•
Direct Acting Instrument:
Selecting SVI as a direct acting instrument or positioner is the right
choice for most applications. Direct action will increase output pressure to the valve for increasing
input signal. Reverse action decreases the output pressure for increasing input signal.
•
Hardware Reverse:
Reverse mode operation may be accomplished by hardware or software means.
Hardware reverse is an option that must be ordered from the factory (the device cannot be hardware
reversed in the field).
•
Software Reverse:
Selecting this option to configure SVI as a reverse acting instrument by software.
In the software reverse mode, care must be taken to insure that the input signal is not less than 3.8
mA. As the input signal drops, the air pressure to the actuator would increase in this reverse mode.
However, below 3.8 mA, the SVI would not have sufficient power to run. When power is insufficient
to run, the output to the I/P would drop to 0 and the output air pressure would decrease to
atmosphere pressure (rather than increase as in the case of Hardware Reverse).
Air-To-Open
Electrical Failure
Air Failure
Sensor Failure
Direct
Closed
Closed
Configurable
Hardware Reverse
Open
Closed
Configurable
Software Reverse
Closed
Closed
Configurable
Air-To-Close
Electrical Failure
Air Failure
Sensor Failure
Direct
Open
Open
Configurable
Hardware Reverse
Closed
Open
Configurable
Software Reverse
Open
Open
Configurable
•
Fail to Low/High Signal (to I/P):
This selection is activated only if Hardware Reverse or Software
Reverse are selected and determines, in case SVI fails, whether it will fail to low or to high signal to
the internal I/P transducer. Fail to Low Signal is the most common choice and is equivalent to fail to
low output pressure. Fail to High Signal option is used almost exclusively for the SVI with the
hardware reverse. For hardware reverse SVI, fail to high signal may be preferred (where fail high
would result in a low actuator pressure). Note, however, that in an electrical failure, the SVI will no
longer operate and will not likely fail to high signal.
49
Reverse mode is a rarely used option and may
cause unexpected results in the event of a power
and/or air failure. Be sure you understand the
operation of the SVI in reverse mode before select-
ing either hardware or software reverse and thor-
oughly test it before use. The following list the
valve positions when electrical failure and/or air
loss occurs for various SVI configurations.
WARNING
