magnitude in ohms varies with the change in temperature, this variation of resistance is interpreted by the transmitter who according to a previous configuration can interpret and transmit the magnitude of the temperature present, additionally have an LCD display for local indication. The transmitter on the right, although it is similar to the previous one, is a device in charge of measuring the variable pressure. It is mounted on a valve used in cases where it is necessary to disassemble the previously depressurized equipment. Inside it has a camera with a diaphragm that is the pressure sensing element and a transducer capable of converting the pressure variation into current variation, it also has a LCD screen for local indication, the main characteristic of the transmitters is the ability to send signals over long distances, signals that are called analog because they are an analog current of the variable you want to monitor taking different values over time as it does this magnitude.
In this image we can see a level transmitter, these have different sensor elements that go from diaphragms to radars, the one we see in the image uses diaphragms like the pressure one, in fact the direct variable that measures is the pressure but
through the pressure formula a clearance is made to calculate height where the height represents the level. These must be installed in the lower part of the tank where the maximum pressure point exists, if it is a closed tank where gases and vapors can be stored, it is necessary to place an inlet in the upper part to subtract the pressure generated by them and thus avoid an erroneous measurement, in these cases they are called pressure transmitters. The operating principle is the same as described in the previous transmitters!
And to complete the four fundamental variables measured and controlled in an industrial system I present the flow transmitter, this is DANIEL brand and works with ultra sound, it is installed directly in the line as you can see and consists of 3 pairs of sensors, each sensor is arranged in front of its partner and sound signals are sent which vary in the presence of flow, based on these variations the device makes the calculations to determine the amount of flow present.
There may be a need in the future to create an article dedicated to each device to delve deeper into them, but for the purposes of this article I just want to present it and describe the general functioning that governs them all.
Analogue signal and operating range
In data transmission the only thing that changes in all these devices is the primary element (sensor) and the transducer due to the nature of the magnitude to be measured and then converted. The operating principle is basically the same, these devices must transmit a standardized electronic signal that varies from 4 to 20 mA and is proportional to the calibration range of the equipment. At this point it is necessary to clarify that the range is what indicates the limits to which the device can operate, if for example we have a pressure transmitter with a range of 0 to 1000 PSI we cannot use it in processes whose pressures are higher otherwise the equipment would suffer damage and possibly the operator.
The calibration range of the device determines the equivalent 4-20mA signal levels at which it will operate. Generally speaking, if a pressure transmitter has an operating range of 0 to 1000 PSI, it can be calibrated to operate within this range, i.e. it can be set so that at a minimum pressure of 100 PSI it sends 4mA representing 0% of the variable and at 200 PSI it sends 20mA representing 100%, for intermediate values between 100 and 200 PSI there will be intermediate values between 4 and 20mA, the calibration ranges may vary as long as they do not exceed the maximum pressure supported by the equipment.
In addition, these devices have a communication protocol that can be hart, fieldbus or profibus that allows all of them to be connected in parallel without their signals interfering with each other. These protocols also determine the intelligent operation of the equipment and the way in which it must be installed and configured, the information on this matter is extensive and I will dedicate an article in the future to explain each one, although a lot of information can be found in the official pages of each protocol.
In the back of the LCD of these devices we will find a cover that covers the connection points in case of not being wireless, we can notice that there are only three points because they are intelligent devices based on communication protocols, only a couple of cables is enough to power them and by the same power cable the information is transmitted, the third connection point is a test point in which the output of 4-20 mA can be measured. The power points are used to connect the calibration equipment that allow us to configure these devices for their correct operation.
These devices can be configured by computer using software and hardware designed for this purpose or by means of calibration equipment, in the following image are shown the most common that are the Emerson 475 that is related to the majority of the hart and fieldbus devices and the fluke 754 that besides the functions of measurement of any multimeter has advanced functions to generate useful process signals to test and simulate the devices, it has function of power supply and communication hart. These devices are used to adjust the operating range of each instrument as well as parameters related to intelligent identification and response.
This concludes my delivery today by thanking you in advance for all your support. All the images are my own, I allow their reproduction as long as they are mentioned to me when using them and the source link from where you have copied them is quoted.
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