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For a few hundred years, mercury-in-glass thermometers were the precise method for measuring temperature. Since German physicist Daniel Gabriel Fahrenheit invented them in 1714, the liquid was the expected element in thermometers, which were the most common instrument for temperature measurement. The phrase “when the mercury rises” references the use of mercury thermometers to gauge temperature.
Yet, mercury is a neurotoxin and can affect vision, mood, and the central nervous system. As a toxic metal, even small amounts can impact the physical and cognitive functioning of the body. As early as 2004, the Environmental Protection agency (EPA) prohibited the sale and distribution of mercury fever thermometers in certain states and by 2008 called for the phase-out of all mercury-containing non-fever thermometers used in EPA labs. Mercury use has since continued to decline. According to the National Institute of Standards and Technology (NIST), professional groups and organizations globally are phasing out mercury thermometers.1 In fact, NIST stopped calibrating mercury thermometers in March of 2011.
As mercury-in-glass thermometers were replaced with alternatives, such as spirit-filled thermometers and other temperature measurement instruments including thermistors and thermocouples, the question remains how accurate are the alternatives? What is the best option for a particular application? How much does accuracy cost?
Digital and Analog Options
Thermistors or Themistor Meters and Probes have exceptional accuracy over ambient and biological temperature ranges, especially when compared to the next two options (RTDs and thermocouples). Outside the biological range, however, they can get damaged. They operate through a decrease in resistance as the temperature increases. This difference is converted and displayed as temperature. Thermistor probes are highly sensitive to small temperature changes and can be fragile. Their use is very limited in the industrial marketplace.
Resistance Temperature Detectors (RTDs) or Platinum Resistance Thermometers (PRTs) offer excellent accuracy, stability, and repeatability over a wide temperature range but are sensitive to mechanical shock. These instruments measure temperature by measuring the change in electrical resistance across a metal wire. Some sources indicate RTDs are the most accurate sensors for use in industrial applications.
Thermocouple Systems (Meters and Probes) use a sensor composed of two dissimilar metals for measuring temperature. This different pairing of multiple metals (such as iron or copper) makes them operable for many different applications and highly versatile. Thermocouples are durable and immune to shock and vibration, although they are less accurate than thermistors or RTDs. Because thermocouples work with a wide range of temperatures at both temperature extremes, they can take readings in applications that thermistors or RTDs cannot handle. Thermocouples are often used in furnaces, ovens, and other spaces in which temperatures tend to rise to 250°C or above.
Infrared Thermometers read infrared energy as emitted from an object, convert it into an electrical signal, and display it as a temperature reading. They provide a fast response time for surface temperatures and also read a wide range of temperatures. Because they are a noncontact technology, they do not contaminate objects from one reading to the next. They are an excellent choice for reading temperatures on moving, vibrating, or rotating parts, high-temperature or too-hot-to-touch objects, electrically active items, or hard-to-reach objects.
Digital Indicator Thermometers are used for general purposes such as monitoring the temperature of freezers, water baths, and incubators. They are also a good option for displaying ambient temperatures for food service purposes. Their accuracy varies by model but they are easy to read and may be battery operated, wireless, solar powered and/or wall mounted.
Organic-Liquid-Filled Glass Thermometers contain nonhazardous liquid in a graduated scale tube which closely resembles their mercury predecessors. The accuracy of a typical glass thermometer is ±1 scale division. These are available in partial immersion or total immersion types and typically have to be considerably longer to achieve the same specifications as a mercury thermometer.
Bimetal Thermometers do not require power and are an economical option. A bimetal element moves the pointer on an analog dial as the temperature changes. They are available in standard, dampened movement, and silicone-filled types. Generally, these thermometers offer ±1% full-scale accuracy.
Greater Accuracy Requires a Bigger Budget, Usually
As is true with most instruments, greater accuracy will likely cost more when measuring temperature. Liquid-in-glass thermometers without mercury are not as accurate, so they should be used when less precision and certainty is acceptable. However, analog alternatives, liquid-in-glass thermometers, and bimetal thermometers are the most economical options, with prices, as of this writing, often under $50.* Digital indicators run approximately $50 to $100. Electronic thermometers cost more than their mercury counterparts, yet offer comparable accuracy. Digital thermometers are typically $100 and up.
Infrared thermometers ace the “cool” factor as an easy-to-use technology that uses lasers to show where the measurement is being taken. They measure surface temperature only and readings can vary based on emissivity. Because of this, accuracy can fluctuate based on the surface being measured. Entry-level infrared thermometers can range from $80 to $120. Of course, with advanced features that heighten performance capabilities, these devices can run as high as $1000.
Thermistors, RTDs/PRTs, and thermocouples all use both a meter and a probe and the combination can create varying pricing structures. For example, when adding multichannel capabilities or data logging features to the meters, their prices increase. For each of these technologies, the probes are the differentiating component and can add substantially to the cost of the system.
Generally, between thermistors, RTDs, and thermocouples, thermocouples are the most cost-effective and measure a wide range of temperatures but are the least accurate of the three. Thermocouples offer the flexibility of being available in a number of configurations and operating from a long distance without signal loss or the need for preamplification. Because of their general benefits, and reasonable-though-not-high accuracy, thermocouples can be seen as quite a value, with prices for meter and probe systems commonly $150 and up.
According to the EPA2, nonmercury PRTs are as accurate as mercury-containing thermometers through a wide temperature range. RTDs/PRTs offer better stability and repeatability and are usually priced similarly to the thermistor for a standard meter and probe, which is about $200 and up. High-accuracy RTDs/PRTs are available, but the price rises steeply at more than $1000 for a system.
Thermistors are more sensitive than standard RTDs and more accurate within the biological temperature range. Thermistor systems are typically $200 and up.
For each of these systems, customized probes can be designed to meet special requirements. The cost of customization differs according to the requested needs.
*Note: Price ranges are subject to change. All suggested ranges are as of this writing.
1Nist.gov, Mercury Thermometer Alternatives. Retrieved from http://www.nist.gov/pml/mercury.cfm on March 7, 2014