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 (http://www.google.com/search?client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&channel=s&hl=en&source=hp&q=measureing+static+pressure&btnG=Google+Search)

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

http://www.dwyer-inst.com/PDF_files/static-pressure-sensors_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 (http://www.professionalequipment.com/tpi-digital-manometer-dual-input-621/manometers/)

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

Dwyer Engineering (http://www.dwyer.com) is a major manufacturer of air velocity and pressure equipment.

I hope I helped.