metrology, calibration, fluke

1. CALIBRATION TIMES
Vo l u m e 2 Issue 3 March 2011
Your Definitive Guide to making the
right choice for your Temperature Readout
Dear Readers,
Selecting the right readout device for your thermometry needs is important and an informed selection of an appropriate
instrument can make your temperature measurement job more efficient, accurate and within the right budget. A good
understanding of the selection criteria needed to select an appropriate readout device will avoid common measurement errors
associated with wrong selection of device.
A lot of general purpose industrial temperature measurement spot checks tend to be done using inappropriate temperature
probes connected to the most handy temperature reading DMM lying around. Users fail to realize the extent of errors in the
measurement leading to nasty surprises in a critical industrial environment. Similarly in a secondary or primary calibration lab,
there are many more selection criteria that a user may consider before a choice is made. This issue quickly takes you through a
number of important points that any temperature measurement system user would consider well before a buying decision is
made.
Mercury thermometers or even LIG (Liquid in Glass) thermometers, widely used in many industrial environments are rapidly
losing popularity due to environmental safety concerns and lack of convenience of use reasons. A new class of “Stick
Thermometers” are now finding wide popularity. Read on more about it.
Happy Reading!
Joey Joseph
Editor & Publisher
T h e Mo n t h l y N e w s Ma g a z i n e o n Me t r o l o g y f r o m T T L Te c h n o l o g i e s

2. CALIBRATION TIMES
Making the Right Choice for Your Temperature Readout
When performing temperature calibrations, the right choice of readout for your reference probe and units under test is critical.
Consider the following:
Accuracy
Most readout devices for resistance thermometers provide a specification in parts per million (ppm), ohms, and/or
temperature. Converting ohms or ppm to temperature depends on the thermometer being used. For a 100? probe at 0°C,
0.001? (1 m? ) equals 0.0025°C or 2.5 mK. One ppm would be the same as 0.1m? or 0.25 mK. You should also note whether
the specification is 'of reading' or 'of full range'. For example, 1 ppm of reading at 100? is 0.1m? . However, 1 ppm of full range,
where full range is 400, is 0.4m? . A big difference!
When reviewing accuracy specifications, remember that the readout uncertainty can be a small contribution to the total
calibration system uncertainty and that it may not always make economic sense to buy the lowest uncertainty readout. The
bridge-versus-Super-Thermometer analysis is an excellent case in point. A 0.1-ppm bridge may cost in excess of $40,000,
whereas a 1-ppm Super-Thermometer costs less than $15,000. Reviewing total system uncertainties, it's clear that the bridge
offers very little improvement in this case, 0.000006°C particularly considering its cost.
Measurement Errors
When making the high-accuracy resistance measurements, be sure the readout is eliminating the thermal EMF errors that are
generated at the dissimilar metal junctions within the measurement system. A common technique for removing EMF errors
uses a switched DC or low-frequency AC current supply.
Resolution
Be careful with this specification. Some readout manufacturers confuse resolution and accuracy. Having 0.001° resolution
does not mean the unit is accurate to 0.001°. In general, a readout accurate to 0.01° should have a resolution of at least
0.001°. Display resolution is important when detecting small temperature changes for example, when monitoring the freeze
plateau of a fixed-point cell or checking the stability of a calibration bath.

3. Linearity
Most readout manufacturers provide an accuracy specification at one temperature, typically 0°C. This is helpful, but you
normally measure a wide range of temperatures, so it's important to know the readout accuracy over your working range. If the
readout were perfectly linear, its accuracy specification would be the same across its entire range. However, all readout
devices have some non-linearity component and are not perfectly linear. Be sure the manufacturer provides an accuracy
specification over your working range or provides a linearity specification for you to include in your uncertainty calculations.
Stability
Readout stability is important, since you'll be making measurements in a wide variety of ambient conditions and over varying
lengths of time. Be sure to review the temperature coefficient and long-term stability specifications. Make sure the variations
in your ambient conditions will not affect the readout's accuracy. Reputable readout manufacturers provide a temperature
coefficient specification. The long-term stability specifications are sometimes tied to the accuracy specification for example,
"1 ppm for one year" or "0.01°C for 90 days." Calibration every 90 days is inconvenient, so calculate a one-year specification
and use that in your uncertainty analysis. Be wary of the supplier who quotes 'zero drift' specifications. Every readout has at
least one drift component.
Calibration
Some readout specifications state "no re-calibration necessary." However, under the latest ISO guides, calibration of all
measuring equipment is required. Some readout devices are easier to re-calibrate than others. Look for a readout that can be
calibrated through its front panel without special software. Some older readouts hold their calibration data on an EPROM that
is programmed with custom software. This means the readout must be returned to the manufacturer for re-calibrationwhich
could be in another country! Avoid readouts that still use manual potentiometer adjustments, since re-calibration is time-
consuming and expensive. Most DC readouts are calibrated using a set of high-stability DC standard resistors. Calibration of
an AC readout or bridge is more complicated, requiring a reference inductive voltage divider and accurate AC standard
resistors.
Traceability
Measurement traceability is another concern. Traceability of DC readouts is extremely simple through well-established DC
resistance standards. Traceability of AC readouts and bridges is more problematic. Many countries have no established AC
resistance traceability. Many other countries that have traceable AC standards rely on AC resistors calibrated with ten times
the uncertainty of the readout or bridge, which significantly increases the bridge's own measurement uncertainty.
Convenience Features
The push for increased productivity is endless. As a result, you'll need a readout with as many time-saving features as
possible.
Direct display in temperature - Many readouts display only raw resistance or voltage. Temperature is the most useful
display, so look for a readout that converts resistance or voltage to temperature and be sure it offers a variety of conversion
methods ITS-90 for SPRTs, Callendar van-Dusen for industrial PRTs, etc.
Variety of input types - It's highly likely that you'll be calibrating a variety of temperature sensors, including 3- and 4-wire
PRTs, thermistors and thermocouples. A readout that measures multiple input types provides the best value and maximum
flexibility.
Learning curve - Look for a readout that's simple to use. Bridges have been around for many years and provide good
measurement performance, but require a significant investment in training to operate (and an external PC to compute
temperature from resistance).
Multiplexers for expansion - When your calibration work includes batches of the same probe type, the ability to expand the
measurement system with multiplexer units can also improve productivity dramatically.
Digital interfaces - For automated data acquisition and calibrations, computer interfaces are essential. Look for RS-232 or
IEEE-488 interfaces and calibration software that interfaces with the readout and other system components (baths and
multiplexers) for automated calibrations.

4. There are a variety of methods for calculating the true cost of If you are using a dry-well calibrator and you have to calibrate a
downtime. Rather than calculate those costs, why not avoid them. probe that does not fit snuggly into one of the wells, you still have
Preventative maintenance like calibration helps manage the risk some options, but putting the probe into a well that's too large with
of downtime. The ability to calibrate an air gap around the sheath is
quickly is an advantage. Rather than wait not one of them. What you
for calibrators adjust their temperature to need is another insert with a
the next test point, use a dual-block or correctly sized hole.
dual-well dry-well to run two Interchangeable removable
temperatures simultaneously. With one inserts make it possible to
block (or temperature well ) set at your calibrate a wider variety of
high temperature and the other preset at probes without giving up on
a low temperature you can quickly good results. You have the
calibrate all of your RTDs and option of ordering
thermocouples without waiting for the interchangeable inserts with
block to change temperatures. That's smart. any FlukeHart Scientific dry-
well except the 9100S.
There's more than one reason why measuring multiple thermometers at the same time could
be a good idea. One is that you can be more productive if you can calibrate multiple devices in
parallel rather than sequentially. Another reason for simultaneous measurement can be
accuracy. If your temperature source is not very stable try measuring the reference and the
device under test at the same time. Measuring both at the same time can reduce the
uncertainty in your measurement by eliminating time dependent temperature differences. With
four independent measurement circuits the FlukeHart Scientific 1529 reads up to 4 RTDs,
thermistors, or thermocouples simultaneously.
Take the “Stik”
Thermometer
with you for
accurate
measurements
anywhere.
Fluke Calibration. Precision, performance, confidence.™
Articles reprinted under permission of Fluke
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