Download Relative Humidity Sensor

Relative Humidity Sensor
(Order Code RH-DIN)
The Relative Humidity Sensor can be used to measure relative humidity in the air as
part of a weather station, or to
• Monitor indoor humidity for health reasons.
• Optimize conditions in a greenhouse or terrarium.
• Determine when static electrical discharges will be a problem.
• Determine the good days for static electricity demonstrations.
• Study transpiration rates of plants by monitoring relative humidity in sealed jars
containing plants.
The Relative Humidity Sensor can be used with any of the following interfaces
available from Vernier Software:
• Universal Lab Interface for Macintosh or IBM-compatible computers
• Serial Box Interface for Macintosh, IBM-compatible, or eMate™ computers
• Texas Instruments Calculator-Based Laboratory™ (CBL™) System (special
information is included for using the CBL with this sensor)
• MultiPurpose Lab Interface (MPLI) for Apple II or IBM-compatible computers
• Voltage Input Unit (VIU) for Apple II or IBM-compatible computers
How the Relative Humidity Sensor Works
The heart of this sensor is the Hy-Cal Engineering IH-3602-L Integrated Circuit
Humidity Sensor, which uses a capacitive polymer to sense humidity. An integrated
circuit then produces an output voltage which varies with relative humidity.
The holes in the box are to provide air circulation, which is, of course, essential. The
response time of the unit in moving air is much shorter than in still air. In some cases,
you may want to create air currents (by moving the sensor, or using a fan) to speed up
the response of the sensor.
The Integrated Circuit Humidity Sensor is housed inside a small, black, plastic box.
This box not only protects the sensor, but also shields it from light. The sensor is
slightly light sensitive if the light strikes it in just the right way. The box housing is
designed to minimize the amount of light that can penetrate the sensor openings.
The calibration of the sensor is somewhat affected by temperature. This effect is
negligible at the low relative humidity readings, but increases at high humidities. If
you want to correct for this error, you can create different calibration files for different
temperatures. In most cases, this is unnecessary.
The IH-3602-L Integrated Circuit Humidity Sensor is somewhat static sensitive, so if
you open the box, use care to avoid static discharges near the sensor.
Specifications
Range: 0% to 100%
Power: 200 µA @ 5 VDC
Response Time (time for a 90% change in reading):
In still air: 60 minutes (typical)
With vigorous air movement: 40 seconds (typical)
Resolution: (with ULI II or Serial Box Interface) 0.04% RH
(with CBL) 0.17% RH
Specifications for the IH-3602-L Integrated Circuit Humidity Sensor (at 25°C and 5.0
VDC) are given below:
• Total Accuracy (with saturated salt calibration): ±2%
• Total Accuracy (with standard calibration): ±10% RH
• Operating Temperature Range: 0 to 85°C
• Temperature Effect on 0%RH voltage: ±0.007% RH/°C (negligible)
• Temperature Effect on 50%RH voltage: -0.11% RH/°C
• Temperature Effect on 95%RH voltage: -0.22% RH/°C
• Supply Voltage Requirement: 4.5 to 6.4 VDC. Output is ratiometric with power
supply voltage.
NOTE: This product is to be used for educational purposes only. It is not appropriate
for industrial, medical, research, or commercial applications.
Calibration
In most cases, you can simply load an experiment file that is designed for use with the
Relative Humidity Sensor and calibration is taken care of. Calibration files for use
with this sensor are provided on all Vernier Software lab interfacing disks. If you have
an old version of our program and you do not find a file for the Relative Humidity
Sensor, contact Vernier Software for a free disk of calibration files.
For greatest accuracy, this Relative Humidity Sensor can be calibrated. Calibration
can be done by comparison to another instrument that measures relative humidity
(hygrometer or psychrometer). Another way is to use salt solutions. You would do a
two-point calibration as you probably have done with other sensors, such as temperature probes or pH sensors. The salt solutions are used to maintain environments with
different known relative humidities. If you place moist salts in a sealed container, the
air above it will reach a known relative humidity. A table of values is shown below.
The relative humidity above the salt depends slightly on the temperature, so the table
also lists temperature.
Here is a step-by-step procedure:
• Place a handful of salt in the bottom of a jar (quart or liter size is fine).
• Add a little water to the jar so that the salt is wet. The goal is to end up with wet
salt, not to totally dissolve the salt.
• Place the Relative Humidity Sensor in a jar. Do not get the salt or salt solution on
the sensor.
• Seal the jar. We usually use plastic wrap and rubber bands.
• Start the program and allow some time (2-6 hours) for the air inside the sensor to
reach the proper relative humidity level.
• Go through the procedure for the first point of calibration. Type in the relative
humidity for the salt you used as determined from the table.
• Repeat the procedure for the second calibration point using a different salt. Be sure
to allow enough time for the Relative Humidity Sensor to adjust for the change in
humidity.
Even though none of the salts listed here are especially dangerous, use normal
cautions with these chemicals.
2
Here are the relative humidity readings to use when calibrating with salts. These
numbers were taken from Hy-Cal Engineering IH-3602-L data sheets.
All Data in %
Lithium Bromide
Lithium Chloride
Potassium Acetate
Magnesium Chloride
Potassium Carbonate
Magnesium Nitrate
Potassium Iodide
Sodium Chloride
Ammonium Sulfate
Potassium Chloride
Potassium Nitrate
15°C
6.86
11.3
23.40
33.3
43.15
55.87
70.98
75.61
81.70
85.92
95.41
20°C
6.61
11.31
23.11
33.07
43.16
54.38
69.90
75.47
81.34
85.11
94.62
25°C
6.37
11.3
22.51
32.78
43.16
52.89
68.86
75.29
80.99
84.34
93.58
30°C
6.16
11.28
21.61
32.44
43.17
51.4
67.89
75.09
80.63
83.62
92.31
35°C
5.97
11.25
32.05
49.91
66.96
74.87
80.27
82.95
90.79
Using the Relative Humidity Sensor with the CBL System
This sensor can be used with the Calculator-Based Laboratory System from Texas
Instruments. Here are some suggestions for using it with the CBL:
• Connect the CBL unit to the TI graphing calculator using the link ports located on
each unit. Be sure to push both plugs in firmly.
• Using a CBL-DIN adapter, connect the sensor to any of the analog input ports on
the top or left side of the CBL unit (CH1, CH2, or CH3). In most cases, CH1 is
used. The CBL-DIN adapter is available from Vernier Software for $5.
• Run a program to monitor the signal from the sensor. We strongly recommend using
programs developed specifically for use with this sensor. Vernier Software has a
number of programs which support this sensor. One program we recommend is
CHEMBIO from the Vernier Software collection of programs.
The Vernier programs for the CBL are available on the Internet or they can be
obtained in disk form. Once you have these programs, it is a simple matter to load
them into a calculator using TI-GRAPH LINK™.
• To obtain our programs through the World Wide Web, download the files from
our web site:
www.vernier.com
• These programs can also be obtained on disk. The programs are a part of our
CBL Made Easy! Package, which includes Macintosh and IBM-compatible
disks and our CBL Made Easy! user’s guide (order code CMEP, $12).
Other notes for using this sensor with the CBL System
1. The resolution of this probe when used with the CBL System is 0.17% RH.
2. If you write your own program for use with the Relative Humidity Sensor, keep in
mind the following:
• The voltage produced by this probe is linear with respect to the relative humidity
it senses. The slope and intercept for calibrating this sensor are:
Intercept (k0): -22.8
Slope (k1) : 35.1
If you use these values in your program, you will get reasonably accurate relative
humidity readings. Add or modify the command 4 line in your program so that it
reads:
{4,channel number,1,1,-22.8,35.1}→L6
• In place of “channel number”, put a 1, 2, or 3 for the channel the probe will be
connected to on the CBL.
• This probe should be used with the CBL set for the 0 to 5 volt range (operation 1
in command 1).
• For more information on writing programs for TI graphing calculators and the
CBL, see your calculator manual and the CBL System Guidebook.
Vernier
SOFTWARE
8565 S.W. Beaverton-Hillsdale Hwy.
Portland, Oregon 97225-2429
(503) 297-5317 • FAX (503) 297-1760
[email protected] • www.vernier.com
VS
Rev. 3/10/99