Model 657C-1 Relative Humidity/Temperature Transmitter
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RECEIVER
®
Specications - Installation and Operating Instructions
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Bulletin E-101-A
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The Dwyer Model 657C-1 Relative Humidity/Temperature Transmitter provides two
4 to 20 mA channels to produce separate output signals for both relative humidity and
temperature. This unit is housed in a die cast aluminum 1/2˝ LB conduit enclosure
with internal screw terminals. A 5/16˝ x 9-1/10˝ stainless steel sensing probe extends
through a side hub. Sensor can be inserted directly into ducts and secured by optional
mounting kits A-158 or A-159. The A-158 kit consists of a split ange which is clamped
around the probe and then attached to the duct exterior with sheet metal screws. A
rubber gasket is included to seal the connection. The A-159 kit includes a bushing with
1/2˝ NPT male threads which is inserted through a hole in the duct. It is then secured
from the duct interior with a washer and locknut. Alternatively, it can be threaded into a
standard pipe ange mounted outside the duct.
The sensor employed in the 657C-1 features a state-of-the-art integrated polymer lm
relative humidity sensor and a thin-lm platinum temperature sensor. These sensors
were designed to provide high reliability and stability over years of continuous service.
The relative humidity sensor uses a special polymer lm which absorbs water vapor at
a precisely known rate which is dependent on the relative humidity of the surrounding
air. This lm is deposited on a CMOS integrated circuit that measures and conditions
the sensor output. The semiconductor and advanced lm technology ensure long
term stability and accuracy. The temperature sensor couples the highly accurate
and predictable temperature characteristics of platinum with a cost effective thin-lm
technology.
Installation
1. Location: Select a clean, dry location for the enclosure where the temperature
will not exceed the limits of 32 to 158°F (0 to 70°C). The transmitter can be
located any distance from the receiver provided the total loop resistance does
not exceed the limits as explained under “Wire Type and Length”. The probe
should be located where conditions are representative of the overall environment
being monitored. Avoid locations where stagnation or rapidly uctuating
conditions might occur. Also avoid areas where water mist or condensation exist
which could cause erroneous full scale humidity readings.
2. Position: The probe and transmitter are not position sensitive and can be
mounted in any orientation required. However, if the formation of condensation
within the conduit is possible, locate the Model 657C so that moisture will drain
away from the enclosure.
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SPECIFICATIONS
Relative Humidity Range: 0-100%.
Accuracy: ±2% (10-90% RH), ±3% (0-10% and 90-100% RH).
Operating Temperature Range: 32 to 158°F (0 to 70°C).
Temperature Measurement Range: 32 to 212°F (0 to 100°C).
Temperature Accuracy: ±1°F (0.5°C).
Output: 2 channels, each 4 to 20 mA, loop powered on RH channel.
Power Supply: 10-35 VDC.
Ambient Operating Temperature Limits: 32 to 158°F (0 to 70°C).
Storage Temperature Limits: -40 to 176°F (-40 to 80°C).
Housing: Die cast aluminum with 1/2˝ conduit connection.
Sensor Probe: 5/16˝ x 9-1/10˝ (0.8 x 25.4 cm) stainless steel.
Electrical Connection
Caution: Do not exceed the specied supply voltage rating. Permanent damage, not
covered by warranty, may result. This unit is not designed for AC voltage operation.
Receiver-Transmitter Connection - The Model 657C-1 transmitter is designed as
a two-wire 4 to 20 mA device with two channels. The channels are common on the
negative side of the current loops. Sensor excitation power is derived from the relative
humidity channel, so power must always be applied to that channel. If the temperature
channel is not used, it can be left disconnected. The basic two-wire connection is
shown in Figure B. Terminal 4 can be used as a tie point since it is not used internally.
However, the voltage on the terminal should not exceed the 35 VDC limit. Never
connect AC or voltages exceeding 35 VDC to this terminal.
MODEL 657C
RH/TEMP
TRANSMITTER
1/2 NPT
POWER
SUPPLY
RH
RECEIVER
TEMPERATURE
3. Mounting: To mount the probe, use optional kits A-158 Split Flange or A-159
Duct Mounting Gland or equivalent. Be careful to avoid excess stress on the
sensing tube and/or housing which could cause the probe and housing to
separate.
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Figure B
Power Supply - The transmitter requires a minimum of 10 and a maximum of 35
VDC at its connection for proper operation. Choose a power supply with a voltage and
current rating which meets this requirement under all operating conditions. If the power
supply is unregulated, make sure voltage remains within these limits under all power
line conditions. Ripple on the supply should not exceed 100 mV.
Loop Resistance - The maximum allowable loop resistance depends on the power
supply voltage. Maximum loop voltage drop must not reduce the transmitter voltage
below the 10 VDC minumum. Maximum loop resistance can be calculated with the
following equation. VPS is the power supply voltage.
V
ps - 10.0
max =
R
20 mA
Some receivers, particularly loop powered indicators, may maintain a xed loop
voltage to power the device. This voltage drop must also be subtracted from the power
supply voltage when calculating the voltage margin for the transmitter. The following
equation takes this into account. V
V
ps - 10.0 - Vrec
Rmax =
20 mA
rec is the receiver xed voltage.
To determine the maximum wire length, follow this procedure:
1. Determine whether the receiver is purely resistive or has a xed voltage drop,
then select the corresponding equation.
2. Determine the minimum power supply voltage that will be used. If the power
supply is unregulated, use the voltage corresponding to the low-line condition.
3. Select the wire size and determine, from Table 1, the resistance per foot for that
wire size. If the wire will be exposed to a wide temperature range, use the
resistance corresponding to the highest expected temperature.
4. Calculate the maximum length. If the required distance is greater than the
calculated value, choose a larger wire size and recalculate the maximum length.
Wire Resistance for Various Wire Sizes
AWG
16
18
20
22
24
26
Ohms/ft
@ 32°F
.03983
.06332
.10069
.16010
.25459
.40486
Ohms/ft
@ 68°F
.04495
.06873
.10928
.17375
.27628
.43930
Ohms/ft
@ 122°F
.04833
.07684
.12218
.19426
.30886
.49114
Ohms/ft
@ 167°F
.05256
.08360
.13291
.21132
.33596
.53445
Table 1
Wire Type and Length - Wire selection is often overlooked or neglected and thus can
contribute to improper or intermittent operation. Although 4 to 20 mA current loops are
relatively immune to wiring related problems, for some systems, proper wiring can be
an important factor in ensuring satisfactory system operation.
Twisted conductors are usually immune to most stray electric and magnetic elds, and
to some extent electromagnetic elds such as interference from RF transmitters. Avoid
use of at or ribbon cable which has no regular conductor twist. Where interference
is possible, use shielded wire. The shield must not be used as one of the conductors
and should be connected to ground at one end only - generally at the power supply.
Similarly, the conduit should be connected to protective ground as dictated by
applicable code and the signal wiring must not be connected to the conduit at more
than one point or as specied by code.
The maximum length of wire connecting the transmitter and receiver is a function of
wire and receiver resistances. Wire resistance is negligible in most installations with
shorter runs, typically under 100 feet. Generally, wire resistance should contribute no
more than 10% to total loop resistance. Where long runs are necessary or unique
application conditions exist, wire resistance must be carefully considered. Used the
following equations to determine maximum wire length.
For a receiver with a pure resistive load, the maximum wire length is:
V
Lc max =
ps - 10 V - 0.2Rr
0.10Rco
For a receiver with a voltage drop specication, the maximum wire length is:
V
Lc max =
ps - 10 V - Vr
0.10Rco
Where:
c max = Maximum wire length in feet
L
ps = Minimum power supply voltage
V
r = Receiver resistance
R
r = Receiver voltage drop
V
co = Wire resistance per foot from Table 1
R
Multiple Receiver Installation
An advantage of the standard 4 to 20 mA DC output signal used in the Model 657C-1
transmitter is the compatibility with a wide range of receivers. Devices such as the
A-701, A-702 and A-705-20 digital readouts, chart recorders and other process control
equipment can be easily connected and used simultaneously. It is only necessary
that all devices be designed for standard 4 to 20 mA input, the proper polarity of input
connections be observed and the combined receiver resistance or loop voltage not
exceed the maximum for the current loop. If any receiver indicates a negative or
downscale reading, the signal input leads are reversed.
MAINTENANCE
After nal installation of the Model 657C-1 transmitter, no routine maintenance
is required. These devices are not eld repairable and should be returned to the
factory if recalibration or other service is required. After rst obtaining a Returned
Good Authorization (RGA) number, send the material, freight prepaid, to the following
address. Please include a clear description of the problem plus any application
information available.
Dwyer Instruments, Inc.
Attn: Repair Department
102 Highway 212
Michigan City, IN 46360
This symbol indicates waste electrical products should not be disposed
of with household waste. Please recycle where facilities exist. Check with
your Local Authority or retailer for recycling advice.
This equation includes a 20% safety factor and accounts for the common lead carrying
the current for both temperature and humidity channels.
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