Welcome to What In The Tech — a LabCertTech blog series where we pull back the curtain on the instruments field certifiers carry into your facility. If you have ever watched a technician walk through your buffer room with a piece of equipment and wondered what it actually does, this series is for you.

Episode 1: The TSI VelociCalc 9565 with Model 964 Probe

If you spot a field certifier walking through your cleanroom or compounding suite holding what looks like a handheld device with a thin metallic wand attached by a coiled cable, there is a good chance you are looking at a TSI VelociCalc — one of the most widely used multifunction ventilation meters in the certification field.

This particular model, the TSI VelociCalc 9565, paired with the TSI Model 964 straight probe, is not a single-purpose tool. One instrument, one probe, and it is simultaneously capturing several of the measurements your certification requires.

What Is It Actually Measuring?

The TSI 9565 with the Model 964 probe measures four parameters at once:

• Air velocity (in feet per minute or meters per second)
• Temperature (in °F or °C)
• Relative humidity
• Differential pressure (in inches of water column or Pascals)

The probe itself is a thermal anemometer — it works by measuring how much electrical energy is required to keep a heated sensor at a constant temperature as air flows across it. The faster the airflow, the more cooling effect the air creates, and the more energy is needed to maintain that temperature. That relationship is how velocity is calculated. Built into the same probe are sensors for temperature and relative humidity, while the differential pressure measurement is captured through a separate port on the instrument body itself via tubing connected to pressure taps in the room.

Where You Will See This Being Used

USP <797> and USP <800> Cleanroom Suites

This is where the VelociCalc earns its keep in pharmaceutical certification work. USP <797> §5 requires that airflow testing be performed as a mandatory part of every recertification cycle. That testing must document three specific values: the ACPH contributed by the HVAC system, the ACPH contributed by the PEC, and the total ACPH for the room. All three must appear on the certification report.

In most certified compounding facilities, airflow volume at ceiling HEPA supply outlets is measured using a capture hood (flow hood) placed directly over each filter. The capture hood collects all air from the outlet and measures volume in CFM directly — which is then used to calculate ACPH for the room. CETA CAG-003 §8.1.1 notes that volume measurement is preferable to velocity measurement in ISO classified rooms as the more representative test of final filter air supply.

Where the VelociCalc comes in for this measurement is as a secondary method — specifically in situations where a ceiling HEPA filter cannot be physically reached with a capture hood due to ceiling height or ceiling tile configuration. In those cases, the thermal anemometer probe is used to take velocity readings across the filter face, and those readings are combined with the filter’s effective face area to calculate supply volume in CFM. It is a valid approach, but the flow hood is the preferred primary method when access allows.

The temperature and relative humidity readings from the same probe pass give you the environmental data required by CETA CAG-003 §8.1.7 — buffer room temperature should be 68°F or cooler, and relative humidity should be below 60%. One instrument pass, multiple required data points captured simultaneously.

The differential pressure capability is used to verify the pressure relationships between classified spaces — the 0.020-inch water column minimum between the buffer room and anteroom, the anteroom and unclassified space, and the negative pressure differential of 0.010 to 0.030 inches water column required for hazardous drug compounding rooms under USP <800>.

Laminar Airflow Workbenches (LAFWs)

For LAFW certification under CETA CAG-003 §8.4, air velocity testing is part of the mandatory minimum test set. The certifier uses the Model 964 probe to take a grid of velocity readings across the HEPA filter face, typically starting 6 inches from the inner edge of the frame and spaced no more than 12 inches apart across the work surface. The FDA-recommended face velocity for LAFWs is 90 fpm ± 20%, which yields a working range of 72–108 fpm. Manufacturer specifications govern where they are more specific than that range.

Biological Safety Cabinets (BSCs)

BSC field certification follows NSF/ANSI 49 Annex N5, and airflow measurement is central to that standard. For inflow, the primary measurement method uses a capture hood placed at the sash opening to measure volume directly in CFM. The VelociCalc’s thermal anemometer probe serves as a secondary method for inflow when a flow hood cannot be used. Downflow velocity across the work zone is measured using the thermal anemometer probe at a grid of points across the work surface, with results compared against the manufacturer’s listed nominal set point for that specific cabinet model.

Chemical Fume Hoods

Face velocity measurement is the primary field test for chemical fume hood performance under ASHRAE 110. The certifier measures average face velocity at the sash opening, typically targeting 100 feet per minute as a general benchmark, though your institution’s safety office or the manufacturer’s specification may set a different target. The VelociCalc handles this measurement the same way it handles LAFW velocity — a grid of readings across the sash opening, averaged for the result.

What the Display Is Showing

One of the things that makes the TSI 9565 useful in the field is that it displays all active measurement channels simultaneously on a single screen. When you glance at that display, you are seeing real-time velocity at the top, with temperature, humidity, and differential pressure reading out below it at the same time. The certifier is not switching modes between measurements — all of that data is being logged continuously with each pass of the probe.

The instrument also stores readings internally and can be downloaded to reporting software, which is how those data tables in your certification report get populated. That date/time-stamped data trail is part of what makes a calibrated, documented instrument different from a simple handheld meter.

Why Calibration Matters

CETA CAG-003 §8.5 requires that every piece of test equipment be listed on the certification report with make, model, serial number, and calibration date. All instrumentation must be calibrated per IEST-RP-CC013 or manufacturer recommendation and be NIST-traceable where possible. For the TSI 9565, that means the instrument should have a current NIST-traceable calibration certificate on file — and a certifier who cannot produce that documentation when requested is not meeting the standard.

When LabCertTech brings this instrument into your facility, it arrives calibrated, documented, and ready to produce data you can stand behind in an inspection.

That is Episode 1 of What In The Tech. Next time we will look at another instrument from the field certifier’s kit and break down exactly what it is doing in your space. Have a piece of equipment you have always wondered about? Let us know.