Differential (or not) flowmeter?
Differential (or not) flowmeter?
(OP)
I am trying to simulate the use of a pressure differential flowmeter with the gas air. The flowmeter has an upstream pressure tapping, a variable orifice and another pressure tapping downstream. The flow goes off to atmosphere. This is not a pressurised flow. Tests to back up my simulatio show that the downstream pressure tapping shows virtually zero, particularly at low flows. Therefore it would appear that its possible to charactetrise the flow simply by the upstream pressure gradient. Is it possible to remove the downstream pressure tappng in this situation? What are the implications of doing this? Thanks in advance for any advice.





RE: Differential (or not) flowmeter?
People do this all the time and many are not very careful about the downstream friction and checking for choked flow. They don't get very good numbers, but they still use them.
David
RE: Differential (or not) flowmeter?
>
>I have an accurate inline flowmeter upstream of the upstream pressure
>tapping. After the upstream pressure tapping going downstream is the
>'choke' which is a movable element. The moving element aims to give a
>more linear flow - pressure characteristic. Downstream of the 'choke'
>is the downstream pressure tapping. I am measuring flows up to 100
>L/min with a low differential pressure.
>I am using the accurate inline flowmeter to 'calibrate' the measured
>flow to the mbar(g) pressure measured at the upstream pressure tapping
>only. Although I could calibrate the accurate measured flow to the
>differential pressure across the 'choke' I am trying to understand why
>I cannot simply use the mbar(g) upstream. As an example, the pressure
>levels at say 50 L/min air flow at 20C and local pressure 1013mbar
>would be:
>Upstream: 3 mbar(g)
>Downstream: 0.2 mbar(g).
>I appear to get good repeatabilty by simply using the upstream
>pressure. However, its clear the flow is not choked by comparing the
>ratio of absolute pressures upstream and downstream. The upstream
>pressure is fed to a differential pressure sensor a distance away from
>the flow measurement point.
>
>Therefore I have 2 main questions:
>1) As the downstream pressure is very low compared to the upstream
>pressure, can it be ignored? What are the implications of ignoring it?
>2) Without the downstream pressure tapping connected to the remote
>pressure sensor, will temperature / humidity variations between the 2
>locations lead to inconsistencies in the readings?
>
>Many thanks for any advice offered.
RE: Differential (or not) flowmeter?
RE: Differential (or not) flowmeter?
Well, let's take a stab at it, using your numbers.
The measured dP between the taps is 2.8 mbar. If you ignore the downstream tap, the measured dP would be 3.0 mbar, a 0.2 mbar error. The flow rate calculated as roughly proportional to the square root of dP, so the ratio of the reported flow rates would be equal to the root of (3.0/2.8), or about 1.035. So, ignoring the downstream tap would give you about a 3.5% error in flow reading (high).
RE: Differential (or not) flowmeter?
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: Differential (or not) flowmeter?
I am measuring a single pressure, the upstream pressure and feeding this pressure to the +ve side of a pressure transducer. I am then measuring the flow independently using an 'off the shelf' thermal inline flowmeter positioned upstream of the pressure tapping and plotting actual flow vs upstream pressure. This then gives me a very accurate linear relationship between just the upstream pressure and actual measured flow.
The downstream pressure is, in effect, a constant zero as it is the -ve side of the pressure transducer.
I can then use this relationship to predict the flow at different flow rates.
This is why I was querying the use of the downstream pressure tapping.
How would a temperature / humidity difference between the actual location of the upstream pressure tapping and the -ve side of the pressure transducer located 2m away affect the stability / error?
With thanks
RE: Differential (or not) flowmeter?
You mean an external change (outside the pipe) or internal? For external, look at the data sheet for your transducer(s). For internal, dunno, what is the mechanism causing that change?