Can You Calibrate your RTD Devices Correctly with your RTD

Practical Instrument Electronics Application Notes

Can You Calibrate your RTD Devices Correctly with your RTD
Calibrator?


The basic concept behind RTDs (Resistive Temperature
Detectors) is a very simple one. A current is passed
through a material that has a stable and defined change in
resistance versus a change in absolute temperature. The
most common example of this is the Platinum (Pt) 100 curve
equal to 0.00385Ω/Ω/°C. This Pt 100 curve has a base
resistance of 100Ω at 0°C and is described typically
according to the Callendar-Van Dusen equation with the
associated coefficients. At 100°C the resistance is equal
to 138.50Ω (ITS-68) or 138.51Ω (ITS-90). The name of the
curve is typically the ratio of resitance at 100°C divided
by the resistance at 0°C. The curves are very reliable and
can be improved depending on the purity of the materials
used. RTDs are typically used for the most demanding and
accurate industrial measurements as well as many laboratory
measurements. While there are many methods of measuring
temperature the RTD remains the most practical for the high
accuracy, mid range process temperatures ranging from about
-100°C to 800°C. Outside these ranges thermocouples and
other devices should be considered more appropriate.
Going back to the basic property that makes RTDs
useful is the ease of measuring the inherent change in
resistance proportional to a change in temperature.
Obviously this is where they get the name RTD. The basic
principle required to read an RTD is based on the simple
Ohm’s Law equation. I = V/R, where I is electrical current,
V is voltage and R is resistance. Conveniently rearranging
Practical Instrument Electronics Application Notes

the Ohm’s Law equation to solve for R (an RTD in our case)
results in R = V/I. From this it should be apparent that if
a KNOWN current is driven through an unknown resistance BUT
the resulting voltage is monitored the unknown resistance
can be easily calculated. This is the basic principle for
all devices that measure resistance including multi-meters,
RTD thermometers, temperature transmitters etc. While the
basic theory of measuring the output of an RTD is
relatively simple, there are countless subtle variations on
the theory in application. In our experience these methods
are as varied as the engineers’ imagination and the
applications they are designing to. A partial list of these
applications include low measurement currents, high
measurement currents, pulsed and intermittent currents,
switched PLC input currents, split and equal lead currents
(typically used for compensating 2, 3 and 4 wire
connections), voltage excitation (not current). Sometimes
all these methods may be combined with polarity switching
(for offset voltage cancellation). Some combination of
these may be present depending on the particular (or in
many cases peculiar) applications for monitoring RTDs. The
applications range from well thought out and critical to
the just plain cheap and dirty implementations. As a user
looking at a device who would know all this is going on.
Unfortunately the myriad of implementations pose a serious
challenge to the users and designers of calibration
equipment. Making matters worse many calibrator designers
themselves are unaware of the techniques used in various
equipment. We have learned these through years of
experience. The variety of techniques unless they are
Practical Instrument Electronics Application Notes

properly addressed can often cause very serious problems.
This is especially true when considering that these are
typically high accuracy and high repeatability requirements
to justify the use of RTDs. Probably the most common
results are poor accuracy or repeatability resulting in
replacing transmitters, receivers, etc. as the
inconsistencies are discovered and troubleshooting the
problem begins. There are many calibrators today that are
not designed for operation with all these devices and
expect the technicians or engineers to know when and where
to use their calibrators based on the application. This is
very difficult to do since how the transmitter accomplishes
its specification is rarely disclosed. That manufacturer is
interested in relaying information on its specifications
not HOW it meets its specifications.



Signal Polarity

Many calibrators are mistakenly designed for
currents flowing in only one direction through the device.
They are not bipolar or bidirectional with respect to
current. RTDs and resistors are. In this case the
calibrator won’t act like a true resistor. This design
approach would theoretically be fine in an ideal world if
the user or the calibrator knew this and could always
guarantee which direction current would flow through the
calibrator. The reality is unless you know and account for
this during hookup this may be a significant problem.
Typical problems that might be experienced range from
Practical Instrument Electronics Application Notes

catastrophic errors, which are generally easier to find to
the more insidious linearity errors in the calibrator due
to internal circuit saturation or leakage currents affect
the calibrator. This often leads to the technician or
engineer to incorrectly assume and blame the transmitters
or receivers. Replacing those repeatedly in a hope to fix
the problem can be costly especially as it relates to down
to process.
Even if the calibrator is designed for bidirectional
currents it is important that any internal offset voltages
need to be very low. If they are not this may lead to
inconsistent results depending on which polarity the
calibrator is connected from one calibration cycle to the
next. This is very important on transmitters or receivers
in applications that may use switched polarity readings
used to cancel thermocouple effects imposed on RTD probes
with low current excitation. This technique is most
commonly found only for the most accurate devices and as a
result makes it that much more critical that the offsets
are accounted for.


Pulsed Currents


Pulsed currents are a relatively new feature in field
devices’ ranging back to about 1990’s and have become
increasingly more popular. Pulsed currents have been
implemented for various reasons. Some of the reasons behind
pulsed currents range from reducing self heating in the RTD
Practical Instrument Electronics Application Notes

probe due to the required excitation current, low power
applications in remote battery powered devices such as
stand alone gauges, PLCs that scan several inputs and may
only look at the RTD for brief period and switch to another
input. While these are clearly all valid applications they
require special attention to assure proper operation and
calibration. Making things even more confusing is the field
devices requiring calibration often don’t indicate that
they use pulsed or intermittent currents. As a result this
probably accounts for more misunderstandings, time lost and
field problems than any other all by itself. This is
because the calibrator may often appear to work and may
manifest itself in noisy readings, inconsistent reading or
even as a temperature offset. The reasons for these
manifestations are beyond the scope of this paper and could
be the subject of a whole paper on its own. However because
of this confusion the most likely thing done in the field
is to replace the installed field devices such as
transmitters, controllers or receivers only to find that
the problem persists.
Many RTD calibrators don’t specify operation with
pulsed currents. Other calibrator manufacturers specify
compatibility with pulsed or intermittent currents but
state a duty cycle such as 10mS/Second. This can be very
misleading. Be sure to check that they respond to
repetitive pulses less than 10mS to the full rated accuracy
of the device.

Current Ranges

Practical Instrument Electronics Application Notes


Another area often overlooked that leads to many field
problems is current ranges allowed. Many of the RTD
calibrators available specify very narrow bands of
excitation current from the transmitter or receiving
device. Many RTD calibrators on the market will specify
current ranges as narrow as 0.5mA to 3mA. This is
insufficient for compatibility with many new devices. Many
of the newer devices use currents from 0.1mA and lower.
Devices that measure lower temperatures or other low
resistance curves like Cu10 use 10mA or higher currents.
Practical Instruments manufactures RTD calibrators that go
from low micro-amps to 20.00mA. This is the widest in the
industry without question and this has been done for a
reason.







Banana Plugs


Be careful with banana jacks. Many calibrators are
equipped with just two double insulated banana jacks for
simulating RTDs with 2, 3 or 4 wire connections. Fluke is a
champion of this method. These are convenient for two wire
connections but for the more accurate 3 and 4 wire
connections may introduce significant errors. So care must
Practical Instrument Electronics Application Notes

be used when using these. The contacts or plugs will be
stacked by its nature to achieve 3 and 4 wire connections.
This will introduce unbalanced paths in 3 and 4 Wire
connections. Another concern with this approach is if there
is any corrosion or contamination this can also introduce
both resistive mismatch errors as well as thermocouple
errors. The thermocouple error for copper corrosion is
quite high. Depending on the amount of excitation current,
this may approach the total accuracy of the calibrator
itself if it is not accounted for. Obviously these
connections are convenient but be careful. Our suggestion
is to find a calibrator with four rather than two banana
connections for simulating RTDs. The reason is with four
protected banana it reduces the possibility of mismatched
corrosion, signal path length and thermocouple effects for
high accuracy measurements at lower currents.

RTD Decade Boxes

While a decade box may seem like a good compromise
there are many things to consider when using a decade box
as a calibrator.
The first issue is they are scaled in ohms. They are
very expensive to approach the equivalent accuracies.
Unless you have all the necessary tables, they are not
useful since they still need to be converted to
temperature. The conversion from resistance to temperature
is very error prone especially in the process environment.
Any one that has done calibrations this way has experienced
making table conversion errors or transposed numbers, etc.
Practical Instrument Electronics Application Notes

This method can potentially be catastrophic if not highly
suspect for minor errors. For any errors that are caught
there will be equally many that are not.
Another insidious error with a decade box is subtle
damage due to resistor heating and overload. The same
decade box that is used for precision RTD calibration may
at times be used in a 4-20mA loop as a current limiter or
voltage divider or some other unintended use. This often
damages the precision resistors within a decade box. This
will cause internal resistance values of one or many of the
resistors to shift and will likely not be discovered until
the decade box goes out for recertification. A decade box
typically cannot be recalibrated. They have many fixed
resistors that are switched in by mechanical switches. Once
one of the resistors is damaged they are damaged forever or
the parts need to be replaced and may never be returned to
original specifications. Most often an offset at different
error values will accompany the certification. These error
values are intended as a correction that needs to be
remembered during calibrations. This will further
complicate the calibration process since these will
hopefully be remembered and added or subtracted correctly
during the conversion to temperature. Most electronic
calibrators are protected from over current and over
voltage conditions. Typically even if an electronic
calibrator is damaged they are easily repaired to original
specifications. Practical Instrument Electronics
calibrators are protected against misconnection to ±60VDC.

Practical Instrument Electronics Application Notes

Why Practical Instrument’s RTD Calibrators are Preferred

Practical Instrument's RTD calibrators are engineered
specifically to address all of the "technology concerns"
discussed above and many more. Our engineering
designs minimize the potential for misdiagnosed field
devices and the costly downtime that is often associated
with troubleshooting or replacing transmitters, receivers,
controllers etc. Our calibrators are intended to be simple
in operation and use, while providing the highest level of
engineering integrity available. We believe if you take the
time to actually compare our calibrators with any in the
industry and understand the subtle details associated
with making precision RTD calibrations, you will find our
accuracies and field compatibility second to none.



With Regards,

Sebastian

G.M. Technical

Nunes Instruments

645 Hundred Feet Road,

Coimbatore. 641012.

Tamil Nadu

India,

Web: www.nunesinstruments.com

Mail: info@nunesinstruments.com



Mobile: 09345226022