Thermometry Basics

Learn More About Thermometry Basics

Thermometers are made to measure different types of physical characteristics, but the five most common are: bi-metallic devices, liquid expansion devices, resistance temperature devices - RTDs and thermistors, thermocouples and infrared radiation devices.

Thermometer Technologies explained

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Have dial displays. The dial is connected to a spring coil at the center of the probe. The spring is made of two different types of metal that, when exposed to heat, expand in different but predictable ways. Heat expands the spring, pushing the needle on the dial. Bi-metal thermometers are cheap and typically take minutes to come to temperature. Not to mention, their entire metal coil has to be immersed in the material being measured to get an accurate reading (usually more than an inch or two).

And bi-metals are mechanical thermometers needing no electricity to function. Bi-metal thermometers lose calibration very easily and need to be re-calibrated weekly or even daily using a simple screw that rewinds the metal coil.

 

RTDs, thermistors and thermocouples - measure the effects of heat on electronic current. Resistance devices, RTDs and thermistors, take advantage of the fact that electrical resistance reacts to changes in temperature along predictable curves.

Both the relatively inexpensive thermistor and the high-precision RTD - measure resistance in a resistor attached to an electronic circuit to measure temperature.

Thermistors typically use ceramic beads as resistors, while RTD's often use either platinum or metal films.

With thermistors, resistance decreases with temperature and with RTDs, resistance increases.

Both thermistors and RTDs may have a higher degree of accuracy than thermocouples, but their range is limited by comparison and they are generally not as fast.

Thermocouples work on the principle, that when connected to two different metals across a span with a temperature difference, an electronic circuit is generated.

The voltage of the generated circuit changes with variations in temperature in predictable ways.

Common thermocouples weld together nickel and chromium - Type K, copper and constantan - Type T or iron and constantan - Type J and place the weld at the very tip of the thermometer probe.

Since thermocouples only generate voltage if there is a difference in temperature along the circuit (and the difference in temperatures needs to be known to calculate a temperature reading), thermocouples either have a cold junction where part of the circuit is brought to the ice point (0°C/32°F) or an electronic cold junction compensation that aids in the calculation. Thermocouples can detect temperatures across wide ranges and are typically quite fast.

 

A type of thermometry that measures the amount of infrared energy being emitted by a substance and compares that value to a predictable curve to calculate temperature.

Thermometry Concepts

speed

Speed, or response time, is another important consideration when choosing a thermometer. Some thermometer technologies are faster than others and depending on the application, additional seconds - or fractions of a second can make all the difference.

Generally, electronic thermometers are faster than mechanical thermometers like liquid mercury or dial thermometers. Thermocouple sensors are faster than resistance sensors like the Thermistor or the RTD, and reduced tip probes are faster than standard-diameter probes because the sensor is closer to the material being measured and the mass of the sensor is smaller and therefore more responsive to changes in temperature. 

The real response time of a thermometer varies depending on the particular substance, and the range of temperatures being measured.

accuracy

Any thermometer is only as good as the temperatures it takes, so thermometer accuracy is of the utmost importance. Slight increases or decreases in temperature can have profound effects upon the growth of bacteria, the pliability of plastics, the interaction of chemicals, the health of a patient, etc, and electronic thermometers with digital displays make it easy to measure temperature within a tenth of degree or less.

Accuracy is usually expressed as ± a certain number of degrees or ± a certain percentage of the full reading.

The United Kingdom Accreditation Service  (UKAS) provides a way for calibrated thermometers and their temperatures to be traceable to a national standard, thereby giving the user a guarantee of accuracy.

resolution

Thermometer resolution refers to the smallest increment of measurement readable from it. 

A thermometer that displays temperature readings to the hundredth of a degree, e.g. 30.26° has a greater resolution than one that only shows the tenths of a degree - e.g. 30.2°, or whole degrees 100°.

Although resolution differs from accuracy, the two should be thought of as going hand in hand. A thermometer that is accurate to ±0.05° wouldn't be half as useful if its resolution were only in tenths of a degree, e.g. 0.1°. Likewise, it could be misleading for a thermometer to show hundredths of a degree on its display, if it's traceable accuracy were only ±1°.

range

Range describes the upper and lower limits of a thermometers' measurement scale. Different thermometer and sensor types tend to perform best in different ranges of measurement. Some specialise at extremely hot temperatures or very, very cold ones. Some have a broader range. Often a thermometer will have different accuracy or resolution specifications in the center of its range than it does at its outer limits.

Specification tables require careful reading. The better idea you have of the temperature range you're most likely to be measuring, for example, baking temperatures at 149-204 °C, the more easily you'll be able to select a technology that performs best in that range.

Learn About Thermometer Features

Thermometers can have many different features that make monitoring and recording temperatures easier; the ones you'll require usually depend on your application. Learn more about each feature to find which ones are best for you.

thermometer Features explained

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Recording maximum and minimum temperatures is a very helpful feature, particularly when trying to determine if a target has been kept within designated temperature boundaries over an extended period of time - as with data logging.

Thermometers with Max/Min functionality display the highest and lowest temperatures encountered. Some mechanical thermometers do this with physical markers that get pushed up or down over time, but Max/Min is more common with electronic instruments. *Note that electronic instruments with Max/Min often do not have Auto OFF  feature since turning an instrument off resets its Max/Min recordings.

Hold is a feature that allows you to freeze a displayed measurement (usually a digital reading) for later consultation.

Differential Recordings - Diff, display the product of subtracting the minimum temperature encountered from the maximum temperature encountered, showing the range of deviation over a span of time.

 

Average temperature recordings - Avg, simply averages all the measurements encountered over a span of time.

 

High and low alarms - Hi/Lo, alert you by blinking, beeping or even sending you an email or text message when a measurement has gone above or below a certain preset temperature.

 

Auto OFF is a feature that shuts the instrument off after a specified amount of time to protect battery life. Some units also come with the ability to disable and edit the time period when the thermometer turns off. Use this feature for more extended measurements.

 

Learn About Sensors

The sensor is the type of probe. There are three main types, and which one you choose typically depends on what kind of accuracy, reliability, and temperature range you require.

thermocouple

The sensor of a thermoelectric thermometer, consisting of electrically conducting circuit elements of two different thermoelectric characteristics joined at a junction.

A common thermocouple sensor pairing two wires made mostly of nickel and chromium and using variance in voltage to calculate temperatures known for its wide temperature range and affordability, typical in industrial applications.

Accuracy Specifications

All type K thermocouple probes/sensors are manufactured from Class 1 type K thermocouple wire as detailed in the British Standard BS EN 60584-1:2013, and meet the following accuracy specification:

±1.5 °C between -40 & 375 °C
±0.4 % between 375 & 1000 °C

High Accuracy Type K Thermocouple Probes/Sensors (indicated in relevant product pages with the 'high accuracy' icon)
ETI high accuracy type K probes are manufactured from Class 1 type K thermocouple wire which is chosen for improved accuracy and performance and meet the following accuracy specification:

±0.5 °C between 0 & 100 °C

A more specialized thermocouple sensor pairing two wires made mostly of copper and constantan and using variance in voltage to calculate temperatures known for greater accuracy and durability, typical in medical or pharmaceutical applications.

Accuracy Specifications

All type T thermocouple probes/sensors are manufactured from Class 1 type T thermocouple wire as detailed in the British Standard BS EN 60584-1:2013, and meet the following accuracy specification:

±0.5°C between -40 & 125 °C
±0.4% between 125 & 400 °C

A specialized thermocouple sensor pairing two wires made mostly of iron and constantan and using variance in voltage to calculate temperatures—more limited in its range to higher temperatures but known for its sensitivity.

RTD/PT100

An acronym for Resistance Temperature Detection. RTD/PT100 probes consist of flat film or a wire wound platinum resistance sensor element. The measurement value changes in line with the temperature being measured. 

These probes use variance in resistance (typically in platinum) to calculate temperatures known for high accuracy over a wide range of temperatures and low drift, typical in high-precision applications like calibration.

 

PT100/RTD Probes/Sensors are manufactured from Class A PT100/RTD 100 Ω (ohms) detectors as detailed in the IEC 60751 (2008) standard, and meet the following accuracy specification:

±0.15 °C ±0.2 % between -200 & 600 °C

Themistor

A common thermal sensor that uses the predictable variance in the resistance to an electrical current with changes in temperature to calculate temperatures

NTC Thermistor Probes/Sensors for all ETI manufactured Thermistor Probes is as follows:

±0.4 °C between -20 & 100 °C
±0.3 °C between -10 & 0 °C
±0.2 °C between 0 & 70 °C
±0.4 °C between 70 & 100 °C

Infrared Basics

Infrared thermometers are very fast, typically giving a reading in a fraction of a second, or the time it takes for the thermometer's processor to perform its calculations. Their speed and relative ease of use have made infrared thermometers invaluable public safety tools in the food service industry, manufacturing, HVAC, asphalt & concrete, labs and countless other industrial applications.

 

Infrared thermometers are ideal for taking surface temperature measurements from a distance. They provide relatively accurate temperatures without ever having to touch the object you're measuring.

 

Watch the video about Infrared Basics.

Infrared Technologies explained

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Mica lens thermometers such as the  RayTemp 38 are the most common type used in industrial settings. They have more rigid mineral-based ground lenses. 

This allows them to:

• Take accurate measurements at much higher temperatures, above 1000°C
•Be about half as susceptible to the thermal shock effects caused by sudden swings in ambient temperature as the fresnel lens thermometers
•Be most accurate at greater distances - above 20:1 distance to target ratios

 

Mica lens thermometers often come with one or two lasers to help guide both the aiming of the thermometer and the estimation of the field of view being measured. Mica lens thermometers are the most fragile of the infrared technologies, however. They often come with carrying cases as they are more likely to crack or break when dropped. They are typically the most expensive and still need to acclimate to extreme ambient temperatures for 10 minutes or more before giving accurate readings.

Fresnel lens thermometers, like the RayTemp 8 are the most common type used in the food industry.

Unlike the mica lens, the fresnel thermometer lens is typically plastic, which offers several key advantages:

• Less xpensive than mica lens thermometers
• More durable and will survive drops better than mica lens thermometer
• Can offer tight spot diameters at a greater distance than no-lens thermometers
•Typically more accurate at a 6" to 12" distance than other technologies

 

Fresnel lens thermometers often come with laser guides to help you aim your measurement. However, the plastic fresnel lens has a narrower temperature range than the more versatile mica lens. It is also more susceptible to inaccuracies due to sudden swings in ambient temperature, called thermal shock, than the other types of infrared thermometers.

If, for example, you carry your fresnel lens thermometer from room temperature into a walk-in freezer to take frozen food measurements, the sudden drop in temperature can actually change the shape of the lens as the plastic contracts with cold. Most fresnel lens thermometers will display error alerts when this happens and will give faulty readings until the lens has had a chance to acclimate to the new environment. Similar distortions occur at the upper range of temperatures in a fresnel lens thermometer's specification.

The good news is that allowing your fresnel lens thermometer to rest in the new ambient temperature for 20 minutes or longer before taking your measurements can dramatically reduce the distortions due to thermal shock.

No-lens thermometers, like the IR Pocket Infrared Thermometer, use a reflective funnel design to focus infrared energy against the thermopile rather than a lens. 

Not having a lens at all offers its own distinct advantages:

• Typically cost less
• More durable 
• Usually smaller and easier to handle 
• Most accurate in cold spaces

Since there is no lens between the electromagnetic waves being emitted by a surface and the thermometer's thermopile, there are no significant contraction or expansion effects on no-lens thermometers. In most units, an internal sensor compensates for the ambient temperature effect on the electronics themselves, so you can literally walk from a warm room directly into a sub-zero freezer and begin taking measurements without waiting.

The important caveat with no-lens thermometers is that their distance to target ratio or DTR is always 1:1 or lower. That means that you should hold no-lens thermometers as close to the target surface as possible when taking measurements. No-lens thermometers are not as well suited for taking measurements at a distance.

Learn More About Bluetooth Features

Secure temperature data transmission is vital for the safety of food processing and food service operations.
This is what makes Bluetooth thermometers an ideal choice, and ETI has plenty of solutions to choose from in our BlueTherm portfolio. Our range offers food industry professionals speed, accuracy and reliability when it comes to keeping digital records of temperatures – an absolute must for firms to operate safely and remain compliant.

Bluetooth Features explained

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Recording maximum and minimum temperatures is a very helpful feature, particularly when trying to determine if a target has been kept within designated temperature boundaries over an extended period of time - as with data logging.

Thermometers with Max/Min functionality display the highest and lowest temperatures encountered. Some mechanical thermometers do this with physical markers that get pushed up or down over time, but Max/Min is more common with electronic instruments. *Note that electronic instruments with Max/Min often do not have Auto OFF  feature since turning an instrument off resets its Max/Min recordings.

Hold is a feature that allows you to freeze a displayed measurement (usually a digital reading) for later consultation.

Differential Recordings - Diff, display the product of subtracting the minimum temperature encountered from the maximum temperature encountered, showing the range of deviation over a span of time.

 

Average temperature recordings - Avg, simply averages all the measurements encountered over a span of time.

 

High and low alarms - Hi/Lo, alert you by blinking, beeping or even sending you an email or text message when a measurement has gone above or below a certain preset temperature.

 

Auto OFF is a feature that shuts the instrument off after a specified amount of time to protect battery life. Some units also come with the ability to disable and edit the time period when the thermometer turns off. Use this feature for more extended measurements.

 

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