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Theory of Operation
The system
produces proper humidity values by continuouslym onitoring
chamber conditions with a very accurate internal probe. The
probe measures both the RH and temperature, then sends this
information back to microprocessor based controllers.
Internally there are 2 pumps connected to a closed-loop
system, one pump for dry air and the second for wet-air. The
controllers call for wet or dry air and the pumps respond
accordingly. This results in very precise, very fast
response for chamber stability. A high volume turbo-axial
fan inside the chamber gives the uniformity of RH and
temperature.
System Description
The Model
2000 uses digital control for fast response and stability.
The controllers are microprocessor based and are programmed
for optimum performance for the application. A very accurate
probe is installed inside the chamber to measure humidity
and temperature. The probe is calibrated against a
2-pressure calibrated standard.
The chamber
is sufficiently large to accommodate several data loggers
and also can accept multiple probes for simultaneous
calibrations. Dimensions are 6" (125mm) in diameter and 9"
(225mm) overall depth, with a useable depth of 6" (125mm).
To reduce the effects of outside temperature the chamber is
wrapped with several layers of foam insulation. Chamber
doors are interchangeable and are easily removed and
installed.
Air within
the chamber is circulated by a turbo-axial fan having high
CFM air flow. This air movement assures optimum uniformity
of the humidity and temperature within the chamber and
prevents any possibility for temperature stratification.
The
saturator (wet air) and desiccant (dry air) are connected as
closed-loop systems that recirculate the air in the chamber
on a demand basis. This results in longer periods between
the replacement or regeneration of wet/dry components.
System Performance
Typically,
the Model 2000 will reach stability within 5 minutes for a
30%RH step change after reaching temperature equilibrium. As
the humidity level increases the system must work a little
harder therefore the settling times will increase to about 8
minutes. There is very little difference in the response
times for increasing or decreasing humidity values, even
when operating at a high controlled humidity for long
periods.
Response
time for temperature will vary depending on an increase or
decrease in settings. Measurements have shown that when
starting at 23C the rate at which the temperature decreases
will be about 1 degree C/minute. The response rate when
increasing from 23C to the highest value will be about 3
degrees C/minute.
Chamber Calibration of Humidity and Temperature
Prior to
installation all probes that are used within the chamber are
calibrated against a reference standard that has been
calibrated using a 2-pressure system with an accuracy of
+/-0.5%RH. Calibration is done by an Accredited NVLAP
Certified laboratory. The operating and control probes are
measured at 3 values of humidity, low, mid and high, and
adjusted to be within +/-0.2%RH of the standard. Temperature
is also adjusted to the accuracy of a precision thermometer
or precision calibrated sensor.
After the
probe is installed inside the chamber a multi-port test door
is mounted. This door is designed to check the uniformity
and stability of the chamber, and to do a final It systems"
calibration. The procedure is to install the "primary"
standard into the chamber to a depth of 6" (125mm). The
chamber control is then set at a mid RH value and the
chamber is stabilized for 30 minutes when control is within
+/-0.1. The reading of the standard probe is then compared
with the reading of the internal probe as seen on the
controller display. If there is any deviation the chamber
reading is adjusted using the calibration capabilities of
the controller. The chamber readings are then checked at the
low and high RH values, allowing at least 1 hour at each
point to assure stabilization. Readings are typically less
than +/-0.2 over the entire range of measurement.
The same
procedure is used for setting the temperature of the
chamber. The temperature set point is 23C. Any deviation
from the standard reading is corrected with adjusted of the
controller to be within +/-0. 1 of the reference standard.
Chamber Uniformity Test
After the
chamber has been calibrated for humidity and temperature at
the 3 RH values the chamber is brought back to 23C and
50%RH. Again the system is stabilized for 1 hour. The test
door has 4 positions to check each quadrant of the chamber.
The primary standard probe is moved to 5", 4" and 3" depths
at each position and after chamber stabilization readings
are again taken. Uniformity of chamber is typically +/-0.1C
in temperature and +/-0.2%RH.
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External Reference Instruments
The
procedure for calibrating the Model 2000 is based on the use
of an accurate reference instrument. For factory calibration
the NVLAP calibrated primary probe is used. However, it is
possible for the customer to establish a calibration of the
chamber using in-house standards. An example would be to use
a chilled-mirror instrument that is connected to the
sampling ports of the Model 2000. In this instance a sample
of the chamber conditions is pulled through the chilled
mirror and RH is read as a computation from the dew point
reading. As with any method that requires a conversion to
compute another parameter, there is additional error because
of the calculation formula.
For this
reason it is recommended that the Model 2000 use a direct
reading of RH instead of a conversion, and if dew point is
the required parameter that a dew point instrument be used
to give a direct reading.
Model 200OREF Instrument
The Model
200OREF is a versatile instrument that automatically
calculates dew point based on the RH and temperature input.
Although this reference does not have the highest dew point
accuracies, as the expensive chilled-mirrors or similar
systems, the accuracy is often suitable for routine tests
and checks of dew point transmitters and probes. The Model
200OREF is very inexpensive, costing less than 1/10th than
the cost of dew point reference instruments.
The Model
200OREF consists of a RH/T probe calibrated against the
NVLAP standard probe and a hand held display instrument with
internal microprocessor to convert the RH/T values to dew
point. The algorithm used is based on ASTM formula.
Annual Calibration Procedures and Recommendations
There are
several methods for having the annual calibration performed.
These are the procedures that can be followed.
The
internal probe used in the chamber has been selected because
it is ASIC based and offers a unique advantage. The
electronics is ASIC (Application Specific Integrated
Circuit) with memory to store calibration data. This enables
the user to just send the probes to a laboratory already set
up to perform the calibration of such probes.
Thunder
Scientific has a calibration procedure for these probes. The
calibration uses the 2-pressure generator and compares "as
found" and "as left" data. Thunder can supply all the
necessary documentation for Certification and Calibration
Certificates.
For
simplified calibration procedures use of the Model 200OREF
offers convenience and flexibility. It involves sending the
entire instrument to a laboratory qualified and certified to
perform a humidity calibration or because it is a
self-contained reference package calibration can be done
using in-house standards (such as a Thunder Scientific
2-pressure generator). The laboratory inserts the probe into
the humidity chamber and using the internal software a
complete check and calibration of the Model 200OREF can be
done. All "as found" and "as left" tests can be done
internally.
Data Output and Serial Communications
The
standard auxiliary output of the Model 2000 is a 0-5V analog
output. The output shown is the measurement of the internal
reference probe. The 0-5V signal corresponds to 0-100%RH and
-40 to +60C. The output will drive a recorder or similar
instrument.
As an
option the Model 2000 can be supplied with RS-485 output.
This enables the customer to integrate their own software
and programming to satisfy the measurement requirements for
linearity or calibration of devices.
When the
RS-485 option is purchased the user can also purchase the
software and programming for specific tests at specific
cycles. An extremely wide range of procedures for automatic
linearity tests can be easily entered by the customer. The
program is very easy to operate and offers an enormous
amount of flexibility to the metrology group.
Calibration Certificates and Documentation
The primary
standard used at the factory for all calibrations are the
probes that have been calibrated by Thunder Scientific in
the 2-pressure generator. Once again, Thunder Scientific is
a Certified Accredited NVLAP company. The primary standards
are used to perform a final system calibration. A
Certificate of Calibration for the internal probe and the
primary probes are included with each system.
Summary
The
Model 2000 is a high accuracy system with specifications and
performance that is equal to or better than some systems
costing considerably more. It offers simplicity of
operation, fast response, high accuracy, low maintenance and
affordability. Because of increased productivity due to
faster turn around times the Model 2000 offers the user
considerable cost savings for everyday, routine humidity
calibrations.
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