CALIBRATION TIMESVo l u m e 2 Issue 3 March 2011 Your Definitive Guide to making theright 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
CALIBRATION TIMES Making the Right Choice for Your Temperature ReadoutWhen performing temperature calibrations, the right choice of readout for your reference probe and units under test is critical.Consider the following: AccuracyMost readout devices for resistance thermometers provide a specification in parts per million (ppm), ohms, and/ortemperature. 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 whetherthe 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 totalcalibration system uncertainty and that it may not always make economic sense to buy the lowest uncertainty readout. Thebridge-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, its clear that the bridgeoffers very little improvement in this case, 0.000006°C particularly considering its cost. Measurement ErrorsWhen making the high-accuracy resistance measurements, be sure the readout is eliminating the thermal EMF errors that aregenerated at the dissimilar metal junctions within the measurement system. A common technique for removing EMF errorsuses a switched DC or low-frequency AC current supply. ResolutionBe careful with this specification. Some readout manufacturers confuse resolution and accuracy. Having 0.001° resolutiondoes not mean the unit is accurate to 0.001°. In general, a readout accurate to 0.01° should have a resolution of at least0.001°. Display resolution is important when detecting small temperature changes for example, when monitoring the freezeplateau of a fixed-point cell or checking the stability of a calibration bath.
LinearityMost readout manufacturers provide an accuracy specification at one temperature, typically 0°C. This is helpful, but younormally measure a wide range of temperatures, so its important to know the readout accuracy over your working range. If thereadout were perfectly linear, its accuracy specification would be the same across its entire range. However, all readoutdevices have some non-linearity component and are not perfectly linear. Be sure the manufacturer provides an accuracyspecification over your working range or provides a linearity specification for you to include in your uncertainty calculations. StabilityReadout stability is important, since youll be making measurements in a wide variety of ambient conditions and over varyinglengths of time. Be sure to review the temperature coefficient and long-term stability specifications. Make sure the variationsin your ambient conditions will not affect the readouts accuracy. Reputable readout manufacturers provide a temperaturecoefficient 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 specificationand use that in your uncertainty analysis. Be wary of the supplier who quotes zero drift specifications. Every readout has atleast one drift component. CalibrationSome readout specifications state "no re-calibration necessary." However, under the latest ISO guides, calibration of allmeasuring equipment is required. Some readout devices are easier to re-calibrate than others. Look for a readout that can becalibrated through its front panel without special software. Some older readouts hold their calibration data on an EPROM thatis programmed with custom software. This means the readout must be returned to the manufacturer for re-calibrationwhichcould 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 ofan AC readout or bridge is more complicated, requiring a reference inductive voltage divider and accurate AC standardresistors. TraceabilityMeasurement traceability is another concern. Traceability of DC readouts is extremely simple through well-established DCresistance standards. Traceability of AC readouts and bridges is more problematic. Many countries have no established ACresistance traceability. Many other countries that have traceable AC standards rely on AC resistors calibrated with ten timesthe uncertainty of the readout or bridge, which significantly increases the bridges own measurement uncertainty. Convenience FeaturesThe push for increased productivity is endless. As a result, youll need a readout with as many time-saving features aspossible.Direct display in temperature - Many readouts display only raw resistance or voltage. Temperature is the most usefuldisplay, so look for a readout that converts resistance or voltage to temperature and be sure it offers a variety of conversionmethods ITS-90 for SPRTs, Callendar van-Dusen for industrial PRTs, etc.Variety of input types - Its highly likely that youll be calibrating a variety of temperature sensors, including 3- and 4-wirePRTs, thermistors and thermocouples. A readout that measures multiple input types provides the best value and maximumflexibility.Learning curve - Look for a readout thats simple to use. Bridges have been around for many years and provide goodmeasurement performance, but require a significant investment in training to operate (and an external PC to computetemperature from resistance).Multiplexers for expansion - When your calibration work includes batches of the same probe type, the ability to expand themeasurement 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 orIEEE-488 interfaces and calibration software that interfaces with the readout and other system components (baths andmultiplexers) for automated calibrations.
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 adowntime. Rather than calculate those costs, why not avoid them. probe that does not fit snuggly into one of the wells, you still havePreventative maintenance like calibration helps manage the risk some options, but putting the probe into a well thats 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 withblock to change temperatures. Thats smart. any FlukeHart Scientific dry- well except the 9100S. Theres 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 In India (Sales, Service & Calibration) TTL Technologies Pvt. Ltd. # 3071, 10th Cross, 11th Main Road, HAL II Stage, Bangalore - 560 008 Tel : 080 - 2526 0646 / 2525 1859 Fax : 080 - 2529 1285 TTL Helpline (Toll Free) Email : firstname.lastname@example.org Website : www.ttlindia.com Sales : 1800 425 3200 / Service : 1800 22 1243