What to do if the medium density of a differential pressure flow meter changes?
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The last edit to this post was made by FightingNow on 2015-6-2 at 22:13. 1. For the orifice flow meters that we commonly use, since only the differential pressure is measured on-site, it is necessary to convert this value into an electrical signal for use in the control system; the control system then converts this signal into a flow rate value. Is the setting of the medium’s density also done within the control system? Does our on-site transmitter only measure the differential pressure, while everything else needs to be handled within the control system? Is that so? What if the medium density changes? ....The field instruments actually have nothing to do with the density of the medium; density is only needed in the control system when converting differential pressure into mass flow rate 2. Also, apart from rotameters, do all positive displacement flowmeters have nothing to do with the density of the medium, and is density only used when converting it to mass flow rate?
For differential pressure, density shouldn’t be a factor, I’m not sure either, but no density setting was visible in the computer
When using an orifice plate to measure flow rate, temperature and pressure compensation is required; in essence, this is to compensate for changes in the density of the fluid being measured.
If volumetric flow rate is displayed, density is not required; but if mass flow rate is output, density must be taken into account. When the mass of the fluid medium remains constant, its density changes as temperature and pressure change. In this case, the value of the mass flow rate output will change, and this constitutes inaccurate measurement. However, when using orifice plates to measure mass flow rate, temperature and pressure compensation is generally applied, with the density being adjusted accordingly based on these values. This ensures that the measurement value does not change with variations in density.
Orifice flow meter: the differential pressure generated by the orifice represents the flow rate. Moreover, the orifice plate measures mass flow rate, which is converted into other types of flow rates as needed in practice. However, the mass flow rate measured by a orifice flow meter is obtained under the assumption that temperature, pressure, viscosity (Reynolds number), and enthalpy in thermodynamics are all constant. Changes in these parameters will affect the measurement results. Therefore, orifice plate flowmeters are all designed (calculated) directly based on the operating conditions. When the operating conditions change, and if the difference is significant, the orifice plate should be recalculated ； If the changes are minor and it is possible to deduce the changes in other parameters from the changing parameter, the measurement results can be processed mathematically, such as by performing temperature and pressure compensation. Under normal operating conditions, no special calculations are required for orifice plate flowmeters. For the reasons, please refer to the link: http://bbs.hcbbs.com/thread-1290852-1-1.html. The direct parameter measured by a volumetric flow meter is the volumetric flow rate; density is used when converting it to a mass flow rate. A rotameter is not a volumetric flowmeter. (It is a area-type flow meter)
Thank you, you understand what I meant by my question! ...So that’s how orifice plate flow meters work!
For differential pressure, pipe density is certainly necessary; it is used in the calculations during design. Different densities mean different media. As for the flow meters used to measure volume flow, aside from float flow meters which need to take density into account, the others indeed do not require considering density
Hello, I found your reply very helpful. There is one question I haven’t understood, so I would like to ask for clarification. According to you, a differential pressure flow meter actually measures mass flow rate, but what is obtained in the calculation is ρV2, that is, density multiplied by the square of the volume flow rate – an expression that contains unknowns. It is actually mass flow rate multiplied by volume flow rate; density is also required to obtain the mass flow rate. Then why can it be said that a differential pressure flow meter measures mass flow rate? (Although in general, during measurement operations, the volume flow rate, or more precisely the velocity flow rate, is calculated by first determining a fixed medium density or by using temperature and pressure compensation to obtain the compensated density.)
Agree with what was said on the fifth floor. If there are no significant changes, let’s go with it. If the changes are significant, it might be advisable to consider performing temperature and pressure compensation using a DCS. Additionally, there are now transmitters that have built-in temperature and pressure compensation, allowing them to output the compensated mass flow rate directly. For example, Rosemount’s multi-parameter flow transmitters.
With an orifice plate, it definitely measures the volumetric flow rate; density is not needed. Calculating mass would result in too large errors
The density of a liquid should be fairly similar to the density specified in your design; whereas the density of a gas is greatly affected by temperature and pressure, so temperature and pressure compensation is necessary.