For many people, the words "thermometer" and "mercury" are practically synonymous. That association is unfortunate: 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 in consumer, professional, and industrial applications.
Replacement is easy. (For information on safe disposal, visit the EPA site.) Digital thermometry technologies are plentiful, versatile, and generally superior to modern variations on the mercury-in-glass design invented by German physicist Daniel Gabriel Fahrenheit in 1714. Many of these digital devices [see options at Selecting an Alternative, top right] have wider effective temperature ranges, and nearly all of them equilibrate about 10 times faster than Hg devices. (The element mercury's symbol, Hg, comes from hydrargyrum, a Greek-Latin word meaning "liquid silver.") Their accuracy typically equals or exceeds that of mercury instruments.
NIST began an active mercury-reduction campaign in 2007, and stopped calibrating Hg thermometers entirely on March 1, 2011. A full range of thermometric calibration services continues for non-mercury devices.
Mercury is not a standard
Today, there is no scientific or metrological reason to employ mercury thermometers for any application. That may come as a surprise. Fahrenheit chose mercury because it gave more precise readings over a wider range than the alcohol mix he used in his first thermometer. And now, three centuries later, many people assume that Hg thermometers must be the ultimate benchmark for temperature because the specifications for some procedures and processes in commerce and industry still refer to mercury analog instruments. Professional groups and standards organizations, however, are rewriting their specs to reflect the global phase-out of Hg thermometers.
In the process, they are often getting better data. Hg units are typically less accurate than their digital counterparts. Indeed, the authoritative worldwide standard – the International Temperature Scale, last revised in 1990 and known as ITS-90 – makes no mention of mercury thermometers for measurement.
In fact, the only defining instrument specified in ITS-90 for the range of temperatures from -259.35 oC to 961.78 oC is the platinum resistance thermometer, a solid-state, digital device. (See more about ITS-90 here.)
Numerous government entities are encouraging or mandating alternatives. In early 2012, for example, the U.S. Environmental Protection Agency (EPA) promulgated a new rule providing flexibility in replacing mercury-containing industrial thermometers while remaining in full compliance with EPA regulations.
The high cost of remediation
There are also practical reasons to avoid mercury thermometers, including the considerable financial and administrative burdens they can impose in the case of a spill or other accident. Laws and regulations regarding mercury disposal and remediation vary from place to place (for a representative list, visit the EPA site), and most are quite exacting. Clean-up costs in the thousands -- or even tens of thousands -- of dollars are not uncommon.
In addition, numerous organizations including NIST, NIH, CDC, EPA, ASTM International, the Northeast Waste Management Officials' Organization (NEWMOA) and the related 15-member Interstate Mercury Education & Reduction Clearinghouse (IMERC), and others are involved in mercury reduction and phase-out campaigns. In 2013 the United Nations Environment Programme (UNEP Hg) is expected to adopt the International Treaty on Hg, which calls for eventual elimination of Hg products.
The resources featured on this and connected pages can help users make intelligent and informed decisions about replacing Hg thermometers, acquiring the right kind of digital thermometer for a specific application, ensuring that the devices meet the latest standards, and disposing of mercury.
Past NIST Training Events
Selecting and Using Alternative Thermometers
Sept. 19-20, 2012 NIST, Gaithersburg, MD
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