Application Note

4 Fluke Corporation Eliminating sensor errors in loop calibrations
screen below. With an analog
transmitter, you will need to
mechanically adjust the Zero
and Span adjustments when
sourcing the appropriate tem-
perature values. The 754 has
a convienent menu key that
allows you to easily set the cor-
rect value on the dry-well with
a single button press.
Calibrating and adjusting
measurement systems using
characterized sensors and
calibration constants
Another method of reducing
uncertainty and optimizing tem-
perature measurement systems
is to carefully characterize the
temperature sensor, calculate
correction coefficients and load
these correction coefficients into
the measurement equipment.
This is the method used in the
Rosemount 644H example on
the previous page. This method
does a better job of reducing
the error in the measurement
system that comes from the
sensor. But it requires trans-
mitters that have a correction
or linearization algorithm that
can accommodate the sensor.
For example, Platinum RTDs
typically use the Callendar-
Van Dusen (CVD) equation for
linearizing the sensor’s output.
A characterized sensor will
provide unique CVD coefficients
that can be input into the trans-
mitter, allowing its conversion
algorithm to more closely match
the unique characteristics of the
sensor.
The Fluke 754 connected
with a dry-well can help
to collect the necessary
information to characterize the
sensor, but additional software
and resources will be needed
to take this data and generate
new CVD constants. Examples
of the required software include
Fluke Calibration’s TableWare.
Other software that could
be used include Mathcad,
Mathematica, Maple or Excel.
But these packages require
considerable knowledge of
the equations used to linearize
a sensor and the ability to
perform curve-fitting of the
collected data.
The method of character-
izing a probe is similar to the
procedure above, but rather
than measuring the output of
the transmitter, the output of
the sensor is connected directly
to the 754. An example of data
collected by a 754 on a temper-
ature sensor is shown below.
Data like this can be entered
into Fluke Calibration’s software
using the screens in Figure 6
and then unique CVD constants
calculated for that probe.
These coefficients can then
be entered into a suitable
measurement device that allows
its linearizations to match the
characteristics of the probe.
Summary
Using a dry-well in combina-
tion with a process calibrator
allows measurement systems
to be verified and adjusted to
optimize measurement perfor-
mance. By verifying the entire
measurement system, unique
characteristics of the sensing
element can be combined with
the measurement electronics
Figure 6. TableWare software from Fluke Calibration calculates unique
CVD constants that match the characteristics of the probe.
to minimize measurement
error. This can result in a
significant reduction in mea-
surement errors. The Fluke 754
Documenting Process Calibra-
tor, combined with a Fluke
Calibration dry-well makes this
process faster and easier.
Fluke Calibration
PO Box 9090,
Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
In the U.S.A. [phone] or Fax [phone]
In Europe/M-East/Africa [phone] or Fax [phone]
In Canada (800)-36-FLUKE or Fax [phone]
From other countries [phone] or Fax [phone]
Web access: http://www.flukecal.com
©2004-2011 Fluke Corporation. Specifications subject to change without notice.
Printed in U.S.A. 11/2011 2148146C A-EN-N
Modification of this document is not permitted without written permission from Fluke Corporation.
Fluke Calibration. Precision, performance, confidence.
™