The traditional choice for accurate pressure and vacuum measurement has been mercury-filled glass instruments, since their invention over 350 years ago by Evangelista Torricelli. Relatively easy to construct, these gauges feature no moving parts other than the mercury itself, making the instruments extremely reliable, unless, of course, the glass breaks, which it often does.
Elemental mercury presents a serious health hazard.
Formerly thought of as relatively safe, due to its low toxicity by ingestion, vapor from liquid mercury is easily inhaled, and extremely toxic. As it slowly vaporizes, it recondenses into an invisible film on surfaces. Once released, mercury can be extremely difficult and expensive to clean up. Additionally, there is the continuing cost of monitoring to ensure that even trace amounts of the metal have been removed. Because of the hazards presented by mercury, many states are instituting laws banning products with as little as one gram (less than 75µL or two to three drops) of added mercury.1 As a point of reference, a typical 4-foot fluorescent light bulb can contain as much as 60mg of mercury.2
Mercury spill clean-up is expensive
- Recently in Stoughton, Wisconsin, a lab mercury spill of approximately 50 millilters (1.5 pounds) required an estimated clean up cost of $65,000.3
- A spill of a vial of mercury in a Hartford, Connecticut school in 2001 cost the school district $50,000 to clean up, monitor, and evaluate nearly 150 students for exposure.4
While these examples may seem extreme, clean-up costs exceeding $25,000 are not uncommon.
5 The cleanup costs of even small mercury spills are measured in the thousands of dollars.
Typical mercury filled vacuum gauges can contain anywhere from ¼ lb up to four pounds of mercury. Even a small, inexpensive mercury-filled gauge typically costs approximately $550. In one of the larger instruments, the cost for the mercury alone, (without the gauge) is over $1,000. Because of the hazards of mercury described above, disposal costs for a mercury-filled gauge, even unbroken, can add considerably to the purchase price. Many universities, hospitals and research centers have organized campaigns to remove mercury from the workplace, not only for safety reasons, but also for the potential liability and cleanup costs. For example, Los Alamos National Laboratory has an official program to not only remove mercury-containing instruments when alternatives can be found, but also to officially phase out mercury-containing fluorescent light bulbs.6
The safe, electronic alternative
Fortunately, there are alternatives to mercury-containing gauges for vacuum measurement. For applications in the “rough vacuum” range—from atmosphere down to 1 mbar/torr—capacitive gauges are an excellent choice. Capacitive gauges, like mercury gauges, measure pressure directly, and can provide accurate, reproducible measurement. Recent advances in the technology have made available instruments with transducers constructed of corrosion-resistant AlO3 ceramic for chemistry applications, as well as extending the measuring range by another order of magnitude, down to 10-1 mbar/torr. Nearly all laboratory applications can be served by these gauges. Better electronic gauges combine digital and analog displays, so that the direction of a rapid pressure change can be detected easily, overcoming a shortcoming of traditional digital-readout gauges.
For applications in the “fine vacuum” range (between 1 mbar/torr and 10-3 mbar/torr), indirect measurement is required. These instruments exhibit gas-dependent measurement; in other words, the pressure displayed will vary slightly compared to the true pressure, with the variance dependent on the molecular weight of the gas. A Pirani gauge is an excellent example of this, measuring pressure by thermal conductivity. While somewhat less accurate than direct-measuring technology, depending on the application, the reproducibility of Pirani gauge measurements make it suitable for a variety of applications with pressures as low as 10-3 mbar/torr.
Summary
With the modern electronic alternatives available, it makes both good financial and environmental sense to replace mercury gauges where possible. Remember, it is always more expensive to wait until a mercury-filled gauge breaks before replacing it, than it is to replace it before it breaks. Can you afford to take the health, cost and legal risks of delay?
Footnotes
1 Source Connecticut Department of Environmental Protection
2 Inform, Inc. website, http://www.informinc.org/fact_P3fluorescentlamps.php#look, accessed August 23, 2005
3 Wisconsin Department of Natural Resources website, http://www.dnr.state.wi.us/org/caer/cea/mercury/schoolspills.htm, accessed July 13, 2005
4 State of Connecticut Department of Environmental protection website, dep.state.ct.us/wst/mercury/mercreport.pdf, accessed July 13, 2005
5 Youngstown State University Environmental and Occupational Health and Safety website: http://cc.ysu.edu/eohs.bulletins/mercury.htm, accessed July 11, 2005
6 Los Alamos National Laboratory 2002 Pollution Prevention Awards, http://www.lanl.gov/orgs/pa/newsbulletin/2002/04/30/finalP2winners.pdf, accessed 7/13/2005
