Airflow Measurement

Case Study: Retrofitting Legacy Airflow Measuring Technology in a Surgical Suite

April 17, 2026·5 min read·TPI Technical Team

Erratic measurements from a Differential Pressure (DP) sensor were negatively impacting the control of Outdoor Air (OA) dampers associated with an HVAC system serving a surgical suite. An independent contractor identified that the DP technology could not meet the operational demands of the site-specific operating conditions. The airflow measuring technology would need to be retrofitted to assure proper HVAC operations.

"The customer selected NJK Precision because the legacy airflow measuring technology available in the marketplace could not sustain the performance demands or accommodate the physical limitations of their existing HVAC system."
— NJK Precision Case Brief

Building Environment & Existing Technology

Building Environment

Critical Environment

Surgical Suite

HVAC System

Air Handling Unit

100% Outside Air

Existing Technology

Differential Pressure

DP Sensor — Failed

The Challenge of Retrofitting

Other "legacy" airflow measuring technologies — Tube Style (TS) with hot wire sensor probes — were evaluated as potential solutions for retrofitting the HVAC system. However, they could not meet the physical constraints of the OA unit.

Locating the legacy TS technology in the 30" × 108" opening would require a 1/4 or 2/8 tube sensor array — blocking physical access to the mixing plenum entirely. The legacy options failed on two fronts simultaneously: performance and fit.

Why Legacy Technology Failed Here

DP sensor could not meet site-specific operating conditions — producing erratic OA damper control readings.

Tube-style hot wire arrays in the 30" × 108" opening would block physical access to the mixing plenum.

Legacy placement rules require 7½ equivalent duct lengths from any elbow, louver, or damper — impossible in this layout.

No legacy option could deliver both the required accuracy and the required physical clearance.

The NJK Precision Solution

NJK Precision airflow measuring technology utilizes a wing-shaped airfoil Flow Frame and Thin Film (TF) sensor technology. Unlike legacy technologies, NJK Precision is highly accurate regardless of flow rate, non-uniform, non-laminar, or turbulent air.

Flow Frame fabrication easily accommodates custom size requirements and utilizes a single sensor within a frame-mounted Flow Measuring Module (FMM). The Flow Frame and Flow Measuring Module design allows installation next to an HVAC elbow, louver, or damper — and does not create drag, airflow restrictions, or impede physical access.

Custom Fabrication

NJK fabricated a custom Flow Frame to fit the 30" × 108" opening. The frame-mounted Flow Measuring Module assured free flow of air and unobstructed access into the mixing plenum. NJK's design assures accuracy regardless of flow rate or turbulence.

NJK Precision vs. Legacy Technology

CategoryLegacy TS / DPNJK Precision

Custom SizingStandard sizes onlyCustom fabrication for any opening

Placement Flexibility7½ duct lengths from any eventNext to elbow, louver, or damper

Sensor DesignMulti-probe array — aggregated dataSingle sensor — no degrading probe averaging

Accuracy in TurbulenceDegrades in non-laminar flowAccurate regardless of flow rate or turbulence

Annual CalibrationRequired — factory recalibrationNot required — inspection & cleaning only

MaintenanceComplex — full array removalPlug-and-play FMM — 4 screws, pop out, blow down

Physical AccessArray blocks mixing plenum accessNo obstruction — full plenum access maintained

Airflow RestrictionAdds drag across duct cross-sectionNo drag, no restriction

Operations & Maintenance Advantages

No Annual Calibration

Does not require annual factory calibration services. Only requires annual inspection and cleaning.

Plug-and-Play FMM

Flow Measuring Modules are plug-and-play. Annual maintenance only requires removing 4 screws, popping out the module, and providing a light blow-down to remove dust.

Extended Sensor Life

Operational life-cycle of sensors is extended through low-temperature operations — reducing replacement frequency.

Single Sensor Accuracy

Single sensor design avoids aggregate data inclusive of data from degrading sensors — ensuring readings stay accurate over time.

Installation Advantages

Custom fabrication for project-specific sizing needs — no standard-size compromise.

Ultimate placement versatility — can be mounted next to an elbow, louver, damper, or any other event.

Does not require placement "7½ equivalent duct lengths away from an event" — the rule that makes legacy retrofits impossible.

Single sensor design reduces installation costs versus multi-probe tube arrays.

Precision accuracy regardless of flow rate, non-uniform, non-laminar, or turbulent air.

Conclusion

NJK Precision technology met the performance demands and overcame the physical limitations where legacy airflow measuring technologies could not. In a surgical suite — where HVAC performance is directly tied to patient safety — there was no room for compromise. NJK delivered a custom-fabricated solution that fit the exact opening, maintained full mixing plenum access, and restored accurate OA damper control from day one.

For any retrofit application where legacy sensors have failed on performance, fit, or both, NJK Precision is the solution worth specifying.