Watlow Series 922 - Manual (Page 22)

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WATLOW Series 922 User's Manual How to Install and Wire, Chapter 4
Install/Wire
Contact Gordon at:
5710 Kenosha St., Box 500
Richmond, IL 60071
Phone: 815/678-2211
Excessive lead length in a two-wire RTD sensor can create indication errors. To
combat this, use a three wire sensor in long lead applications.
RTD - The Dry Bulb Installation
We suggest you mount the dry bulb RTD at a location in your process or
system where it reads an average temperature. Air flow past this sensor should
be moderate. The sensor should be thermally insulated from the sensor
mounting.
RTD - The Wet Bulb Installation
One way to prepare the RTD wet bulb sensor for reading relative humidity on
Channel 2 is to slip a cotton wick over the sensor sheath, Watlow Part # 0830-
0111-0000 or an equivalent. Put one end of the wick in water. Some systems
use a small trough of water with a float valve to replenish the evaporating liquid.
Use distilled water to avoid mineral deposits on the wick that can change the
%RH reading. The wicking distance should be as short as practical. The
cotton wick must be free of any sizing and starch. The "float valve-trough"
systems should also remove the water when the ambient temperature goes
below freezing or above boiling.
To obtain valid %RH readings, air flow past the %RH sensor must be sufficient
to evaporate the maximum amount of water from the wick without drying it out.
Evaporation cools the sensor; the amount of cooling relates directly to the
relative humidity present.
Thermally insulate the wet bulb sensor from the sensor mounting. This pre-
vents mounting surface heat from flowing into the sensor sheath and changing
the wet bulb temperature. (The result of such heat flow will be a humidity
reading higher than actual).
Thermocouple Installation Guidelines
Locate the thermocouples in your process or system where they read average
temperatures. Air flow past these sensors should be moderate. The sensors
must be thermally insulated from the sensor mounting. Both thermocouples
must be ungrounded to obtain correct readings.
Event Input and Remote Hold Input
These inputs accept a switch closure to digital common shown on Page 27.