Omega Products RH70 Installation Manual

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User’s Guide

RH70 and RH70-AC

Handheld T emperature/

Humidity/Dew Point

Indicators

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MODE MODE MODE

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It is the policy of OMEGA to comply with all worldwide safety and EMC/EMI regulations that
apply. OMEGA is constantly pursuing certification of its products to the European New Approach
Directives. OMEGA will add the CE mark to every appropriate device upon certification.

The information contained in this document is believed to be correct, but OMEGA Engineering, Inc. accepts
no liability for any errors it contains, and reserves the right to alter specifications without notice.
WARNING: These products are not designed for use in, and should not be used for, patient-connected applications.

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WARRANTY/DISCLAIMER

OMEGA ENGINEERING, INC. warrants this unit to be free of defects in materials and
workmanship for a period of 13 months from date of purchase. OMEGA’s Warranty
adds an additional one (1) month grace period to the normal one (1) year product
warranty to cover handling and shipping time. This ensures that OMEGA’s
customers receive maximum coverage on each product.

If the unit malfunctions, it must be returned to the factory for evaluation. OMEGA’s
Customer Service Department will issue an Authorized Return (AR) number immediate­ly upon phone or written request. Upon examination by OMEGA, if the unit is found to
be defective, it will be repaired or replaced at no charge. OMEGA’s WARRANTY does
not apply to defects resulting from any action of the purchaser, including but not limit­ed to mishandling, improper interfacing, operation outside of design limits, improper
repair, or unauthorized modification. This WARRANTY is VOID if the unit shows evi­dence of having been tampered with or shows evidence of having been damaged as a
result of excessive corrosion; or current, heat, moisture or vibration; improper specifi­cation; misapplication; misuse or other operating conditions outside of OMEGA’s con­trol. Components which wear are not warranted, including but not limited to contact
points, fuses, and triacs.

OMEGA is pleased to offer suggestions on the use of its various products.
However, OMEGA neither assumes responsibility for any omissions or errors nor
assumes liability for any damages that result from the use of its products in
accordance with information provided by OMEGA, either verbal or written.
OMEGA warrants only that the parts manufactured by it will be as specified and
free of defects. OMEGA MAKES NO OTHER WARRANTIES OR REPRESENTA­TIONS OF ANY KIND WHATSOEVER, EXPRESS OR IMPLIED, EXCEPT THAT OF
TITLE, AND ALL IMPLIED WARRANTIES INCLUDING ANY WARRANTY OF MER­CHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE HEREBY DIS­CLAIMED. LIMITATION OF LIABILITY: The remedies of purchaser set forth herein
are exclusive, and the total liability of OMEGA with respect to this order, whether
based on contract, warranty, negligence, indemnification, strict liability or other­wise, shall not exceed the purchase price of the component upon which liability
is based. In no event shall OMEGA be liable for consequential, incidental or spe­cial damages.

CONDITIONS: Equipment sold by OMEGA is not intended to be used, nor shall it be
used: (1) as a “Basic Component” under 10 CFR 21 (NRC), used in or with any nuclear
installation or activity; or (2) in medical applications or used on humans. Should any
Product(s) be used in or with any nuclear installation or activity, medical application,
used on humans, or misused in any way, OMEGA assumes no responsibility as set
forth in our basic WARRANTY/DISCLAIMER language, and, additionally, purchaser will
indemnify OMEGA and hold OMEGA harmless from any liability or damage
whatsoever arising out of the use of the Product(s) in such a manner.

RETURN REQUESTS/INQUIRIES

Direct all warranty and repair requests/inquiries to the OMEGA Customer Service
Department. BEFORE RETURNING ANY PRODUCT(S) TO OMEGA, PURCHASER
MUST OBTAIN AN AUTHORIZED RETURN (AR) NUMBER FROM OMEGA’S
CUSTOMER SERVICE DEPARTMENT (IN ORDER TO AVOID PROCESSING DELAYS).
The assigned AR number should then be marked on the outside of the return package
and on any correspondence.

The purchaser is responsible for shipping charges, freight, insurance and proper
packaging to prevent breakage in transit.

FOR WARRANTY

RETURNS, please have
the following information available
BEFORE contacting OMEGA:

1. Purchase Order number under which

the product was PURCHASED,

2. Model and serial number of the product

under warranty, and

3. Repair instructions and/or specific

problems relative to the product.

FOR NON-WARRANTY REPAIRS,

consult
OMEGA for current repair charges. Have
the following information available
BEFORE contacting OMEGA:

1. Purchase Order number to cover the

COST of the repair,

2. Model and serial number of the product,

and

3. Repair instructions and/or specific

problems relative to the product.

OMEGA’s policy is to make running changes, not model changes, whenever an improvement is possible.
This affords our customers the latest in technology and engineering.

OMEGA is a registered trademark of OMEGA ENGINEERING, INC.
© Copyright 2001 OMEGA ENGINEERING, INC. All rights reserved. This document may not be copied,

photocopied, reproduced, translated, or reduced to any electronic medium or machine-readable form, in
whole or in part, without the prior written consent of OMEGA ENGINEERING, INC.

UUSASA

MADE

ININ

NOTES

26

i

ii

NOTES

25

NOTES

24

SECTION 1 INTRODUCTION

The OMEGA® RH70 Handheld Temperature /Humidity /
Dew Point Indicator is a microprocessor-based digital
hygrometer / thermometer with built-in date storage and an
RS-232-C interface. The RH70-Ac can be operated using a
rechargeable Ni-Cad battery.

The RH70/RH70-AC Indicator is used for measuring
humidity and temperature in heating, ventilation and air
conditioning industries. The RH70)/RH70-AC calculates the
dew point and displays in either °C or °F. The RH70/RH70-AC
features 2% reading accuracy for humidity and 0.2%
reading accuracy for temperature. From the front
keyboard, the user can select English or Metric Units,
operating mode and data storage controls.

This handheld indicator can store up to 999 lines of data.
Each single line of data contains one temperature and one
relative humidity reading. The dew point reading can not
be saved or transformed (it can only be read on the
display).

Figure 1.1 shows the RH70 Indicator.

Figure 1.1 RH70 Temperature/Humidity/Dew Point Indicator

1

Two different models of the RH70 are available from

OMEGA Engineering

PART
NUMBER

DESCRIPTION

RH70

RH70-AC

PORTABLE HYGROMETER/
THERMOMETER WITH 9V
BATTERY

RH70 WITH RECHARGEABLE
BATTERY AND CHARGER

For accessories, refer to Section 10.

SECTION 2 UNPACKING

Remove the Packing list and verify that all equipment has been
received. If there are any questions about the shipment, please
call OMEGA Customer Service Department.

Upon receipt of shipment, inspect the container and equipment
for any signs of damage. Take particular note of any evidence of
rough handling in transit. Immediately report any damage to the
shipping agent.

NOTE

The carrier will not honor any claims
unless all shipping material is saved for
their examination. After examining and
removing contents, save packing material
and carton in the event reshipment is
necessary.

2

23

SPECIFICATIONS ( Cont’d)

TEMPERATURE

RANGE: -5 TO 175°F (-20 to 80°C)
ACCURACY: 0.2% reading ± 1 digit
RESOLUTION: 0.7°F (0.4°C)
SENSING ELEMENT: 100 ohms Pt RTD sensor

RESPONSE TIME: 60 seconds

SEPARATE RTD PROBE

RANGE: -148 to 572°F (-10 to 300°C)
ACCURACY: 0.2% of reading ±1 digit

OPERATING TEMPERATURE

INSTRUMENT: 32 to 125°F (0 to 50°C)
RH/ TEMP PROBE: -5 to 175°F (-20 to 80°C)
WITH SEPARATE RTD PROBE: -148 to 572°F (-100 to 300°C)

POWER SUPPLY

RH70: One 9V battery
RH70-AC: One 9V Ni-Cad battery with

AC charger

BATTERY LIFE

9V ALKALINE: 78 hours
Ni-CAD RECHARGEABLE: 15 hours
(FULLY CHARGED)

GENERAL

BAUD RATE: 1200 Baud
DISPLAY: 1/2” LCD, 4 digit
ANALOG OUTPUTS

RELATIVE HUMIDITY: 0 to 2 volts
TEMPERATURE: 0 to 2 volts
DIGITAL OUTPUT: standard RS-232-C
DATA STORAGE: Store up to 999

measurements for both
humidity and temperature.

DIMENSIONS

INSTRUMENT: 5.9” H x 3.15” W x 1.2” D
PROBE: 6” L x 0.93 diameter
PROBE CABLE: 5 feet

WEIGHT

INSTRUMENT: 9.49 oz (269 gm)
PROBE AND CABLE: 2.75 oz (78 gm)

22

9.2 REACHING SATURATION LEVELS

The measuring range for Relative Humidity is from 5% to 95%.
The characteristics are not altered when operating in conditions
close to saturation; however, exposure to high humidities and
the moistening risk of the humidity sensor can momentarily falsify
the measurements. For example, a permanent measurement for
an extended duration measurement (>1/2 hour) at higher than
90% RH leads to a phenomena of secondary absorption.

This results in an evolution able to reach approximately +6% of
a %RH measurement close to saturation. The evolution is memorized
by the humidity sensor and it can take up to 24 hours of drying
to return the sensor to its original
characteristics.

SECTION 10 ACCESSORIES

SECTION 11 SPECIFICATIONS

TEMPERATURE

RANGE: 5 to 95%RH
ACCURACY: 2%RH ± 1 digit
RESOLUTION: 0.1%RH
TEMP DRIFT: ±0.5% RH per 10°C
RESPONSE TIME: Up to 90% of fluctuations in 10 s
SENSING ELEMENT: Thin film capacitive type sensor

DEW POINT

RANGE: -5 to 175°F (-20 to 80°C)
SENSING ELEMENT: Mathematically calculated

from RH and temperature
readings (using a look-up table)

PART NUMBER DESCRIPTION

RH70-RP Replacement Humidity/

temperature probe with
5 ft cable

RH70-SF Sinter Filter Cap

Please note that the following items should be in a box:

MODEL RH70

QTY DESCRIPTION QTY DESCRIPTION

MODEL RH70-AC

1 RH70 Indicator
1 Humidity/ Temperature Probe

with 5 ft cable
1 RS-232 Cable (5 ft)
1 Analog Output Cable (5 ft)
1 9V Battery

1 Rugged Carrying Case
1 Operator’s Manual

1 RH70-AC Indicator
1 Humidity/ Temperature Probe

with 5 ft cable
1 RS-232 Cable (5 ft)
1 Analog Output Cable (5 ft)
1 9V Rechargeable Ni-Cad

Battery

1 Rugged Carrying Case
1 Operator’s Manual

SECTION 3 DISPLAY AND KEYPADS OPERATION

Figure 3-1. RH70/RH70-AC

3

ITEM KEY DESCRIPTION(refer to Figure 3

1 -- Connector for the RS-232 Cable

2 -- Connector for the Humidity/Temperature Probe
3 -- Connector for the Analog Output Cable (0-2V)
4 -- Instrument LCD Display (4 Digits)
5 °F Pressing the “°F” key displays a temperature

reading in degrees Fahrenheit. The reading is
updated 2.5 times per second.

6 °C Pressing the “°C” key displays a temperature

reading in degrees Celsius. The reading is
updated 2.5 times per second.

7 MAX Pressing the “MAX” key displays the highest value

in either Temperature or Relative Humidity. This
feature is not applicable to the Dew Point values.
The maximum value (since turning on the RH70)
alternates with the flashing”H”. To exit from the
MIN/MAX mode, and continue to monitor
temperature, press the “TEMP” key. To exit from
the MIN/MAX mode and continue to monitor
humidity, press the “%RH” key.

8 MIN Pressing the “MIN” key displays the highest value

in either Temperature or Relative Humidity. This
feature is not applicable to the Dew Point values.
The maximum value (since turning on the RH70)
alternates with the flashing”L”. To exit from the
MIN/MAX mode, and continue to monitor
temperature, press the “TEMP” key. To exit from
the MIN/MAX mode and continue to monitor
humidity, press the “%RH” key.

9 The RH70 is turned ON by pressing the“ON/OFF”

key. Pressing it again turns the RH70 off.

10 SAVE Pressing the “SAVE” key stores the current temperature

and relative humidity readings at the instant the
key is pressed, i.e. 98.0°F and 33.2% RH. Dew Point
values are NOT saved. See NOTES on next page

ON
OFF

4

21

PROBLEM CONDITION SOLUTION

“E-04” on display No communication Check RS-232

through the RS-232 connection and
data link with a peripherals

“E-06” on display Relative Humidity

lower than measuring ----

“E-07” on display Relative Humidity

higher than measuring ----

“E-08” on display Temperature lower

“E-09” on display Temperature higher

than measuring range. Connect probe

“E-10” on display Relative Humidity not

within range for ----
calculating the Dew
Point (3.0% to 97.0% RH)

“E-11” on display Temperature not within

range for calculating the ---­Dew Point (-5 to 175°F
or -20 to 80*C)

“E-XX” on display Error code for 50 or Contact OMEGA

higher (shows internal
errors)

“E-51” on display Some hardware has Contact OMEGA

come loose internally.

20

PROBLEM CONDITION SOLUTION

“L” on display Weak battery Replace battery

(less than 6V) (refer to Section

8.1 on how to
replace battery)

“E-02” on display Instrument has Clear memory

reached max. using “DUMP”

key (See Section
allowable number 5.3.3).
of stored data
points (at 999).

“E-03” on display Storage memory Press “SAVE”

is empty key to store

measurements.

SECTION 8 POWER SOURCE INFORMATION

8.1 BATTERY REPLACEMENT

To replace the battery, gently open the battery cover on the
back of the unit (use a small screwdriver, if necessary). Plug
in the 9V battery observing polarity, place in compartment
and gently close the cover.

8.2 RH70-AC POWER INFORMATION

Check to see that a Ni-Cad battery is in the RH70-AC. Plug
in one side of the AC Charging Adapter into the socket on
the side of the RH70-AC and the other end to a 110VAC
outlet.

The RH70-AC will NOT work with the AC charging adapter
alone. The rechargeable Ni-Cad battery must be installed in
order to operate the unit.

ITEM KEY DESCRIPTION(refer to Figure 3

The stored values are retained,

even when the unit is turned
off.
Pressing the “SAVE” key while
viewing a temperature reading
automatically saves the corresponding
humidity reading at that moment and
vice versa.

11 DUMP Pressing the “DUMP” key down-loads the

stored measurements to a connected printer or
computer (IBM XT, AT, or compatible) through
the RS-232 cable.

TO CLEAR STORED DATA: Data is maintained
in the RH70 memory for a minimum of 1 year or until
it is cleared. Clearing the memory is done by pressing
and holding down the “DUMP” key while turning ON
the RH70.

12 %RH Pressing the “5RH” key displays the humidity reading in

% Relative Humidity (5 to 95% RH). The reading is updated

2.5 times per second. Refer to Section 9.2, Reaching
Saturation, if the sensor becomes saturated.

Notice that the RH reading is left
always displayed by the LEFT
most digits of the meter display.

13 Pressing the “DEW PNT” key displays a calculated Dew

Point reading. The user can toggle the Dew Point value
between °F and °C. Pressing the “DEW PNT” key and
then the “°F” key will display the Dew Point in degrees
Fahrenheit. Pressing the “DEW PNT” key and then the
“°C” key will display the Dew Point in degrees Celsius.

14 TEMP Pressing the “TEMP” key displays the temperature. When

the RH70 is first turned on, it defaults to degrees Fahrenheit.
Press “°F or °C” to select the desired units for displaying
temperature.

DEW
PNT

5

NOTE

NOTE

6

SECTION 4 ASSEMBLY

4.1 PROBE ASSEMBLY

Connect the Humidity/Temperature Probe to the middle
connector on top of the RH70. Refer to item #2 in Figure 3-1
for the location of the probe connector. The male connector
on the cable and the female connector on the instrument are
keyed, so make sure they are lined up before connecting to
each other. Tighten knurled threaded fitting to secure the
cable to the instrument.

4.2 BATTERY ASSEMBLY

Gently open the battery cover on the back of the unit (use a small
screwdriver, if necessary). Plug in the 9V battery observing
polarity, place in compartment and gently close the cover.

4.3 AC CHARGING ADAPTER (RH70-AC ONLY)

1. Install the rechargeable Ni-Cad battery as described in
Section 4.2

2. Plug in one side of the AC Charging Adapter into the socket
on the side of the RH70-AC and the other end to a 110VAC
wall socket. Refer to 3-1 for the location of the socket on the

meter.

NOTE

The RH70-AC will NOT work
with the AC charging adapter
alone. The rechargeable Ni-Cad
battery must be installed in
order to operate the unit

WARNING

DO NOT OPERATE THE RH70-AC
WITH A NON-RECHARGEABLE
BATTERY AND THE AC CHARGING
ADAPTER TOGETHER. USE OF A
NONRECHARGEABLE BATTERY AND
THE RECHARGER CAN CAUSE THE
BATTERY TO EXPLODE!!

19

2. Carefully fit the probe tip in the closed glass container with

the solution simulating 11% relative humidity. Make sure the
assembly is sealed. Let it sit for approximately one (1) hour.
Adjust the pot P4 until the display reads 11.0. Refer to Section 7.2

3. Repeat step 2 with the sodium chloride solution.

Carefully fit the probe tip in the closed container with the
solution simulating 76% RH for approximately one (1) hour.
Adjust P3 until the display reads 76.0.

4. Repeat steps 2 and 3 until 11% and 76% relative humidity

can be achieved without resetting pots P3 or P4.

5. Different relative humidities can be simulated by using any

of the following saturated solutions.

TEMPERATURE VS RELATIVE HUMIDITY

Ammonium Sulfate
Sodium Chloride
Ammonium Nitrate
Potassium Carbonate
Potassium Acetate
Lithium Chloride

81
76
65
44
22
11

80
75
62
43
22
11

SALT

SOLUTION

% RH AT

68°F (20°C)

% RH AT

77°F (25°C)

7.4 TEMPERATURE CALIBRATION

Temperature is measured using a precision RTD sensor
which is very stable by nature. The sensor electronics
generally do not go out of calibration.

Periodic checks can be performed to determine the
equipment accuracy in the temperature range. This is done
by comparing the RH70 readings to known temperature
values.

In the event that the RH70 is out of specification, contact
OMEGA. Temperature calibration can not be performed by
the user. Due to the complexity of the procedure, it is
required that the RH70 be returned to OMEGA Engineering
for recalibration. Do not attempt to recalibrate the RH70

yourself.

18

7.3 HUMIDITY CALIBRATION

All calibrations should be made at room temperature (68°F to
77°F (20°C to 25°C)). Refer to Figure 7-1 for the location of the
calibration potentiometers (pots).

1. Connect the probe and the battery as described i

1. Carefully remove the sensor cap from the probe handle.
This is done by grasping the sensor cap and the lower
end of the probe enclosure firmly. Carefully twist in
opposite directions of each other and pull. Do not bend
the assembly since you could damage it. Be careful not to touch
the sensors.

7

SECTION 5 OPERATING INSTRUCTIONS

5.1 PRINCIPLES OF OPERATION

RELATIVE HUMIDITY (%RH): This relates to the ratio of
actual water vapor pressure in the air to the vapor pressure
in saturated air at the same temperature. This unit measures
from 5% to 95%. Refer to Section 8 if saturation occurs.
Typical applications: monitoring computer rooms,
test chambers, pharmaceutical and food packaging places.

DEW POINT/FROST POINT: The dew point is the temperature
at which water vapor begins to condense. If the dew point
occurs below 32°F, it is called the frost point. Typical
applications: Heat treating, annealing atmospheres, dryer
control, instrument air monitoring, meteorological/environmental
measurements, and external paintwork.

5.2 MEASUREMENT PROCEDURES

1. Connect the probe and the battery as described in Section 4.

2. Turn on the RH70-AC by pressing the “ON/OFF” key. The

unit will display for two seconds each of the following.
“88.8.8” (this is an internal test) then,
the battery voltage, then “P” followed by three numbers
where the 3-digit number indicates how many data points
have been “saved” in the memory, +1. For example, when
3 readings are stored in memory, the display will show
“P004”. These three stored values are retained in memory,

even after shut-off.

NOTE

The RH70-AC is fully
operational while the Ni­Cad battery is recharging.
After 0ne (1) hour the bat-

Refer to Section 3 for complete
operating details of each key on

NOTE

3. Now you must instruct the RH70 what measurement to
display.

For TEMPERATURE: Press the “TEMP” key.
Pressing the “°F” or “°C” key will cause the display
to show the Temperature in the desired units.

For %RH: Press the %RH key.

For DEW POINT: Press the “DEW PNT” key.

Pressing the “°F” or “°C” key will cause the display
to show the Dew Point in the desired units.

4. Press the “ON/OFF” key to turn off the RH70.

5.3 SAVE/DUMP/CLEAR MEMORY

5.3.1 The Save Feature

Pressing the “SAVE” key will store BOTH the humidity AND the
temperature values the moment the key is pressed. The two are
simultaneously saved as one line of data. This means that pressing
the “SAVE” key while viewing the temperature reading automatically
saves the corresponding humidity reading AT THE SAME INSTANT.

Notice that when the “SAVE” key is pressed, the display will flash the
letter “P” followed by a 3-digit number (i.e. P204). This number indicates
how many data points have been saved in the memory, plus 1.

For example, when the FIRST data point is saved, “P002” will flash on
the display. This indicates that the RH70 has saved the reading and the
memory is ready and pointing to the next available address. However,
when the saved data is later down-loaded through the RS-232
connector, that first point will be labeled as “P001”. All the rest of the
points will follow in sequence. Refer to Section 5.5, “Sample RS-232
Data Format”.

Data is maintained in the RH70 memory for a minimum of 1 year or
until it is cleared. Refer to Memory section below for instructions to
clear the memory. A maximum of 999 temperature/humidity readings
can be saved.

Note that the Dew Point reading CAN NOT be saved or output to a
computer. The Dew Point can only be read on the display.

8

17

It is important that the solutions must remain saturated for
those relative humidities to be valid. Increases in ambient
temperature will require agitation of the solution to ensure
saturated conditions. To prevent this from becoming a problem,
the solutions, once prepared, should be kept in a
temperature-controlled room.

7.2 PREPARING A SALT REFERENCE

To calibrate the humidity portion of the RH70, it is necessary
for the user to prepare two saturated salt solutions - one to
simulate 11% RH and one to simulate 76% RH (at 68°F).

Equipment Needed:

1. 2 glass containers (fitted to probe end without endcap)

and lid with a good seal.

2. Distilled water

3. Lithium Chloride (LiCl) (for 11% RH) and

4. Sodium Chloride (NaCl) (for 76% RH).

To simulate 11% RH, fill the bottom of two glass containers
approximately 1/4 full of distilled water. Add lithium chloride
until the solution is saturated (solution is saturated when an
additional crystal of solute which is added to the solution does
not enter into the solution). Keep the chamber closed for at least
one hour, with the temperature of the solution constant to prevent
the concentration of the salt solution form changing ( 68°F or 20°C).

To simulate 76% RH, follow the same procedure as above.
But in this case, in the second glass container, add sodium
chloride to the distilled water (until saturated). Let the solution
sit for at least one hour.

16

6.3 HUMIDITY SENSOR HANDLING

Due to its design, and the ease of access, the humidity sensor
must be handles with care. AVOID ANY CONTACT with
fingers or with products which could be harmful to the good
permeability of the electrode and dielectric. The sensor must
not come in contact with dirt or other foreign material.
Incorrect readings can be caused by build-up of material,
which can increase or decrease the RH reading.

6.4 HUMIDITY SENSOR CLEANING

Use no solvents stronger than DISTILLED WATER to clean
the humidity sensor. If necessary, rinse the sensor with
distilled water, being careful not to touch it with your fingers.
After rinsing, the sensor should air dry for at least 24 hours
(preferably over an air flow register).

SECTION 7 CALIBRATION

The RH70 is calibrated at the factory.
The warranty is void if the label on the
probe handle is tampered with. Contact
OMEGA Engineering for service information

The following procedures are necessary
only if the unit is operating out of
specification and is no longer under
warranty. These procedures should be
performed by a qualified technician.

7.1 SALT REFERENCES

When a closed air space is maintained in equilibrium with a
saturated aqueous salt solution, the relative humidity of the
enclosed air remains constant as long as the temperature and
pressure do not change.

NOTE

WARNING

DO NOT EXPOSE THE RH70 HUMIDITY SENSOR

TO TEMPERATURES OVER 175°F (80°C) !

DOING SO , WILL DESTROY THE SENSOR.

9

There is no internal clock in the RH70 or RH70-AC for record
keeping purposes. It is up to you to keep track of the time
when the readings are saved.

5.3.2 The Dump Feature

Pressing the “DUMP” key will down-load (or dump) the
measurements stored to a connected serial printer or computer
(IBM XT, AT, or compatible). These points are still retained in
memory, however, even after being dumped through the
RS-232 connector. Refer to Section 5.5 for complete details on
the RS-232 feature, including more “DUMP” details.

5.3.3 Clear Memory

To clear the memory, press and hold down the “DUMP”
key while turning ON the RH70. Doing this will erase all

the “saved” data points and get the instrument ready for new
readings. The erased data can not be retrieved once it is
cleared.

5.4 ANALOG OUTPUTS

The RH70 features two 0-2 volt analog output signals on
demand. Both voltages are present at the analog output
connector at all times. These two signals correspond to the
displayed temperature and humidity readings and can be
observed as analog voltages on a printout or on the
computer screen. A typical printout looks like the following:

NOTE

If the display reads “E-04”, when the
“DUMP” key is pressed, the RH70 is
indicating that nothing is connected
to the RS-232 port or that there is a
possible transmission failure. Check
all the connections from the RH70 to
the printer/computer.

0.670 0.579

0.671 0.588

0.676 1.011

0.682 1.047

0.677 1.877

The first column represents temperature. The second
column represents relative humidity.

These two analog signals can be connected to a variety of
equipment, including chart recorders, remote displays and
an analog input card in a personal computer. The three wires
in the cable allow for continuous monitoring of relative
humidity and temperature simultaneously.

The RH70 analog outputs for temperature and humidity are
transmitted via single-end outputs with a common ground.
Locate the five-foot cable with 3 exposed wires (one grey,
one green, and one brown) at one end and a connector at the
other end. To access both analog output signals on the RH70,
plug the connector side of the cable into the analog output
connector on top of the RH70 (right hand connector #3 in
Figure 3-1). Referring to Figure 5-1, connect the tree exposed
wires to the instrument/recorded that will accept a 0-2 volt
analog signal.

5.4.1 Temperature Analog Signal

The 0-2 volt analog output signal on the RH70 is accessed
by the BROWN wire (green is ground) of the analog output
cable. The 0-2 volt range of the analog output corresponds
to the full temperature range of the instrument itself (when
used with a separate RTD probe). This equivalent temperature
range is -148°F to 572° F (-100° C to 300°C).

WARNING

DO NOT EXPOSE THE STANDARD RH70

HUMIDITY/TEMPERATURE PROBE TO TEMPERATURES

OVER 175°F (80°C) !

DOING SO , WILL DESTROY THE HUMIDITY SENSOR

!!

10

Once the handshaking is established, the ASCII data is
transferred at 12 baud. Each line (record) of data is
terminated with a “carriage return and line feed”. At the end
of the data transmission, the RH70 will generate an End of
File (“EOF”) character. The PC program can detect the EOF
character and return to the operating system.

SECTION 6 SENSOR INFORMATION AND

MAINTENANCE

6.1 TEMPERATURE SENSOR

The temperature sensor in the Humidity/Temperature probe

consists of a Platinum RTD sensor (100 ohm). It conforms

to the European (E) curve where o = 0.00385 ohms/ohm/°C.

An RTD (resistance temperature detector) operates on the
principle of change in electrical resistance in wire as a
function of temperature. These sensors are desirable when
accuracy over a wide temperature range is important. RTD’s
are stable over long periods of continuous use, which makes
them very reliable.

6.2 HUMIDITY SENSOR

The relative humidity sensor is a thin film capacitance
sensor. It consists of a single capacitor, capacitance of
which varies according to the water molecules absorbed by
the active polymer.

The characteristics are not altered when operating in
conditions close to saturation. However, using the sensor at
high humidity and the moistening risk of the sensitive
element can momentarily falsify the measurements.

A permanent measurement or an extended duration
measurement (>1/2 hour) higher than 90% leads to a
phenomena of “secondary absorption” which results in an
evolution able to reach about ±6% of a relative humidity
close to saturation. This evolution is memorized by the
sensor and a return to ambient conditions (40% up to 50%
RH) of 24 hours might be necessary to find again the origin
characteristics.

15

(

5.5.4 Basic Emulator Program

5.5.5 RS-232 Signal Details

The RS-232 signal voltage levels are +9 volts and -9 volts. A
signal of +9 volts represents a space (logical “0”). A signal of ­9 volts represents a mark (logical “1”).

Protocol is 1 start bit, 7 data bits, 1 parity bit, 1 stop bit Data.

Format is ASCII.

Parity is odd parity

Baud Rate is 1200 baud.

When the “DUMP” key is pressed on the instrument a RTS
signal becomes available on the RS-232 plug (logical “1”).
The PC program or printer must respond with a CTS signal
(logical “0”). If this CTS signal does not come within 1
second, the instrument will display “E-04” (error 4) meaning
no data communication possible and terminates the dump
mode.

14

The following formula is then used to convert the analog
output voltage signal to an equivalent temperature value:

Temp (°F) = (360 x analog voltage) - 148
Temp (°C) = (200 x analog voltage) - 100

5.4.2 Humidity Analog Signal

The 0-2 volt analog output signal on the RH70 is accessed
by the grey wire (green is ground) of the analog output
cable. The 0-2 volt range of the analog output corresponds
to the humidity range of the electronics of the instrument
itself: 0% RH to 100% RH

The following formula is used to convert the humidity analog
signal to the equivalent humidity value:

Humidity (%RH) = 50 analog voltage

5.5 RS-232 Outputs

The RH70 can be used as a portable “data logger” to store
as many as 999 temperature and relative humidity values.
This feature is very simple to use. Press the “SAVE” key to
store current readings. When it is time to down-load the
information, simply connect the RS-232 cable and press the
“DUMP” key. A sample printout you can expect to see on a
computer screen or printer is shown below:

SAMPLE RS-232 PRINTOUT

5.5.1 RS-232 Set-up Details

To use the RS-232 feature, first “SAVE” a number of data
points into the RH&) memory. Refer to Section 5.3.1, Save
Feature. Next, connect the RS-232 cable to an RS-232 port.
Plug one end of the RS-232 cable into the left-most
connector on top of the RH70. (Refer to Figure 3-1 for the
RS-232 connector location - #1). Connect the other end of
the cable to a serial printer or computer (IBM XT, AT or

compatible).

11

001 72.5 dgF 35.1 %RH
002 73.2 dgF 71.9 %RH
003 78.7 dgF 92.4 %RH
004 81.5 dgF 94.1 %RH

Refer to Figure 5-2 for the pin-outs on the RS-232 connector

12

13

5.5.2 Down-Loading to a Serial Printer

1. To output data to a serial RS-232 printer, connect the
RS-232 cable between the RH70 and the printer port.
(Refer to Table 5-2 for wiring information).

2. Once connected, simply press the “DUMP” key and the
data will print out essentially the same way as shown
in Section 5.5

5.5.3 Down-Loading to a Computer

The stored data points in the RH70 can be down-loaded to
an IBM XT, AT, or compatible personal computer.
(Communication is one-way only!!!)

1. Make sure there is a program in your computer to allow
data to be sent by the RH70 and received by the
computer. A simple emulator program is shown in
Section 5.5.4.

2. Connect the RS-232 cable between the RH-232 and the
computer. (Refer to TAble 5-2 for wiring information.

3. Once the communication is established and the RS-232
cable is connected, simply press the “DUMP” key and the
data will transfer through the cable to the computer and
the printout will essentially look the same way as shown
in Section 5.5

Refer to the OMEGA Data
Acquisition Handbook for software
that has advanced data analysis
capabilities. An example could be

Labtech

®

Notebook software.

NOTE

Where Do I Find Everything I Need for

Process Measurement and Control?

OMEGA…Of Course!

Shop online at www.omega.com

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