• Other Fluke companies:
  • Fluke
  • Fluke Biomedical
  • Fluke Networks
  • Fluke Process Instruments
See more Fluke brands
Accelix Amprobe Beha-Amprobe Comark Emaint Landauer Pacific Laser Systems Pomona RaySafe Schad
Home
Precision, Performance, Confidence.
Fluke Calibration: Europe
  • Register
  • Log in
  • Contact Us

EU - English [Change]

 

Get Quote

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:

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 100W probe at 0°C, 0.001W (1 mW ) equals 0.0025°C or 2.5 mK. One ppm would be the same as 0.1mW 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 100W is 0.1mW. However, 1 ppm of full range, where full range is 400W , is 0.4mW . 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.

Sources of Uncertainty - Comparison Calibration of PRTs –196°C to 420°C
SPRT 0.001000°C 0.001000°C
1-ppm Super Thermometer (1 ppm) 0.000250°C  
0.1-ppm Bridge   0.000025°C
Bath Uniformity / Stability 0.005000°C 0.005000°C
Estimated Total Uncertainty (k=2)* 0.005105°C 0.005099°C
*RSS, assuming uncertainty components were statistically evaluated.

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.

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.

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.

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.

Some readout specifications state "no re-calibration necessary." However, under the latest ISO guides, calibration ofall 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-calibration—which 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.

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.

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.

Convenience Features

Traceability

Calibration

Stability

Linearity

Resolution

Measurement Errors

Accuracy

  • Home
  • Products
    • New Products
    • Electrical Calibration
      • Electrical Standards
      • Electrical Calibrators
      • Bench Multimeters
      • Electrical Calibration Software
    • RF Calibration
      • RF Reference Sources
      • RF Calibration Accessories
      • RF Calibration Software
    • Data Acquisition and Test Equipment
      • Bench Multimeters
      • Data Acquisition
      • Data Acquisition Software
    • Temperature Calibration
      • ITS-90 Temperature Standards
        • ITS-90 Fixed-point cells
        • Standard Platinum Resistance Thermometers
        • Maintenance Apparatus
        • Liquid Nitrogen Comparison Calibrator
        • Resistance Bridges
        • Standard Resistors
      • Calibration Baths
        • Compact Calibration Baths
        • Standard Calibration Baths
        • Special Application Baths
        • Bath Accessories
        • Bath Controllers
        • Bath Fluids
      • Industrial Calibrators
        • Field Metrology Wells
        • Metrology Wells
        • Handheld Calibrators
        • Field Dry-Block Calibrators
        • Micro Baths
        • Infrared Calibrators
        • Thermocouple Furnaces
        • Dual Block Dry-Well
        • Zero-point Dry-Well
      • Probes / Sensors
        • Platinum Resistance Thermometers (PRTs)
        • Thermistors
        • Thermocouples
      • Digital Thermometer Readouts
      • Multifunction Calibrators
      • Temperature Calibration Software
    • Humidity Calibration
      • Humidity Generators
      • Humidity Data Loggers and Monitors
    • Pressure Calibration
      • Piston Gauges
        • NMI Piston Gauges
        • Absolute Piston Gauges
        • High Pressure Pneumatic Piston Gauges
        • Hydraulic Piston Gauges
        • Piston Gauge Accessories
      • Automated Pressure Controller / Calibrators
        • Low Pressure Controllers / Calibrators
        • Pneumatic Pressure Controllers / Calibrators
        • High Pressure Pneumatic Controller / Calibrators
        • Hydraulic Controller / Calibrators
      • Pressure Monitors
        • Low Pressure Monitors
        • Digital Pressure Gauges
        • Reference Pressure Monitors
      • Deadweight Testers
        • Pneumatic Deadweight Testers
        • Oil Deadweight Testers
        • Water Deadweight Testers
        • High-Pressure Hydraulic Deadweight Testers
        • Deadweight Tester Accessories
      • Manual Pressure Calibration
        • Pressure Calibrators
        • Pneumatic Pressure Control
        • Hydraulic Pressure Comparators / Pumps
      • Handheld Pressure Calibrators
      • Air Data Calibration
      • Environmental Monitors
      • Pressure Calibration Accessories
      • Pressure Calibration Custom Systems
      • Pressure Calibration Software
    • Flow Calibration
      • Gas Flow Standards
      • Gas Flow Accessories
      • GFS Primary Gravimetric Flow Standard
      • Flow Calibration Software
    • Process Calibration Tools
      • Temperature Calibrators
        • Handheld Temperature Calibrators
        • Dry-Block Calibrators and Micro-Baths
        • Precision Digital Thermometers
        • Temperature Probes
        • Infrared Calibrators
        • Hygro Thermometer with Data Logging
      • Pressure Calibrators
        • Digital Pressure Calibrators
        • Handheld Pressure Calibrators
        • Deadweight Testers
        • Precision Digital Pressure Gauges
        • Calibration Hand Pumps
      • Multifunction Calibrators
      • mA Loop Calibrators
      • Process Calibration Software
    • Calibration Software
      • MET/CAL® Software
      • MET/CAL® Support
      • Asset Management Software
      • Temperature Calibration Software
      • Pressure Calibration Software
      • Flow Calibration Software
      • Mechanical / Dimensional Calibration Software
    • Service and Support
    • All Calibration Instruments
    • Handheld Test Tools
  • Purchase Info
    • Where to Buy
    • Request a Quote
    • Request a Demo
    • Request a Sales Consultation
    • Certified Pre-Owned Equipment
    • General Services Administration (GSA)
    • Financing Program
    • National Stock Numbers (NSNs)
    • Payment Options and Tax Info
  • News
    • Press Releases
    • Promotions
    • Industry Links
    • Metrology Salary Survey
  • Training and Events
    • Conferences and Exhibitions
    • Training Courses
    • User Group Meetings
    • Web Seminars
      • Live Seminars
      • On-Demand Seminars
  • Literature and Education
    • Education Hub
    • About Calibration
    • Articles and Education
    • Product Literature
    • Product Manuals (User Guides)
    • Videos and Virtual Demos
    • Resource Centers
    • Blog
  • Service and Support
    • Service Request (RMA)
    • Service Plans
    • Technical Support
    • Knowledge Base
    • Accreditations
    • Authorized Service Centers
    • Calibration Certificates
    • Community Forum
    • My MET/SUPPORT
      • Activate
      • Procedures
      • Software
      • Technical Bulletins
      • Priority Support
      • Manuals
    • Product Manuals (User Guides)
    • Safety Data Sheets (SDS)
    • Recycle Program
    • Safety, Service, and Product Notices
    • Software Downloads
    • Warranties
    • Tools
      • PT100 Calculator
      • ITS-90 Reference Function Calculator
      • Thermocouple Table Voltage Calculator
      • Thermocouple Voltage to Temperature Calculator
  • About Us
    • Contact Us
    • Visit Us
    • Employment
    • Fluke Calibration
      • Hart Scientific Temperature Calibration
      • DH Instruments Pressure and Flow Calibration
      • Pressurements Pressure Calibration
      • Ruska Pressure Calibration
    • Fluke Companies
    • Integrity and Compliance
    • Frequently Asked Questions
    • Why buy Fluke Calibration ?
Home|Literature|Articles and Education|Temperature Calibration|Papers / Articles|Making the Right Choice for Your Temperature Readout
©1995-2022 Fluke Corporation
             

Secondary menu

  • Disclaimer
  • Privacy Statement
  • Terms of Use
  • Terms of Sale