What You'll Learn

• What a calibrated nozzle is and the measurement principle behind it
• When to use nozzle testing vs. TrueFlow vs. duct traverse
• Equipment required for nozzle testing
• Step-by-step setup and measurement procedure
• How to read and record nozzle test results in measureQuick
• Accuracy advantages and practical limitations

What You'll Need

• Device: iPhone (iOS 15+) or Android phone (Android 10+) with measureQuick installed
• Account: measureQuick account with active subscription
• Calibrated nozzle plate: Factory-calibrated orifice with known pressure-flow curve
• Pressure gauge: TEC DG-1000 or equivalent digital manometer
• Adapter or mounting hardware: To install the nozzle plate at the duct opening
• Sealing materials: Painter's tape, foam weatherstrip, or gasket material
• Prerequisite knowledge: Total external static pressure (E9) and TrueFlow Grid theory (M1)
• Time: 15-30 minutes for setup and measurement

What Is a Calibrated Nozzle?

A calibrated nozzle is a precision orifice - a plate or fitting with a carefully machined opening of known geometry. When air passes through the nozzle, the pressure drop across it is directly related to the airflow volume. The manufacturer tests each nozzle size in a laboratory flow bench and provides a calibration curve (or lookup table) that maps pressure drop to CFM.

The measurement principle is identical to TrueFlow and blower doors: force air through a known restriction, measure the pressure difference, and convert that pressure to airflow using the calibration data. The difference is in the geometry and calibration precision of the restriction itself.

How It Works

1. The nozzle plate is installed at a duct opening, replacing the filter or covering the duct end.
2. The system blower pushes (or pulls) air through the nozzle.
3. Pressure taps on each side of the nozzle connect to a manometer.
4. The manometer reads the pressure drop across the nozzle in inches of water column (IWC).
5. You look up the measured pressure drop on the nozzle's calibration curve to find the corresponding CFM.

The relationship between pressure and airflow through a nozzle follows a square-root function. Doubling the airflow quadruples the pressure drop. This means the pressure signal is stronger at higher airflows, which improves measurement resolution at the flow rates typical of residential HVAC systems.

When to Use Nozzle Testing

Nozzle testing occupies a specific niche in the airflow measurement toolkit. It provides laboratory-grade accuracy in field conditions, but it requires a physical setup that is not always practical.

Choose nozzle testing when accuracy is the priority and you have access to install the nozzle at a duct opening. For routine service calls, TrueFlow is faster. For commissioning work, warranty verification, or situations where the airflow measurement is being used to resolve a dispute, the nozzle's higher accuracy justifies the setup time.

Equipment Details

The DG-1000 as a Nozzle Manometer

The TEC DG-1000 digital pressure gauge is commonly used for nozzle testing. It reads pressure differential to 0.001 IWC resolution and auto-zeroes to eliminate drift. The same instrument serves double duty for static pressure measurement, TrueFlow readings, and nozzle testing.

Connect the high-pressure tap (upstream side of the nozzle) to Channel A and the low-pressure tap (downstream side) to Channel B on the DG-1000. The display shows the differential pressure directly.

Nozzle Plates

Nozzle plates are available in several sizes to match common duct openings. Each size has its own calibration curve. Using the wrong size nozzle for the duct opening, or using a nozzle without its specific calibration data, invalidates the measurement.

Standard nozzle plates are made from machined aluminum or precision-molded plastic. The critical dimension is the nozzle bore diameter, which determines the flow range the nozzle can measure accurately.

Adapters and Mounting

The nozzle must create an airtight seal at the duct opening. Some nozzle systems include adapter plates for standard filter slot sizes. For non-standard openings, you may need to fabricate a mounting plate from rigid foam board or plywood, with the nozzle installed in a cutout and sealed with gasket material around the perimeter.

Setup Procedure

Step 1: Select the Nozzle Size

Choose a nozzle size appropriate for the expected airflow range. The nozzle calibration curve specifies a valid measurement range (for example, 400-1,200 CFM). If the expected system airflow falls outside that range, select a different nozzle or combine nozzles if the system supports it.

Step 2: Prepare the Mounting Location

The nozzle installs at a duct opening, typically the return filter slot or a duct termination. Remove the filter or grille. Clean the mounting surface so the nozzle plate seats flat.

Step 3: Install the Nozzle Plate

Place the nozzle plate in the opening. If using an adapter, attach the adapter first, then the nozzle. Press firmly against all edges.

Step 4: Seal the Perimeter

Run your hand around the nozzle perimeter with the blower running. Any air escaping around the edges bypasses the nozzle and produces a low reading. Seal gaps with painter's tape, foam weatherstrip, or gasket material. The seal must be airtight.

Step 5: Connect Pressure Taps

Connect tubing from the upstream pressure tap (return side of the nozzle) to Channel A on the DG-1000. Connect the downstream pressure tap (supply/blower side) to Channel B. Verify both connections are secure and the tubing is not kinked.

Step 6: Run the System

Turn the system on in the desired operating mode (cooling high stage, heating, or fan-only). Allow the system to stabilize for 30-60 seconds.

Step 7: Read the Pressure

Record the differential pressure from the DG-1000. Wait for the reading to stabilize; fluctuations of +/-0.002 IWC are normal, but the average should be steady.

Step 8: Convert to CFM

Look up the measured pressure drop on the nozzle's calibration curve or table. Some nozzle systems include software or a calculator app that performs this conversion automatically.

Airflow measurement equipment: TEC DG-1000 manometer reading 0.51 inWC, TrueFlow Grid nozzle plate, and duct tester setup schematic

Recording Results in measureQuick

After obtaining the CFM value from the nozzle calibration curve:

1. Open the test in measureQuick.
2. Enter the CFM in the airflow field as a manual entry.
3. measureQuick records this with airflow_source = "manual".
4. Note the measurement method (nozzle type and size) in the project notes for future reference.

There is no direct data transfer from a nozzle to measureQuick. The value is entered manually. Despite the manual entry classification, the accuracy of the underlying measurement is higher than an enthalpy estimate.

Accuracy Advantages

Calibrated nozzles tested on laboratory flow benches achieve +/-1-2% accuracy under controlled conditions. In field use, achieving +/-2-3% is realistic with proper sealing and setup. This is better than TrueFlow (+/-5%), capture hoods (+/-5-10%), and significantly better than enthalpy estimation (+/-15-25%).

The accuracy comes from the precision of the nozzle geometry and the quality of the laboratory calibration. Unlike TrueFlow, which uses a perforated plate averaging pressure across many sensing ports, a nozzle concentrates the airflow through a single defined opening with a well-characterized pressure-flow relationship.

Limitations

• Access required. The nozzle must be physically installed at a duct opening. If the filter slot is inaccessible, oddly shaped, or in a tight space, installation may not be feasible.
• Setup time. Nozzle testing takes 15-30 minutes including setup, sealing, measurement, and teardown. TrueFlow takes 5-10 minutes.
• Not practical for every service call. The accuracy advantage is meaningful for commissioning, verification, and dispute resolution. For routine diagnostics, TrueFlow or capture hoods are faster and accurate enough.
• Nozzle size must match the flow range. Using a nozzle outside its calibrated range produces unreliable results. You may need multiple nozzle sizes in your kit.
• Density corrections. Like all pressure-based airflow measurements, nozzle readings assume standard air density. At altitude or extreme temperatures, apply a correction factor.

Tips & Common Issues

Pressure reading is lower than expected for the system size

Check the seal around the nozzle plate. Even a small gap allows air to bypass the nozzle, producing a lower pressure drop and a lower calculated CFM. Re-seal and re-measure.

Pressure reading fluctuates and will not settle

Close interior doors near the measurement location to reduce pressure disturbances. Check for wind effects on the building envelope. Verify the system blower is running at a constant speed; variable-speed motors may ramp while adjusting to the nozzle restriction.

The calculated CFM seems too high

Verify you are using the correct calibration curve for the nozzle installed. Each nozzle size has a unique curve. Using the wrong curve produces incorrect CFM values. Also verify the manometer connections are on the correct sides (upstream vs. downstream).

ECM motor compensation

On systems with ECM or constant-torque blower motors, the motor may increase speed to compensate for the nozzle restriction, similar to the TrueFlow Grid effect described in M1. Run the system in a fixed-speed mode if available, or note that the reading reflects the motor's compensated output rather than normal operating airflow.

Related Articles

Prerequisites (complete these first):

• Total External Static Pressure - Pressure measurement fundamentals
• TrueFlow Grid Measurement Theory - Understanding pressure-based airflow measurement

Follow-up articles (next steps after this one):

• Airflow Documentation Best Practices - Recording and reporting airflow data
• Duct Traverse Measurement - Alternative airflow method using multi-point velocity measurement

Related in other domains:

• CFM per Ton - Target airflow ranges for residential HVAC systems
• TrueFlow Grid Setup - TrueFlow setup for comparison
• TESP Budget & 140% Rule - Understanding pressure budget in the context of added nozzle restriction

Need Help?

If you get stuck or this article does not answer your question:

• Check the Related Articles section above
• Contact measureQuick support: support@measurequick.com