What signals are transmitted to the DCS by differential pressure instruments (such as differential pressure level transmitters and densitometers)? Is it a pressure signal or...
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Hello everyone, I have a question that’s confusing me, and I hope you can help answer it. What values are transmitted to the DCS by differential pressure instruments, such as differential pressure level transmitters and differential pressure densitometers? Is it a pressure value or liquid level and density values? My colleague said it’s a pressure value; the liquid level and density are both calculated within the DCS. If both the liquid level and density are calculated by the DCS, and some instruments need to display the values locally, does the DCS still need to send the calculated values back to the field transmitters? If the DCS does not transmit the calculated values to the field transmitters, then how do the values displayed on those field transmitters come about? Please help me figure this out; thank you all!
This post was last edited by Ku Liang Zhi Le on 2015-7-2 at 15:45. The basic principle of measurement transmitters is to convert the measured variable from 0~100% into a signal value within the 0~100% range. For example, in the case of 4~20mA signals, or when using differential pressure transmitters to measure liquid level, the 0~100% range represents the liquid level itself, while the 0~100% range also corresponds to the differential pressure associated with the liquid level. Once these signals are sent to the DCS, it is necessary to define the variables first; the liquid level defined in this way represents the actual liquid level, and the differential pressure defined in this way represents the actual differential pressure. The DCS cannot send signals back to the transmitter.
The measurement principle of differential pressure instruments is that there is a certain relationship between the parameters of the medium being measured (level and density) and the differential pressure. The differential pressure transmitter measures this differential pressure and converts it into a 4–20 mA signal, which is then sent to the display instrument or DCS. The display instrument or DCS uses corresponding formulas to calculate the relevant parameters of the medium being measured and displays them. If display on-site is required, an additional local instrument can be installed, or a transmitter with a local display can be used (the value displayed by the transmitter is shown by the transmitter’s own circuitry, and it is not transmitted to it by a DCS)
The transmitter converts the parameter to be measured into a 4–20mA current signal, which is sent to the DCS; by setting the same unit as that of the parameter to be measured in the DCS, the value of that parameter can be displayed.
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The transmitter processes the differential pressure signal to convert it into a 4–20 mA current, which is then sent to the DCS
Regardless of the type of instrument, the signals received by the DCS are generally DC signals; of course, there are also other types such as pulses! Where are the density signals?
This post was last edited by jiaguoyun on 2015-7-3 at 15:39. The transmitter sends electrical signals to the DCS – either current (4~20mA or voltage 1~5V; currently, current is more commonly used). As for what should be displayed after the signal reaches the DCS, that’s up to you; of course, it should display whatever is being measured. The range and units must match those of the instruments on-site. Shouldn’t the original poster review ρgh again?
The current mainstream instrument output types are 4-20mA/Hart, that is, an output current value. The DCS has corresponding range settings, such as (0-100) kPa. If the instrument measures a pressure of 0 kPa, the output will be 4 mA; after conversion by the DCS card, this value is displayed as 0 kPa in the DCS. Similarly, if the instrument measures a pressure of 100 kPa, the output will be 20 mA, and after conversion by the DCS card, this value is displayed as 100 kPa in the DCS. The different pressure values measured by the instrument are converted into current values by the instrument itself and sent to the DCS. The DCS cards then convert these values, and based on the range settings defined in the DCS configuration, they transform them into the numbers displayed on the DCS screen. There are specific formulas that can be used for calculation. The numbers displayed locally on the instrument are processed by the instrument’s own module