The panel shows no pressure or flow indication, so a running fan tells you nothing about the actual conveying condition. Install permanent static-pressure taps at component boundaries, read them with one portable draft-range differential gauge, and use a pitot traverse in the cleanest suitable duct section to determine gas velocity. Repeat the test at each powder and flow condition to build separate pressure profiles and system curves.
Read the symptoms first
Start here. Decide whether you need a pressure profile, a component pressure drop, gas velocity, or all three. They require different connections even when one portable instrument supplies every reading.
| Symptom | Likely cause | First check |
|---|---|---|
| Pressure stays near zero at every fan condition | Blocked tap, leaking hose, incorrect gauge port, or unsuitable range | Disconnect the hose and prove the tap and gauge independently |
| Pressure changes sharply across a cyclone or filter | Real component loss, material buildup, loaded filter bags, or poor tap placement | Inspect both taps and repeat the differential reading under stable operation |
| Pitot velocity is unstable across the duct | Swirl, pulsation, nonuniform flow, or a partially plugged probe | Inspect the probe and compare readings across the full traverse |
| Calculated flow disagrees with the pressure profile | Wrong air density, incorrect duct area, or confusion between static and velocity pressure | Check the pressure type, local dimensions, temperature, and absolute pressure |
A flat reading at a dusty tap often means blockage. That is not proof that the duct has no pressure. Clear and verify the measurement path before changing the fan, filter, or process settings.
Separate static pressure from velocity pressure
A wall tap measures static pressure relative to the gauge reference. Two taps connected to opposite ports of a differential gauge measure the pressure difference across a cyclone, filter, pipe section, or other element.
A pitot tube measures total pressure at its forward-facing opening. Pair that value with static pressure at the same section. Their difference is velocity pressure:
q = P_total - P_static
Calculate local gas velocity from:
v = C_p × sqrt(2q / ρ)
Here, v is gas velocity, C_p is the probe coefficient, q is velocity pressure, and ρ is actual gas density. Use compatible units throughout. Obtain the coefficient from the pitot documentation and determine density from the air temperature and absolute pressure at the measurement section.
The result is air velocity, not particle velocity. Slip between powder and air, particle concentration, and disturbed flow can make a gas-only relationship a poor representation of solids motion. If loading materially affects the readings, develop a separate measured curve for each powder and feed condition.
Install the pressure points
Fit permanent test ports even if the budget allows only one gauge. You can move a portable instrument between ports; you do not need a dedicated gauge at every location.
- Place a flush static tap before and after each cyclone, filter bank, major pipe section, and the fan.
- Keep the tap opening flush with the internal wall. A protruding fitting can sense velocity effects instead of true static pressure.
- Avoid elbows, branches, dampers, transitions, and strongly swirling fan connections when selecting measurement planes.
- Where the profile may be asymmetric, compare taps around the duct circumference before accepting one point as representative.
- Label every port and hose so the positive and negative sides of the gauge cannot be reversed accidentally.
The fan is in the middle of the system, so expect suction-side pressure below the local atmospheric reference and discharge-side pressure above it. Select a gauge configuration that can read the required direction and range without being over-ranged.
For a low-cost temporary setup, a draft-range gauge such as a Dwyer Magnehelic is a candidate. Match its range, pressure compatibility, and connection arrangement to the expected service. A transmitter costs more but becomes useful when you need continuous logging rather than manual test points.
Measure pressure and flow
- Define one operating condition. Record the powder, feed condition, fan setting, equipment configuration, filter condition, and any damper positions. Hold them steady during the reading set.
- Zero the gauge. Equalize both pressure ports at the same reference pressure. Check hoses for cracks, loose fittings, liquid, and deposited powder.
- Read the static profile. Connect one gauge port to each tap in sequence and keep the other at the chosen reference. Record the sign as well as the magnitude.
- Read component differentials. Connect the upstream tap to one port and the downstream tap to the other. If the displayed sign is reversed, correct the connection or record the signed result consistently.
- Select the pitot location. Use the cleanest accessible straight section with the most stable profile. A dusty location can plug the total-pressure or static openings and create a false low or drifting result.
- Traverse the duct. Take readings at multiple positions across the cross-section. Convert each velocity pressure to local velocity using local density and the documented probe coefficient.
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Calculate volume flow. Average the traverse according to the applicable method, then calculate
Q = A × v_average. Use the measured internal duct area, not a nominal outside dimension. - Repeat the test. Change one planned operating variable, allow the process to stabilize, and collect another complete pressure-and-flow set.
Do not estimate cyclone or filter loss from adjacent pipe friction. Measure directly across each device. Its loss changes with flow, powder loading, buildup, and filter condition.
Verify the system curves
Plot system pressure requirement against measured volume flow for each fixed process configuration. Treat each powder or materially different feed condition as a separate data series. Mixing them into one curve hides the effect you are trying to measure.
- Check that passive elements lose pressure in the direction of flow.
- Check that the fan produces a pressure rise between its inlet and outlet measurement points.
- Repeat at least one operating point. A repeat that moves beyond normal instrument resolution indicates process drift, fouling, or a measurement problem.
- Compare the integrated component losses with the end-to-end pressure difference. A large mismatch directs you to omitted sections, leaking hoses, blocked taps, unstable operation, or incompatible pressure references.
- Review the traverse profile. One centerline reading is not a valid average when powder, elbows, or fan swirl distort the flow.
If duct area or gas density changes substantially between sections, calculate velocity separately at each location. Volume flow and velocity are not interchangeable across changing areas or density conditions.
Avoid the recurring measurement traps
Adding transmitters everywhere wastes money before the tap arrangement has been proven. Start with permanent ports and one verified portable gauge. Upgrade only the points that need continuous monitoring or control.
Changing fan settings because one tap reads zero also wastes time. Prove the opening, hose, gauge zero, range, and port orientation first. Clean a pitot tube after exposure to powder and inspect every pressure opening before accepting the next traverse.
Do not combine static pressure from one location with total pressure from a different flow regime. Velocity pressure must represent the same measurement section. Do not convert pressure to velocity until you have identified the pressure type and actual gas density.
Keep the raw signed readings. Prematurely converting every value to an absolute pressure loss can conceal a reversed hose, a fan pressure rise, or a wrong reference connection.
FAQ
Can I measure the whole system with one pressure gauge?
Yes. Install labeled static-pressure ports throughout the system and move one portable differential gauge between them. Use paired upstream and downstream ports when measuring a component pressure drop.
Does a pitot tube measure powder velocity?
No. The pitot differential provides gas velocity through v = C_p × sqrt(2q / ρ). Particle velocity can differ from gas velocity because of slip and solids loading.
Can I use one pitot reading at the duct center?
Not when you need defensible volume flow. Traverse multiple positions and calculate the cross-sectional average because elbows, the fan, and powder distribution can produce a nonuniform profile.
Can I continue testing when the readings do not reconcile?
No; first prove the taps, hoses, gauge zero, pressure references, pitot openings, duct area, and density calculation. Stop when readings remain unstable, exceed the instrument range, or indicate an equipment condition outside the approved operating data. Escalate unresolved fan, cyclone, filter, or instrument behavior to the relevant manufacturer’s official technical support.