Mercury Thermometers
Public Act 578 (PDF) of 2002 of the State of Michigan prohibits the sale or offer for sale of mercury thermometers for use in the state after January 1, 2003. The act permits three exemptions:
- A use for which a mercury thermometer is required by state or federal statue, regulation or administrative rule;
- Pharmaceutical research purposes;
- Mercury fever thermometers dispensed by prescription.
If the thermometer use does not meet one of the exemptions, a mercury free alternative will need to be purchased.
Special storage requirements for non-mercury thermometers
When stored horizontally, such as in a drawer, non-mercury thermometers separate more frequently than mercury thermometers. Ideally non-mercury thermometers should be stored with the bulb lower than the expansion chamber, however this is not always possible. Special drawer trays, which will store the thermometers at the correct angle, can be purchased from VWR (catalog numbers 61010-012 or 61222-502). These trays are only 1.5 inches high and will fit in most drawers.
Selecting equivalent non-mercury thermometers
Before purchasing a replacement, take an inventory of the thermometers in your laboratory. Remove mercury thermometers from areas where there is a high risk of breakage such as heating blocks, drying ovens, water baths, and refrigerators. Many times when a thermometer is broken in one of these devices the first available thermometer is grabbed as a replacement. Often it is an expensive thermometer, which may be used as your replacement and it can be replaced with an inexpensive substitute.
Next, identify the properties of the thermometer that was broken:
- Partial immersion/total immersion
- Accuracy
- Range
- Certification yes/no
Then, identify the properties needed for the application. When looking at alternatives the tendency is to buy the most accurate with the largest range. Most non-mercury thermometers do not have as large of a range as mercury. The most cost-effective replacement may not exactly duplicate the properties of the original thermometer but will meet the properties required for the application.
Finally, identify the alternatives:
Red spirit
Cost similar to mercury
Accuracy: Varies, similar to mercury
Range: -100 to 200°C
Comments: Often discounted by suppliers, most cost effective alternative for water baths, ovens, and refrigerators.
Ever-safe
Cost: comparable to, to substantially more than mercury depending on range and accuracy.
Accuracy: Varies, similar to mercury
Range: -10 to 260°C
Comments: Used successfully in many mercury replacement programs.
Enviro-safe
Cost: Comparable to mercury
Accuracy: varies
Range: -10 to 225°C
Comments: Chemistry has attempted to use these as a substitute in their teaching labs. They experienced problems with reuniting separated columns. Some of the liquid may remain on the walls of the capillary tubing resulting in inaccurate readings.
Digital
Cost: Depends on the accuracy, some are comparable to Ever-safe.
Accuracy: Similar to much better than mercury
Comments: We are currently testing Fisher PN #15-078J in Chemistry. With discount pricing it is comparable to the Ever-Safe thermometer however it can be stored in a drawer without the separation problems associated with non-mercury thermometers. Digital thermometers are rapidly improving and decreasing in price. Do not dismiss this alternative without consulting the most current information from vendors.
Some things to consider when purchasing a digital thermometer:
- How often the reading is updated.
- The diameter of the stem may be different than a traditional thermometer, apparatus made need to be modified
- The orientation of the display, it may be difficult to read in some applications
- The thermometer housing or stem may degrade in the experiment.
Recombining separated non-mercury thermometers
Manufacturers recommend two different methods for reuniting non-mercury thermometers. Before trying either of these methods be sure to wear the proper eye and hand protection to protect against broken glass.
The preferred method suggested by manufacturers uses a centrifuge. The centrifuge has to be large enough to ensure that the liquid column will be forced down. If the cup is not deep enough the column will split, forcing part of the liquid down. The remainder of the liquid will be forced up into the expansion chamber, which may break.
Pad the bottom of the centrifuge cup with soft wadding, such as cotton, and insert the thermometer, bulb side down, into the cup. Turn on the centrifuge for a few seconds and the liquid will be forced passed the air gap.
For the second method, hold the thermometer in an upright position. Gently tap on the stem above the separation against the palm of your hand. Be careful! If the thermometer is hit too hard it may break and could cut your hand. As the thermometer is tapped the liquid above the separation will break away from the wall and run down to join the main column.
Total immersion/partial immersion thermometers - what's the difference?
Total immersion thermometers are designed to have the bulb and the liquid column exposed to the temperature being measured. For example, when the thermometer is used in a water bath set to 80 o C, the entire thermometer is immersed to the 80 o line and it is pulled out less than an inch to take a reading. A correction factor may be applied to a total immersion thermometer used in partial immersion applications.
Partial immersion thermometers are designed to have a specific portion of the thermometer exposed to the temperature being measured. The amount of immersion is usually indicated by a ring on the stem or by an inscription on the reverse giving the depth of immersion.
Total immersion thermometers may have "total immersion" inscribed on the back but often they do not. If the thermometer does not say partial immersion or have a ring around the bottom assume that it is a total immersion thermometer.
Calibration/Certification - what do I need?
All thermometers are calibrated but not all are certified.
When a thermometer is manufactured it is calibrated against a reference standard. This is done to correctly identify the scale.
If a thermometer is certified it means that the manufactured thermometer is measured a second time against a reference standard at specific points on the scale. The results are sent from the manufacturer as a calibration certificate and accompanies the certified thermometer when it is purchased. These thermometers will need to be re-calibrated periodically for certain programs such as those administered by the FDA, EPA, some state agencies and for ISO 9000 compliance.
A certified thermometer that is calibrated with an ice point in its scale requires only one complete certification in its lifetime. The re-calibration of the ice point will provide a reliable means for the accurate adjustment of the remainder of the scale.