Fluke 922 Service Guide

HVAC pressure

applications with

the Fluke 922

For contractors troubleshooting pressure,
a lot depends on system peculiarities. Are
the installation instructions with the equip­ment? Is the start-up performance report
with them? Is the TAB (test and balance)
report available? Do you understand the

Application Note

control system? More often than not, it
seems, you can only answer yes to the
last question. You rely on your experience,
knowledge, and tools to check the funda­mentals. And for that much, at least, Fluke
can help.

Pressure measurement

Pressure is measured in sev­eral different scales. Pounds
per square inch (psi), inches of
water column (in. wc), inches
of mercury column (in. hg), and
millionths of a meter of mercury
column (microns) are the most
typical in HVAC work.

Bourdon gauges are the tradi­tional choice for higher pressure
readings in psi. Finer precision
measurements use mercury col­umn scales. And for the most
precise measurements—and most
low pressure measurements in
HVAC—inches water column is
the standard.

The following chart compares
these common pressure scales
and their precision.

Using the Fluke 922 low pressure differential meter in a duct traverse.

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

Atmospheric Pressure 1 psi 1” hg 1” wc

14.696 psia 1 psi 0.019 psi 0.0361 psi

29.921 “ hg 2.036” hg 1” hg 0.0736” hg

406.8” wc 27.68” wc 13.595” wc 1” wc

Of the many different low
pressure measuring instruments
used over the years, electronic
manometers/micromanometers
(very low pressure gauges)
now offer durability, precision,
accuracy, and the significant
time saving convenience of fully
automatic calculations as well as
minimum-maximum-average and
memory functions.

How pressure sensing

Total Pressure

Airflow

Airflow

Pitot-Static Tube

Pitot Tube

Static Pressure

Static Pressure

Total Pressure

Airflow

Airflow

Airflow

Airflow

Static Tip

works

The Fluke 922 is a low pres­sure differential meter used in
traditional manometer or micro­manometer applications. It has
a range of +/-16” wc, resolution
of 0.001” wc, and over-pres­sure protection of 10 psi at each
high and low pressure port.
With a single tube connected to
its high or low port, the meter
will display positive or negative
pressure relative to the ambi­ent pressure at the meter’s open
port. For a pressure differential
at two remote points, use tubing
to connect the higher pressure
sensing point to the “+” port and
the lower pressure sensing point
to the “-” port.

The meter uses a variety of
sensing probes and fittings to
access the type and location of
pressure measurement points.
These can be as simple as a
straight metal tube for check­ing draft, a “Tee” for tapping
into pressure sensing tubing
connected to a pressure switch,
a Pitot tube for sensing total
pressure, a Pitot-Static tube for
sensing both total and/or static
pressure, or a static pressure tip.

Pressure applications

Draft pressures: Draft pressures
are negative relative to ambient
pressures at the burner. Measur­ing and controlling draft is a crit­ical step in combustion tests and
influences net thermal efficiency
of the appliance. Draft is created
by vertical vents or powered
sidewall vent systems connected
to fossil fuel equipment. Like too
much air, excessive draft through
a heat exchanger increases the
velocity of the combustion prod­ucts containing heat, thereby
reducing the contact time in the
heat exchanger. Follow guide­lines from the manufacturer or
standards authority.

Over-fire draft: Power burn­ers typically have specifications
related to over-fire draft, which
relates to the velocity of com­bustion products through the
heat exchanger. This over-fire
draft typically is slightly nega­tive (-0.01” wc to -0.02” wc)
but, depending on burner type
and application, may be slightly
positive (0.01” wc to 0.02” wc).
Follow guidelines from the man­ufacturer or standards authority.

Draft inducer pressure:
Category I fan-assisted gas appli­ances utilize a combustion air
inducer to create draft through
the appliance and deliver the
combustion products to a nega­tive pressure vent. A pressure
switch is normally connected
to the inlet side of the inducer
to ensure a minimum draft has
been established before allowing
an ignition sequence to begin
or continue. For testing, a “Tee”
taps into the pressure sensing
tubing, to monitor the draft
created by the inducer. This is an
important value to benchmark at
appliance installation. While the
pressure switch ratings list cut-in
and cut-out (or differential)
pressures, without a benchmark
you can’t evaluate changes
within the appliance or vent over
time. Follow guidelines from the
manufacturer.

Pressure effects and
measurements

Pressure is an integral part of HVAC
equipment design, system design, func­tion, application, control and diagnostics.
Pressure determines

• cleanliness of airside devices (pressure
drop across filters, evaporators, etc.)

• boiler steam temperature

• the saturation temperature of a fluid
(boiling temperature of a liquid, the con­densing temperature of a vapor)

• the direction of fluid flow (fluids flow
from higher to lower pressures)

• fluid volume flow rate (pressure drop
across an orifice)

• whether or not a control or safety switch
will make or break (high/low pressure
switches, draft proving switches)

• the availability of a fluid to perform work
(oxygen/nitrogen tank pressures, gas
supply pressures, duct static pressures)

Pressure also

• senses zone device positions, modu
lates VFD drives, bypass dampers and
valves

• affects solubility of air in water

• contributes to flue gas velocity
(stack draft)

-

2 Fluke Corporation HVAC pressure applications with the Fluke 922

Options for sensing probes and fittings when accessing pressure measurement points.