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 declineand and is no longer used to measure temperature. In fact, NIST stopped calibrating mercury thermometers in March of 2011.
Many alternatives are available today and provide better data but still questions remain: How accurate are the alternatives? What is the best option for a particular application? How much does accuracy cost? And, is adding the latest IoT technology worth the cost?
Digital and Analog Options
Digital temperature instruments
Temperature Bluetooth® Data Loggers with and without probes monitor shipments while in transit. They track environmental conditions of reagents, clinical samples, vaccines, pharmaceuticals, and food items during transport to any location. Data loggers without probes fit inside of the carton. Once the carton is open, simply access the data from the data logger. When probes are featured on the unit, they fit securely inside the carton while the data logger secures to the outside of the container for easy monitoring and access. There is no need to open the container to read the data logger. These types of data loggers also are available in a single-use models so the end user can discard after one use.
Temperature Wi-Fi Data Loggers Compatible with Cloud Service offer an easy and secure way to connect your smartphone, tablet, or personal computer remotely with your water baths, refrigerators, freezers, and biological/pharmaceutical lab equipment.
Once setup is complete you can securely monitor parameters 24/7, control your alarms remotely, view your data logging history, run reports in real time, and give access to other team members. No local software is required to use the cloud-based data interface. Get mobile push notifications, emails, or texts for out of range conditions, connectivity interruptions, low-battery condition or if your device is due for calibration.
Thermistors or Thermistor 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.
Analog temperature instruments
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.
Temperature Instruments to Fit Any Budget
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 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.
Temperature Data Loggers that work via the cloud and/or equipped with technology, such as Bluetooth and Wi-Fi, doesn’t mean they are the most expensive. They come in a variety of types and range in price from $60 for data loggers for transport to $1,500 (the higher price is based on adding premium monthly services to instruments with the cloud capabilities). And, for the cost, these devices can be invaluable, providing monitoring via the cloud, 24/7, allowing the end user to sleep well knowing the valuable samples in the lab refrigerator are being safely kept at the correct temperature ranges.
*Note: Price ranges are subject to change. All suggested ranges are as of this writing.
1Nist.gov, Mercury Thermometer Alternatives. Retrieved from https://www.nist.gov/pml/mercury-thermometer-alternatives on , January 21, 2020.
2EPA.gov, Phase-Out of Mercury Thermometers Used in Industrial and Laboratory Settings. Retrieved from https://www.epa.gov/mercury/phasing-out-mercury-thermometers-used-industrial-and-laboratory-settings January, 23, 2020