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Display Accuracy
Dry-wells are typically calibrated by inserting a calibrated
PRT into one of the wells and making adjustments to the
calibrator’s internal control sensor based on the readings
from the PRT. This has limited value because the unique
characteristics of the reference PRT, which essentially
become “calibrated into” the calibrator, are often quite
different from the thermometers tested by the calibrator.
This is complicated by the presence of significant thermal
gradients in the block and inadequate sensor immersion into
blocks that are simply too short.
Metrology Wells are different. Temperature gradients,
loading effects, and hysteresis have been minimized to make
the calibration of the display much more meaningful. We use
only traceable, accredited PRTs to calibrate Metrology Wells
and our proprietary electronics consistently demonstrate
repeatable accuracy more than ten times better than our
specs, which range from ±0.1°C at the most commonly used
temperatures to ±0.25°C at 661°C.
An application note is available to help better understand
the uncertainties mentioned above. Click here,
Understanding
the uncertainties associated with the use of Metrology Wells
to download the application note in Adobe Acrobat (.pdf)
format.
For even better accuracy, Metrology Wells may be ordered
with built-in electronics for reading external PRTs with
ITS-90 characterizations. (See, Built in Reference
Thermometry below.)
Built-In Reference Thermometry!
Fluke’s Hart Scientific Division has been making the
world’s best thermometer readout devices for quite some
time. Our Super-Thermometer, Black Stack, and Tweener
thermometers are well-known everywhere. Now we’re making our
proprietary Tweener measurement circuitry available directly
in a heat source — our new Metrology Wells.
This optionally built-in input accepts 100-, 25-, and
10-ohm PRTs. It reads thermometer probes accurately from
±0.006°C at 0°C to ±0.027°C at 661°C, not including errors
from the probe. It is compatible with every PRT sold by Hart
and connects to Metrology Wells via a 5-pin DIN connector.
Two things dramatically differentiate the Tweener
circuit from the measurement electronics built into many
dry-wells. First, it accepts unique ITS-90 characterization
coefficients from reference thermometers, which allow you to
take full advantage of the accuracies of those thermometers.
Second, it comes with a traceable, accredited calibration,
providing you full confidence in the integrity of its
measurements.
Stability
Heat sources from Hart have long been known as the most
stable heat sources in the world. It only gets better with
Metrology Wells. Both low-temperature units (Models 9170 and
9171) are stable to ±0.005°C over their full range. Even the
700°C unit (Model 9173) achieves stability of ±0.03°C.
Better stability can only be found in fluid baths and
primary fixed-point devices. The “off-the-shelf controllers”
used by most dry-well manufacturers simply can’t provide
this level of performance.
Axial Uniformity
The EA-10/13 document suggests that dry-wells should include
a zone of maximum temperature homogeneity, which extends for
40 mm (1.54 in), usually at the bottom of a well. Metrology
Wells, however, combine our unique electronics with
dual-zone control and more well depth than is found in
dry-wells to provide homogeneous zones over 60 mm
(2.36 in). Vertical gradients in these zones range from
±0.02°C at 0°C to ±0.4°C at 700°C.
What’s more, Metrology Wells actually have these
specifications published for each unit, and we stand by
them.
Radial Uniformity
Radial uniformity is the difference in temperature between
one well and another well. For poorly designed heat sources,
or when large-diameter probes are used, these differences
can be very large. For Metrology Wells, we define our
specification as the largest temperature difference between
the vertically homogeneous zones of any two wells that are
each 6.4 mm (0.25 in) in diameter or smaller. The cold units
(9170 and 9171) provide radial uniformity of ±0.01°C and the
hot units (9172 and 9173) range from ±0.01°C to ±0.04°C (at
700°C).
Loading
Loading is defined as the change in temperature sensed by a
reference thermometer inserted into the bottom of a well
after the rest of the wells are filled with thermometers,
too.
For Metrology Wells, loading effects are minimized for
the same reasons that axial gradients are minimized. We use
deeper wells than found in dry-wells. And we utilize
proprietary dual-zone controls. Loading effects are as
minimal as ±0.005 °C in the cold units.
Hysteresis
Thermal hysteresis exists far more in internal control
sensors than in good-quality reference PRTs. It is evidenced
by the difference in two external measurements of the same
set-point temperature when that temperature is approached
from two different directions (hotter or colder) and is
usually largest at the midpoint of a heat source’s
temperature range. It exists because control sensors are
typically designed for ruggedness and do not have the
“strain free” design characteristics of SPRTs, or even most
PRTs. For Metrology Wells, hysteresis effects range from
0.025°C to 0.07°C.
Immersion Depth
Immersion depth matters. Not only does it help minimize
axial gradient and loading effects, it helps address the
unique immersion characteristics of each thermometer tested
in the heat source. Those characteristics include the
location and size of the actual sensor within the probe, the
width and thermal mass of the probe, and the lead wires
used to connect the sensor to the outside world. Metrology
Wells feature well depths of 203 mm (8 in) in the Models
9171, 9172, and 9173. The Model 9170 is 160 mm (6.3 in) deep
to facilitate temperature of –45°C.
Other Great Features
A large LCD display, numeric keypad, and on-screen menus
make use of Metrology Wells simple and intuitive. The
display shows the block temperature, built-in reference
thermometer temperature, cutout temperature, stability
criteria, and ramp rate. The user interface can be
configured to display in English, French, or Chinese.
All four models come with an RS-232 serial interface and
the Model 9930, Interface-it software. All are also
compatible with Model 9938 MET/TEMP II software for
completely automated calibrations of RTDs, thermocouples,
and thermistors (Metrology Wells with built-in reference
input options will be compatible with MET/TEMP II in early
2006).
Even without a PC, Metrology Wells have four different
preprogrammed calibration tasks that allow up to eight
temperature set points with “ramp and soak” times between
each. There is an automated “switch test” protocol that
zeros in on the “dead-band” for thermal switches. And a
dedicated °C/°F button allows for easy switching of
temperature units.
Any of six standard inserts may be ordered with each
unit, accommodating a variety of metric- and imperial-sized
probe diameters (view inserts and sizes in
Ordering Information). And Metrology Wells are small
enough and light enough to go anywhere. |
