How do I measure inches of water column in a exhaust for a reflow oven ?

Do you mean static pressure?

The first two links give you the procedure:

Google

Here is a particular probe and an optional "surge damper" device

http://www.dwyer-inst.com/PDF_files/...ensors_cat.pdf

In a round duct what you will find is that usually near the center the flow is more laminar, stable and near a maximum. Near the edges, you get more of a turbulent flow.

You need to be at least a duct diameter away from say the exit and another duct diameter before it hits the fan.

If your going to be doing this as part of a routine inspection consider installing ports and use a differential pressure sensor.

W.C. is yet another unit like PSI

Example: Digital Manometer, Dual Input -TPI 621 - Mfg# 621

A differential manometer may be more useful. For single ended use, leave one port open to atmosphere.

Dwyer Engineering is a major manufacturer of air velocity and pressure equipment.

I hope I helped.