Dwyer AFG User Manual

Series AFG Averaging Flow Grid

Specifications - Installation and Operating Instructions

SECTION A-A

1/64

[.396]

SCALE 2:1

A

Bulletin F-AFG

The Series AFG Flow Grid is an outstandingly simple yet accu-

rate and cost effective alternative to other duct mounted pressure
sensors. Once installed and connected to a suitable measuring
instrument, the device will provide years of trouble free monitoring
of both air and gas flow. Fitting the AFG Flow Grid is simple, with
the unit supplied in kit form to allow both workshop and on-site
installation into a wide range of square and circular ducts.

The AFG Flow Grid is a fundamental pressure-sensing device
designed to transmit a continuous differential pressure signal.
When this output is connected to a suitable measuring instrument
(i.e. manometer, pressure transducer, etc.) it may be used to
determine air velocity and volume flow rate.

HOW THE AFG FLOW GRID WORKS

The AFG Flow Gird consists of two tubes mounted diagonally
across a square or rectangular duct, or diametrically across a
round duct. The tubes are drilled with a series of equi-spaced
holes.

The holes in one tube face directly upstream and sense total pres­sure, while the pairs of holes in the second tube also face forward
but at an included angle of 79 degrees, sensing static pressure.

The total and (sub) static pressures are averaged along the length
of each tube and provide pressure signals at connectors outside
the duct wall. The pressure differentials across these connectors
constitute the output signal.

CONSTRUCTION

The AFG Flow Grid tubes are stainless, steel, PVC, Polyurethane,
acetyl plastics, and neoprene rubber.

ØA

2

A

B

[27]

[51]

1-3/16

[30.16]

1-11/16

[42.86]

[2.38]

1-1/16

SPECIFICATIONS
Service: Monitor air or compatible gas flow .
Wetted Materials:304 SS, PVC, Polyurethane, acetyl plas-

tics, and neoprene rubber.

Accuracy: ±5%.
Maximum Temperature: 176ºF (80ºC).
Velocity Range:295.2 ft/min to 5904 ft/min (1.5 to 30 m/sec).
Diameter of T ubes:5/16˝ (8 mm) or 5/8˝ (16 mm).
Max Duct Diagonal: 60.4˝ (153.4 cm).
Max Duct Diameter: 59.4˝ (150.9 cm).
Process Connections: 5/16˝ barbed.
Weight: AFG-1: 1 lb (454 g); AFG-2: 3 lb (1361 g).

APPLICATIONS

The AFG Flow Grids will give useful and reliable readings in a wide
variety of ‘in duct’ locations often where other flow rate measuring
devices are found to be unsatisfactory.

However , AFG Flow Grids should not be used in conditions of high
humidity or where material of a sticky nature is present in the air
stream, as a build-up of deposits on the pressure tubes or in the
pressure holes will affect calibration. Where a high dust burden is
likely to be encountered, it is advisable to provide access for
cleaning when installing an AFG Flow Grid.

Positioning in Ducts:
The AFG Flow Grids should be positioned in straight lengths of
duct and at right angles to the axis of the duct. The following cri­teria should be complied with when positioning an AFG Flow Grid:

5/32

The output connectors are 1/4" diameter and are intended for use
with plastic tube.

The AFG Flow Grids must not be used in air temperatures above
176ºF (80ºC).

1. Allow at least 3.5 diameters (3.5D) of duct upstream from 1D
bends and minor obstructions.

2. Major obstructions such as right angled bends and opposed
blade dampers require greater lengths of upstream duct (See the
last page).

3. Allow at least 1D of duct downstream of the AFG Flow Grid
before any change to the duct cross section or obstruction to the
flow. Shorter lengths are likely to increase pressure loss.

DWYER INSTRUMENTS, INC.

Phone: [phone] www.dwyer-inst.com

P.O. BOX 373 • MICHIGAN CITY, INDIANA 46361, U.S.A. Fax: [phone] e-mail: [e-mail]

4. Avoid a sudden expansion immediately before an AFG Flow
Grid (See Fig. 2a).

5. A local contraction at the plane of the AFG Flow Grid is a use­ful technique to increase pressure differential when velocities are
low (See Fig. 2b).

6. Where persistent swirl is expected it is advisable to install an
anti-swirl device at least 1D upstream from the AFG Flow Grid.

Note: For rectangular duct D = Width + Height of Duct/ 2

a) Unacceptable

b) Acceptable

Fig. 2

INSTALLATION

The AFG Flow Grids are supplied in kit form, and tube lengths are
cut to suit the required duct size by the customer.

page 2

Table 2: Included Parts

Description

Adapter & Pressure Tube
Adapter Plate
Duct Cap
Cap Plate
Adapter Drilling Template
Duct Cap Drilling Template
Diaphragm
Elbow
Tube Cap
Grommet
Cap Seal
Adapter Seal
Screws, Self Tapping No. 6 x 5/8˝

Decide on the best location for your AFG Flow Grid.
The AFG Flow Grid is designed to fit into the shortest duct sides

of a rectangular duct or across a duct diameter. Mark out the duct
to the dimensions shown in Fig. 4 and 5. Attach the self-adhesive
templates supplied, and produce the holes to the sizes indicated
on the templates. DEBURR the 1.26˝ (32 mm) diameter holes.

Quantity

2
2
2
2
2
2
2
2
2
2
2
2

16

The AFG Flow Grid is available in two tube diameters and lengths
(See Table 1).

Table 1: Dimensions

Tube & Adapters

Tube Diameter “A”

5/16˝ (8 mm)
5/8˝ (16 mm)

Supplied Length Size “B”

27˝ (688 mm)

59-4/5˝ (1518 mm)

Fig. 3

Fig. 4

Fig. 5

Introduce tubes into the duct and secure at the adapter end using
the clamping plate and No. 6 self-tapping screws supplied (See
Fig. 6 and 7). Note position of tubes relative to the direction of flow
(See Fig. 7).

They may be used in square, rectangular or round ducts.
Unpack your AFG Flow Grid. The kit should contain the following

parts: