Fluke 718, 725, 753, 754, 707 Service Guide

Eliminating sensor

errors in loop

calibrations

Calibrating a loop is more than just

4 mA to 20 mA

Significant performance
improvement can realized by
optimizing the loop calibration
measurement system to better
accommodate the unique char­acteristics of the temperature
sensing element. All tempera­ture probes and their sensing
elements are unique, with
variations in materials, con­struction and usage, or exposure
to different environments. This
uniqueness continues through­out the useful life of the sensor,
in the form of drift due to
mechanical shock and vibra­tion or to contamination of the
materials when exposed to the
material they are measuring.
Only through periodic verifica­tion can these differences and
changes be accommodated,
improving total measurement
performance.

Temperature plays an impor­tant role in many industrial and
commercial processes. Examples
range from sterilization in
pharmaceutical companies,
metal heat-treatment to ensure
optimal strength in aerospace
applications, temperature
verification in a cold storage
warehouse, and atmospheric
and oceanographic research. In
all temperature measurement
applications, the sensor strongly
affects the results; unfortu­nately, many measurements are
made without optimizing the
system to get the best perfor­mance from the temperature
transducer.

The majority of process
temperature measurements are
performed using a sensing ele­ment connected to a transmitter.

Figure 1. Diagram of a typical process temperature measurement system.

Figure 1 shows a diagram of a
common configuration.

In many applications, it is
common to verify the elements
of the measurement system
separately, but in doing so,
significant improvements made
possible by considering the
system as a whole are ignored.
One of the main reasons the
elements are verified or cali­brated separately is that it is
often considered to be more
efficient. Verifying the mea­surement component is done
simply and quickly with an
electronic thermocouple (TC) or

Application Note

resistance temperature detector
(RTD) simulator. This approach
does not verify the performance
of the associated temperature
probe, and assumes all probes
are identical and closely follow
some standard. In practice, no
two probes are identical; they
all vary from the ideal standard,
and over time and usage their
characteristics change. Under­standing how probes vary from
the ideal will allow you to opti­mize the measurement system
to achieve the best performance.

From the Fluke Calibration Digital Library @ www.flukecal.com/library

Ideal curve

Actual curve

Class A tolerance

Class B tolerance

0 °C 400 °C

Figure 2.

System accuracy improved by more than 75 %!

1.2

0.8

0.4

RSS Error ± °C

0

Figure 3. System accuracy improvement achieved with a
calibrated Pt100 Sensor.

DOCUMENTING PROCESS CALIBRATOR

754

Figure 4. Connecting a Fluke 754 to a Fluke Calibration dry-well.

Temperature °C – Input

Standard Sensor Calibrated Sensor

2514 dry-well

interface cable

System accuracy comparison measuring 150 °C using a Pt100 (IEC751)
RTD with a transmitter span of 0 to 200 °C

Standard RTD Accuracy Characterized RTD Accuracy
Rosemount Model 644H ± 0.15 °C Rosemount Model 644H ± 0.15 °C
Standard RTD ± 1.05 °C Matched (calibrated) RTD ± 0.18 °C
Total system ± 1.06 °C Total system ± 0.23 °C

Total system accuracy calculated using RSS statistical method.

Table 1

Rosemount Inc. uses the
example provided in Table 1 for
information on the possible per­formance improvement of their
Model 644H Smart Tempera­ture Transmitter. To achieve
this performance improvement,
the Rosemount 644H is given
information (Callendar Van
Dusen Coefficients) that allows
it to correct for the unique
performance of the temperature
sensing element, in this case a
standard IEC751 Pt100 sensor.

Dry-wells and micro-baths
are good choices for verifying
the performance of tempera­ture probes and other related
sensors. But they do not have
the capability to calibrate the
transmitter’s output or read­out and, by themselves, do
not allow the entire measure­ment loop to be optimized. A
heat source, combined with an
intelligent electronic process
calibrator that is capable of
calibrating the transmitter and
readout, is required if the above
performance improvement is to
be realized and maintained.

By combining the automating
and documenting capabilities
of the Fluke 754 Document­ing Process Calibrator with
Fluke Calibration’s intelligent
and stable family of field dry­wells and micro-baths, you
have the capability to test the
entire loop. This combination of
equipment allows you to easily
verify the characteristics of the
temperature sensor and mea­surement electronics. Using this
information, the entire loop can
be adjusted to optimize system
measurement performance.
Below are some examples of
how to optimize the perfor­mance of your measurement
system using these instruments.

The Fluke 754 is connected
to a Fluke Calibration dry-well
or micro-bath by way of a serial
RS-232 interface cable. Version

2.3 or greater firmware for the
754 is required. The firmware
version is displayed briefly on
the display of the 754 during
power-up. If you do not have
the required firmware, contact
your authorized Fluke distribu­tor for information regarding an
upgrade. The serial cable may
be obtained from either your
authorized Fluke distributor or
directly from your Fluke Calibra­tion representative. The heat

Null modem

Fluke Calibration

Dry-well

(DB9)

source is connected to the 754
pressure port and is accessed
by the 754 TC/RTD source
key. Due to the length of these
tests, it is recommended that a
fully charged battery or battery
eliminator for the 754 be used.
A diagram of the connection of

Fluke Calibration

3.5 mm

interface cable

Fluke Calibration

Dry-well (3.5 mm)

this equipment is pictured in
Figure 4.

In many process applications,
the instrumentation of choice
for temperature measurements

2 Fluke Corporation Eliminating sensor errors in loop calibrations