Instrument Mistake 2: Using Capture Hoods to Measure Return and Exhaust Grilles

I had two new techs with me on a job gathering return and exhaust readings. At lunch, we looked at the readings and realized every fan was 10% to 20% low.

One tech suggested the mechanical contractor hadn’t sealed the return or exhaust duct properly and we were dealing with leaky ductwork. It was a good assumption, and once I’d made early in my career. But I suggested we look further.

I instructed them to traverse the fan for total airflow, then compare it to the capture hood readings. We eventually traversed individual inlets and discovered the capture hood read 7% to 15% lower than the traverse. This varied by grille type and quantity but was consistently low.

We learned instrument mistake #2: Capture hoods read lower airflow on the exhaust and return inlets. Why?

Most capture hoods are designed for supply airflow to travel about 36′ before it hits the velocity grid. This allows the airflow to even out before the grid, giving an accurate measurement. But exhaust and return airflow have a shorter distance from the hood base before the air hits the velocity grid.

To overcome the low airflow readings on our exhaust system, we took traverses of the different types of exhaust grilles and came up with a field derived Ak. This Ak was multiplied with the hood readings to give an accurate measurement.

Not only did we get correct data for the job, but it demonstrated that some problems are not always the mechanical contractor’s fault.

Instrument Mistake 3: Properly Recording Negative Velocity Readings in a Traverse

I teach new technicians to traverse by looking for a straight run of duct, located three-quarters of the way downstream of the last transition. But in the field, technicians often find these conditions are not always available. Sometimes you have to take the traverse in a less-than-ideal location.

Traverse points will mostly give you positive readings, but negative readings do occur. These readings typically do not indicate negative airflow, but a circular air motion pattern in the duct, similar to an eddy in a river or stream.

The flow is circulating and not truly moving in a negative direction. It is common practice to record these readings as zero, not a negative number. When set on pitot tube mode, several meters will record these negative readings as zero. This gives an accurate average velocity.

But as every new technician learns, by using the wrong instrument on a traverse, they can run into trouble.

For example, I was once onsite with a tech who was having trouble finding a straight run of duct for a traverse. We found a workable section, and he drilled the holes. Then he took readings with an airfoil, thinking it would be the same as his pitot tube. Instead, he learned instrument mistake #3.

With his meter connected to an airfoil, the negative readings were not stored as zero fpm. Instead, the meter records and uses the negative reading as part of averaging velocity. This calculates a lower airflow than the fan is actually moving.

To correct this, remove the negative velocity readings from the average and replace them with zero when using an airfoil over the pitot tube. This provides an accurate reading, despite not having the best run of duct for a traverse.

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