Thermometer Image: Courtesy FreeDigitalPhotos.net & mistermongUSA — The USA’s National Institute for Standards and Technology (NIST) began an active mercury-reduction campaign in 2007, and stopped calibrating Mercury (Chemical symbol: Hg) thermometers entirely on March 1, 2011.
A full range of thermometric calibration services continues for non-mercury devices according to the special NIST webpage at: www.nist.gov/pml/mercury.cfm
Mercury is a potent neurotoxin, and every thermometer that contains it is a potential environmental threat. In the 21st century, however, that is a risk that no one needs to take, and a worldwide effort is underway to deploy substitute devices (alternatives) in consumer, professional, and industrial applications.
New analog thermometers with safe filling materials are in production at several companies and recent ASTM standards have been developed to cover them. See ASTM Standards E1 (www.astm.org/Standards/E1.htm) and E2251 (www.astm.org/Standards/E2251.htm).
For more information on each, click each standard’s title (above).
Digital thermometry technologies are plentiful, trials versatile, and generally superior to modern variations on the mercury-in-glass design. Many of these digital devices have wider effective temperature ranges, and nearly all of them equilibrate about 10 times faster than Mercury-filled devices.
There are three classes of sensors that produce signals which can be converted into a digital temperature read-out: thermistors, platinum resistance thermometers and thermocouples. For more information on each, click on the name on on their name.
Each sensor type used is digital thermometers uses a slightly different aspect of a well-characterized relationship between temperature and electrical resistance or induced voltage in certain materials.
The term “thermometer,” when used in the context of digital equipment, refers to electronic systems that capture signals from the sensors, convert them into temperatures using conversion methods compatible with ASTM and/or ITS-90 standards, and then display the result in some format.
The accuracy of digital thermometers thus depends on the sensor type used, the sensor’s quality, its calibration, and conformance to specified standards. Plus, the conversion system’s electronics, it’s calibration and conversion technique used and the unit’s sensitivity to ambient temperature and other conditions result additional sources needing traceable calibration.
In many modern devices, these details can be transparent to the user and summary details are described in the unit’s ‘System” calibration certification.
If not, then it is the responsibility of the user to assure that all major components of the measuring system have certified, traceable calibration and then perform the required calculations to determine the system’s measurement capability and combined measurement uncertainty.
This is not always an easy task, but is quite straightforward, as described in the NIST Publication: NIST/SEMATECH Engineering Statistics Handbook (http://www.nist.gov/itl/sed/gsg/handbook_project.cfm)
Reference webpage: Selection of Alternatives to Liquid-in-Glass Thermometers
Promoting alternatives to mercury thermometers
Thermistors
Thermocouple
Ref: ASTM E230 / E230M – Standard Specification and Temperature-Electromotive Force (emf) Tables for Standardized Thermocouples
Platinum Resistance Thermometers (PRTs)
Standard Specification for Industrial Platinum Resistance Thermometers:
ASTM E1137 / E1137M – 08
Verification Methods for Alternative Thermometers
Background References & Links
(Laws and Regulations),
EPA’s Mercury home page — (www.epa.gov/hg/index.html)
State Regs: 2005 Mercury Compendium www.ecos.org/section/committees/cross_media/quick_silver/2005_mercury_compendium1/
The Environmental Council of the States
50 F Street NW Suite 350,
Washington, DC 20001
Tel: +1 202-266-4920
Fax: +1 202-266-4937
Email: ecos (at) ecos (dot) org
Website: www.ecos.org
