| Introduction
The
Model 2000 is a low cost, high accuracy
alternative to RH calibrations that use
chilled mirror or 2-pressure systems, and
is faster than procedures using saturated
salts. The system incorporates advanced
electronics and innovative approaches resulting
in an overall improvement in performance
and reliability.
The
system design provides a capability to
perform calibrations of chart recorders
with remote probes or do a simultaneous
linearity test of multiple data loggers.
The Model 2000 can also be used for a fast
assessment of dew point instruments.
Unlike
larger systems, the Model 2000 reaches
equilibrium within minutes, thus enabling
a full range calibration in much shorter
times. The flexibility of the system gives
laboratories a means for continuous calibration
while larger systems are being used for
other tasks requiring extended calibration
times.
The
Model 2000 measures and controls RH directly.
This eliminates the need for any conversions
or computations that often lead to additional
errors.
In
general, the Model 2000 Humidity Calibrator
will generate chamber conditions for accurate
and repeatable calibration of instruments,
probes, recorders and data loggers.
Theory
of Operation
The
system produces proper humidity values
by continuously monitoring 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.
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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