Flow transmitter issues

1# jjlhero: The flow rate measured by the flow meter, after passing through the field flow transmitter, is converted into a standard value of 4~20MA.

The flow rate measured by the flowmeter after passing through the field flow transmitter is merely a differential pressure value; based on the square relationship between flow rate and differential pressure, this value is converted into the actual flow rate using the DCS module. After the field transmitter linearizes the flow rate, further linearization of the flow rate by the DCS module has no effect on it.

What was said on the 3rd floor is absolutely correct. A differential pressure flow transmitter measures a differential pressure value. Based on the quadratic relationship between flow rate and differential pressure, if this differential pressure value is taken to the square root by the transmitter, it is not necessary to take it to the square root during DCS configuration; the flow rate can be calculated directly, or it can be connected directly to a secondary display instrument for indication ； If the flow transmitter does not perform square root calculation, the DCS module must carry out this square root conversion to obtain the actual flow rate, or the flow value must be converted through a flow indicator. After the field transmitter linearizes the flow rate, further linearization of the flow rate by the DCS module has no effect on it.

It can be said that the flow rate measured by the orifice plate flow meter, after passing through the field flow transmitter, is the standard flow rate, which is then converted into the actual flow rate by the DCS. The DCS module’s linearization of flow rate has no impact on the display of flow rate

I think there’s a misunderstanding on your part. I know that what was sent to the site was a value in the 4–20 MA range, but I’m not sure what standard this refers to – whether it’s related to the pressure difference across the orifice plate or to the actual flow rate. Let me give you a simple example to illustrate this. Suppose I measure nitrogen with a pressure of 0.8 MPa and a temperature of 30 degrees. I want the DCS to display the flow rate in standard units; the orifice plate indicates a pressure difference of 0.8 MPa at 30 degrees. Where is this conversion from the actual flow rate to the standard flow rate carried out? Is it a transmitter or a DCS block? I checked the DCS configuration; from the transmitter to the data acquisition unit, it is a flow signal, rather than the pressure difference signal mentioned above. I work in the field of manufacturing; I think this shouldn’t be a problem for those who study instrumentation. Thank you!

How to convert a differential pressure signal into an electrical signal – that’s a knowledge point, haha. The conversion is usually carried out at the conversion section of the instrument, that is, at the gauge on site; this gauge functions like a miniature calculator, though with more features. Some intelligent models also come with temperature and pressure compensation; they generate standard current signals that correspond to the actual flow rate. If there is no temperature and pressure compensation, temperature and pressure compensation calculations must be performed at the DCS to obtain the actual flow rate.

. . . After talking for so long, what you’re actually asking is where the differential pressure signal is converted into a flow rate signal, right? Well, it’s converted inside the instrument itself; by the time it becomes a 4–20 MA signal sent to the DCS, it’s already a flow rate signal – it just may or may not include temperature and pressure compensation

Before purchasing or using a flow meter or flow transmitter, it is necessary to understand its working principle～～～ The manufacturer should provide the final calibration table, from which it is possible to determine the relationship between the actual flow rate and the milliampere current or voltage output~~~

Why aren’t you answering people’s questions earlier? ? 1. What is the flow rate of the flowmeter after passing through the field flow transmitter? Is it the actual flow rate or has it been converted to a standard flow rate? ? ? If it’s a transmitter, it can directly convert the value into mass – that’s one of its functions. However, it depends on whether the transmitter has temperature and pressure compensation; if it does, the flow rate indicated will be the weight under standard conditions. This applies to liquids as well, while for gases, the value is usually given in terms of mass. In contrast, rotameters generally provide volumetric flow rates, and the values displayed are also based on standard conditions, that is, they represent the flow rate adjusted to those standard conditions. 2. I know that on-site flow transmitters have a square-root function, but I’m not sure if they also have a conversion function ? ? Modern instruments definitely have conversion functions; they are quite advanced these days, and most of them can be used for conversion (such as flow transmitters). 3. After the field transmitter linearizes the flow rate, does the DCS module’s further linearization of the flow rate affect its display? Theoretically, there shouldn’t be any impact, but some people say there is. What do you all think? What’s displayed on this DCS actually has an impact, in theory, on the scanning time as well. Is there also a time element in DCS data collection? The impact is minimal, though. For flow rates that need to be accumulated, if the DCS performs this accumulation and the field instruments also have a function for displaying cumulative values, you will find that the two figures differ. The value shown on the field instruments represents the instantaneous cumulative total of all flow rates, while the value on the DCS comes from each individual measurement. Considering the scanning time and frequency, the two values are bound to be different; the field instrument readings should be taken as the authoritative source. You can give it a try if you don’t believe me!!

I learned a lot; thanks to everyone for sharing! ! ! :)