Should volume flow rate measurements be corrected for different conditions?

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Discussion Overview

The discussion revolves around the measurement of volume flow rate using a laminar flow element and whether corrections are necessary when operating under different conditions than those used for calibration. The scope includes technical considerations related to calibration, measurement accuracy, and the impact of varying environmental conditions on flow rate readings.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions if the calibration law for the laminar flow element was obtained at standard conditions, suggesting that it may need correction for different conditions.
  • Another participant asserts that calibration is usually done for a range of operating states, implying that corrections may be necessary depending on the specific conditions encountered.
  • A participant emphasizes that the density of air is a significant factor affecting airflow measurements and suggests that the need for correction depends on the magnitude of the error.
  • One participant mentions that even small variations in conditions may require corrections for accurate measurements, especially when high precision is needed.
  • A participant proposes a formula for correcting air volume flow rate based on the inlet pressure ratio, indicating a specific approach to addressing the issue.
  • Another participant warns about the importance of defining "Standard Conditions," sharing an anecdote about how a slight temperature change affected a heat exchanger design.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and extent of corrections for varying conditions, indicating that multiple competing perspectives remain unresolved.

Contextual Notes

There are limitations regarding the definitions of standard conditions and the specific calibration conditions used, as well as the potential for significant errors depending on the application and measurement precision required.

jenSG
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Hi
I have a question about measuring volume flow rate with a laminar flow element.
I read the pressure difference across the element (mmH2o) and I get volume flow rate (cfm) from a calibration law. If that calibration law was obtained at standard conditions, would I have to correct it for different conditions? Thanks!
 
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Not that I don't believe you, but I find it hard to believe that the unit was calibrated at standard conditions. The calibration was probably referenced to standard conditions. You might want to check the documentation with your unit. The ones we have have correction factors for calculating the actual flow for non standard conditions.
 
calb law is not at standard coditions ;-) I meant calibrated at certain conditions and then used at others.
 
It depends. Usually if you calibrate a measurement device its for a range of operating states or measurements. Otherwise there wouldn't be much of a point in calibrating it would there? What kind of change in conditions are you referring too? Pressure, temperature, less gravity, what? Are you talking about a wind tunnel or flow venturi or something?
 
A laminar flow element is a type of air flow meter. They are sometimes used as calibration masters.
 
The biggest thing that typically affects airflow measurements is the density of the air. But what determines if you need to make a correction is how big the error is. If it is less than, say, 5%, then it is probably inside the error margin and repeatability of the device and system.
 
That's one of the things about these particular instruments. They are often used as calibration standards. The 5% number is huge. I have one on my desk right now that measures up to 150 scfm. It's not much but it does require corrections even for small variations if you need that level of accuracy. These guys are well beyond pitot tubes.
 
thanks for replying.
Should I correct the air volume flow rate for the inlet pressure ratio? I mean, if the calib. law is
Q [cfm] = (calib. coeff.) * Delta_h [mmH2O]
should it be Q [cfm] = (calib. coeff.) * Delta_h [mmH2O]* P_upstream/P_atm ?
 
The equations for many properties of air as a function of temperature can be found in ASTM C-680
 
  • #10
Also, be very careful with "Standard Conditions" my "Standard" for air properties is 60 F but with a slight change of 8 degrees I almost lost my whole heat exchanger design once.
 

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