Dwyer HFO User Manual

Series HFO Flow Alarm

Specifications – Operating Instructions

Bulletin F-HFO

Series HFO Flow Alarms are typically used to

make or break a set of electrical contacts to
signal a limit setting. They may be used to turn
on a warning light, sound a bell or horn, or even
to shut down a process. The switches on the
flow alarm can be configured to open or close a
contact for an increasing or decreasing set
point. Decreasing flow set points may be
located anywhere in the lower 2/3 of the scale
while increasing set points may be located
anywhere in the upper 2/3 of the scale.

Note: Refer to Series HF In-Line Flow Monitor,
Bulletin F-55, “Installation and Operating
Instructions” for installation, operation, and
cleaning instructions for the basic flow monitor
cartridge (included). The following instructions
are specifically for the Series HFO Flow Alarm
switches.

PRESSURE DIFFERENTIAL VS. FLOW RATE

1/4˝ FEMALE NPT 1/2˝ FEMALE NPT 3/4˝, 1˝ & 1-1/2˝ FEMALE NPT

SPECIFICATIONS
Service: Compatible gases or liquids.
Wetted Materials: Body: Aluminum, brass or

304 SS; Seals: Buna-N or Fluoroelastomer;
Magnet: PTFE coated Alnico; Other internal
parts: 304 SS.

Viscosity Limit: 500 SSU.
Temperature Limits: 170°F (76°C).
Pressure Limits: See Chart.
Enclosure Rating: NEMA 4X (IP65).
Accuracy: Measuring ±4% FS over entire

range; ±2.5% over center third of the
measuring range.

Repeatability: ±1% of full scale.
Switch Type: SPDT, 10A @ 250 VAC; 0.5A @

125 VDC (resistive), 1/4 hp @ 250 VAC
(inductive); 3A @ 125 VAC “L” lamp load.
Shipping Weight: 1/4 to 1/2˝ female NPT
Models: 3 lb (1.4 kg); 3/4 to 1˝ female NPT
Models: 4.5 lb (2.0 kg); 1-1/2˝ female NPT
Models: 12 lb (5.4 kg).

DWYER INSTRUMENTS, INC.

P.O. BOX 373 • MICHIGAN CITY, INDIANA 46360, U.S.A. Fax: [phone] e-mail: [e-mail]

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

Operation:

44 44

2

222

6666

88

1

100

88

1

100

GGPPMM

P

OINTER

S

S WWIITTCC HH GG LL II DDEE

SS CC RR EE WW

FF LL OO WW RR AA TT EE
SS CC AA LL EE

AA LL AARRMM

S

S WWIITTCC HH

FF LL OO WW IINNDDIICC AA TT OORR

L

L IINN EE

F

F OO LL LLOO WWEERR

Illustration 1 shows the primary mechanism for
a single flow alarm. The HFO dual-switch flow
alarms contain two sets of these same
components (wiring to the DIN connector is
described on page 3). The configuration is such
that the high alarm is for increasing flow, and the
low alarm is for decreasing flow.

Illustration 1

The follower moves in unison with an orifice
plate inside of the unit’s pressure vessel via a
magnetic coupling in order to indicate flow rate.
As the follower moves with changes in flow rate,
the flow rate is determined by relating the
position of the flow indicator line to the
increments on the flow rate scale.
The pointer indicates the set point for the alarm
switch. In Illustration 1, the switch will be
actuated at all flow rates below 4 GPM. To
change the set point, simply loosen the switch
glide screw one (1) turn and slide the switch to
the desired position along the flow rate scale.
When the pointer is pointing to the desired flow
rate, re-tighten the switch glide screw.

Switches:

The switch is a simulated roller, lever operated
low force microswitch. The specifications for this
switch are listed on the first page. The switch is
actuated when movement of the follower
causes the switch lever to be lifted. In Illustration
2, the switch has not yet been actuated, and the
electrical circuit is through the normally closed
(NC) contact. Illustration 3 shows the switch
after it has been actuated. In this scenario, the
electrical circuit is through the normally open
(NO) contact.

Illustration 2

Illustration 3