Ratio Relay; Pneumatic Potentiometer - Honeywell Pneumatic Controls Engineering Manual

Relays and Switches
Figure 45 shows an averaging relay in a typical application with
two thermostat signals as inputs. The average of the thermostat
signals controls a valve or damper actuator.
THERMOSTAT 1
M B
M
Fig. 45. Averaging Relay Application.

Ratio Relay

The ratio relay is a four-port, non bleed relay that produces a
modulating pressure output proportional to the thermostat or
controller branchline output. Ratio relays can be used to control
two or three pneumatic valves or damper actuators in sequence
from a single thermostat. The ratio relay has a fixed input
pressure range of either 3 or 5 psi for a 10 psi output range and
an adjustable start point. For example, in a ratio relay with a 5
psi range set for a 7 psi start, as the input pressure varies from
7 to 12 psi (start point plus range), the output pressure will vary
from 3 to 13 psi.
In Figure 46, three 3 psi span ratio relays are set for 3 to 6, 6 to
9, and 9 to 12 psi inputs, respectively. The thermostat signal
through the relays proportions in sequence the three valves or
actuators that have identical 3 to 13 psi springs.
DA ZONE
THERMOSTAT
M
M B
EXH
M
M
EXH
M
EXH
Fig. 46. Ratio Relays in Sequencing Control Application.
144 customer.honeywell.com
THERMOSTAT 2
M B
P 2
TO
B
ACTUATOR
P 1
AVERAGING
RELAY
C2345

RATIO RELAY 1

3-6 PSI
M
B
E P
HEATING VALVE
3–13 PSI
RATIO RELAY 2
6-9 PSI
M
B
E P
MIXING
DAMPERS
3–13 PSI
RATIO RELAY 3
9-12 PSI
M
B
E P
COOLING VALVE
3–13 PSI
C2370

Pneumatic Potentiometer

The pneumatic potentiometer is a three-port, adjustable linear
restrictor used in control systems to sum two input signal
values, average two input pressures, or as an adjustable flow
restriction. The potentiometer is a linear, restricted air passage
between two input ports. The pressure at the adjustable output
port is a value based on the inputs at the two end connections
and the location of the wiper between them.
Figure 47 shows a pneumatic potentiometer providing an
average of two input signals. The wiper is set at mid-scale for
averaging or off-center for a weighted average. It can be used
this way to average two air velocity transmitter signals from
ducts with different areas by positioning the wiper according to
the ratio of the duct areas. This outputs a signal proportional to
the airflow.
SIGNALS
FROM
SENSORS
Fig. 47. Pneumatic Potentiometer as Averaging Relay.
Figure 48 shows a pneumatic potentiometer as an adjustable
airflow restrictor.

PNEUMATIC POTENTIOMETER

M
Fig. 48. Pneumatic Potentiometer as
Adjustable Airflow Restrictor.
70-6925
PNEUMATIC POTENTIOMETER
WEIGHTED
AVERAGE
SIGNAL
AVERAGE
SIGNAL
C2374
CAP
TO CONTROLLED
DEVICE
C2372