Uncertainties and their estimated maximum errors in field measurements using Pitot tube for air flowrate calculations

Introductory

Air balancing, trouble-shooting, and system evaluation in dust collection and industrial ventilation systems all require accurate measurements of air flowrates.

Normally, the Pitot tube and magnehelic gauge (or manometer) are used to measure the velocity pressure and static pressure inside of a duct (round or rectangular), and then the air flowrates are calculated based on the measured values.

This article talks about 5 kinds of uncertainties that always happen in field measurements using Pitot tube, the estimated maximum errors with some of them, and how to avoid some uncertainties with recommendations from manuals and standards (ISO 3966 and NF X 10-113).

 

Uncertainties, maximum errors, and how to avoid some of them

In addition to the inherent errors from Pitot tube and gauges themselves (calibration, resolution, drift, etc., sometimes with the correction factor of Pitot tube involved), there are other errors we have to pay attention to in the process of measurements.

• The uncertainty in the positioning of the Pitot tube inside the duct. This component can be significant in some cases, so precautions must be taken to minimize this uncertainty component.

a. The stem diameter of the Pitot tube should not exceed 1/30 (1/20 from another source) of the test duct diameter.

   If the stem diameter exceeds 1/30 of the test duct diameter alone, it will introduce at least 0.1% increase of the air flowrate, while 1/20 will introduce at least 0.25% increase of the air flowrate.

b. The misalignment of the probe end with airflow should not be more than 5 degrees. When the Pitot tube is correctly aligned, the pressure indication will be at a maximum.

    If the misalignment of the probe end with airflow is more than 5 degrees, it will introduce at least 0.8% decrease of air flowrate.

So, the air flowrate measurement is more sensitive to misalignment than to the stem diameter of the Pitot tube.

c. A complete and accurate Traverse has to be made. The Pitot tube should move back and forth in line up with the diameter line of the duct.

 

• The uncertainty in picking traverse position.

a. Some manuals state that the readings should be taken 7.5 duct diameters or more downstream and 2 duct diameters or more upstream from any major air disturbance, such as an elbow, hood, transition, takeoff, damper, or branch entry.

  Most of the time, even this recommendation is difficult to apply for measurements on site, because conditions of straight lengths are often not available, and because it’s not always possible due to reasons of size, space, set-up cost etc.

b. ISO 3966 and NF X 10-113 standards recommend the minimum number of points and minimum distance away from disturbances required for circular and rectangular ducts. However, practical ventilation systems rarely have straight lengths able to comply with the conditions described in the standards.

c. An empirical formula was developed to evaluate the maximum error accompanied with the uncertainty of traverse position.

For example, on a 12” diameter duct, the table below shows clearly that, if conditions of straight lengths are not available, then two or more traverses and more measuring points on each traverse can be applied as kind of a compensation to reduce error in the measurements.

In the table:

k is traverse number,

p is measuring point on each traverse,

L is distance between the traverse position to the disturbance upstream, and,

D is the diameter of the duct.

k	p	L/D	Max Error
1	8	7.5	4%
2	8	7.5	1.9%
1	8	20	2%
2	8	20	1%
1	8	4	6%
1	10	4	5%
2	8	4	3%
2	8	2	4.9%

 

• The uncertainty caused by not knowing the humidity in the air flow. Most of the time, humidity is not measured on site, and air flowrate calculations are simply based on the assumption of dry air.

a. For example, saturated air at 135 F, elevation of 719ft, and static pressure of -15” H2Og, if assumed as dry air, then the calculated air flowrate will be about 1.2% lower than the real value.

b. The higher the air flow temperature, the lower the calculated flowrate of the saturated air than the real value.

• The uncertainty of knowledge of the inner duct section

a. Blockages, dirt, ruptures, etc. inside the duct also contribute to wrong Velocity Pressure measurement. VP is a vital parameter and most essential input for proper flow computation.

• The air flowrate measurement scheme proposed in standards is often time-consuming and field technicians prefer to simplify them.

a. Measure only one traverse (most of time, with rotation angle and tilt angle, sometimes not in line up with the diameter line of the duct).

b. Move the Pitot tube inch by inch to get readings without consideration of duct diameters, sometimes, only measure the center velocity pressure.

c. Insert the flexible hose end into the hole to measure static pressure, and this is definitely wrong.

d. Pick a traverse location by experience, sometimes, at random.

In these cases, measurement errors occur, and the estimation of measurement error is not known.

Air Balancing, Trouble-shooting, and System Evaluation services by Airvate

Due to the importance of field measurements using the Pitot tube, it’s important to spend time accurately measuring with it. Airvate LLC offers reliable Pitot tube measuring and also the following services, so feel free to email Airvate at info@airvate.com to get the help you need:

·  Dust collection system and industrial ventilation system testing and Air Balancing

·   Dye test for bag leak

·  Technical services: field survey and dust collection system evaluation and trouble-shooting

·   New system start-up

·   Dust sample analysis

·  Dust collector selection and sizing: baghouse, wet scrubber, cyclone, settling chamber, drop-out box, etc.