Digital thermometers are temperature-sensing instruments that are easily portable, have permanent probes, and a convenient digital display. The way a digital thermometer works depends upon its type of sensor. Sensor types include resistance temperature detector (RTD), thermocouple and thermistor.  Each type has advantages and disadvantages.

Types of Digital Thermometers

Resistance temperature detectors (RTDs) are wire windings or other thin film serpentines that exhibit changes in resistance with changes in temperature.  They measure temperature using the positive temperature coefficient of electrical resistance of metals. The hotter they become, the higher the value of their electrical resistance. Platinum is the most commonly used material because it is nearly linear over a wide range of temperatures, is very accurate, and has a fast response time. RTDs can also be made of copper or nickel, but these materials have restricted ranges and problems with oxidation. RTD elements are usually long, spring-like wires surrounded by an insulator and enclosed in a sheath of metal.

Internal construction of typical RTD Electrical Resistance-Temperature Cure

Advantages of RTDs include their stable output for long periods of time. They are also easy to calibrate and provide very accurate readings.  Disadvantages include a smaller overall temperature range, higher initial cost, and a less rugged design.

Thermocouples are accurate, highly sensitive to small temperature changes, and quickly respond to changes to the environment. They consist of a pair of dissimilar metal wires joined at one end. The metal pair generates a net thermoelectric voltage between their opening and according to the size of the temperature difference between the ends. A temperature reading is made by calibrating the device with known temperatures, then placing one of the metal junctions on ice (or something else of a known temperature) and the other on the object whose temperature needs to be identified. The voltage displayed is read using the calibration formula and the temperature of the object can be calculated.

Advantages of thermocouples include their high accuracy and reliable operation over an extremely wide range of temperatures. They are also well-suited for making automated measurements both inexpensive and durable.  Disadvantages include errors caused by their use over an extended period of time, and that two temperatures are required to make measurements. Thermocouple materials are subject to corrosion, which can affect the thermoelectric voltage.

The basic idea of a thermocouple. Image Credit: explainthatstuff.com

Thermocouples are available in a range of classes, each of which indicates the temperature at which the device is most effective.

Thermistor elements are the most sensitive temperature sensors available. A thermistor is a semiconductor device with an electrical resistance that is proportional to temperature. There are two types of products.

Negative temperature coefficient (NTC) devices are used in temperature sensing and are the most common type of thermistor. NTCs have temperatures that vary inversely with their resistance, so that when the temperature increases, the resistance decreases, and vice versa. NTCs are constructed from oxides of materials such as nickel, copper, and iron.

Positive temperature coefficient (PTC) devices are used in electric current control. They function in an opposite manner than NTC in that the resistance increases as temperature increases. PTCs are constructed from thermally sensitive silicons or polycrystalline ceramic materials.

Advantages include their small size and high degree of stability. NTCs are also long lasting and very accurate.  Disadvantages include their non-linearity, and unsuitability for use in extreme temperatures.

Specifications

Important specifications for digital thermometers include display options, user interface, outputs and operating environment.

Display Options

Display options include:

• Fahrenheit display, display range, and scale divisions
• Celsius or Centigrade display, display range, and scale divisions
• Both Fahrenheit and Celsius displays
• Max/min readings

Display range is the minimum and maximum values of temperature that can be displayed. Scale division is the smallest division of degrees that can be displayed. It is sometimes referred to as resolution in digital instruments.

User Interface

User interface options include:

• Analog or digital front panel interface
• Computer interface
• Serial or parallel interfaces
• Application software

Output Options

Output options influence the user / device interaction.

• Analog voltage represents temperature sensed as voltage, for example 0-10 VDC.
• Analog current represents temperature sensed as current, for example 4-20 mA.
• Frequency or modulated frequency is a signal representing the sensed temperature.
• Switch or alarm signals are designed to alert the user when a reading is finished, or when a temperature value has been reached.

Operating Environment

Operating temperature is the temperature range in which the device will be used. Thermometers are offered in a variety scales which can be used in a wide range of temperatures.

Portability is an important specification to consider if the thermometer will be carried on a person or if will remain stationary (i.e. home weather station)

Explosion proof construction. An explosion proof thermometer is a device that can withstand an explosion of gases within it and prevent the explosion of gases surrounding it due to sparks, flashes, or the explosion of the container itself, and maintain an external temperature, which will not ignite the surrounding gases.

Splash proof or watertight device. Watertight thermometers are rated for washdown or wet environment applications

Sanitization is an important specification if the thermometer is going to be used in a clinical setting or in the food and beverage industry.

Weather exposure could affect the life span of the thermometer. Thermometers used for meteoroidal or in outdoor settings should be sealed against water and humidity as well as study enough to face high winds, snow and direct sunlight. 

Features

In terms of specialty features, products may include or offer:

• Datalogger or data collection capabilities
• Recording of minimum and maximum values
• Internal timers and counters
• Ability to perform math or statistical functions
• Self-test or diagnostic capabilities
• Battery-powered

Applications

Common applications for digital thermometers include:

Medicine — Digital thermometers are often used in clinical settings on patients.

HVAC thermometers — rated for HVAC applications such as duct or flume monitoring.

Sanitary applications — Sanitary thermometers are rated for sanitary use such as food or pharmaceutical applications.

Home use — includes digital thermometers used for home health care, cooking, and monitoring temperature on home appliances such as the refrigerator or swimming pool.

Laboratory — includes monitoring experiments and chemical reactions as well as maintaining an optimal laboratory environment.

Food Service — uses thermometers in identifying completeness of cooking, and sanitation of ingredients.

Meteorological thermometers — give air, atmosphere and water temperature readings.

References

Explain that Stuff: Thermocouples

Explain that Stuff: Thermometers

Engineering Toolbox - Thermocouples

eHow - Difference Between a Thermostat and Thermistor

What are RTDs?

Image credits:

EXTECH | bapanavina.blogspot.com | Industry-medical