Temperature

About Thermometers

Precise process control is an important factor to maintaining high production quality, just as precision and accuracy are the key to research. Temperature is a crucial variable in both production and research.

Glass and metal thermometers use the physical law of thermal expansion to measure temperature. However, this method has limitations in accuracy and range. Glass construction is fragile and comes with risk to a person’s health, as well as to the environment. For these reasons, an alternative way of measuring temperature has become necessary. Hanna electronic thermometers are designed to withstand mechanical stress and extreme environments while maintaining high accuracy.

Electronic thermometers provide the versatility, speed and accuracy requested by operators in all areas of temperature measurement. Speed is important when the reactions being monitored change rapidly. Small, compact sensors are preferable for tightly arranged areas, such as electronics and other miniature applications. Electronic thermometers allow users to monitor maximum, minimum and even average temperatures.

Dedicated research teams, precision process control, integrated production facilities, and an overall team effort is required to meet the demanding applications of our users. Hanna’s extensive professional thermometer line constitutes the true dedication Hanna commits to thermometer design and production.

Achieving Thermometer Accuracy

Even though it is easy to show resolutions of 0.1 °C with digital thermometers, there is no relationship between resolution and accuracy of measurements. Below is a list of the main causes that can impact accuracy of temperature measurements:

  • Instrument: The instrument may have an extended scale and 19,000 points of measurement may be obtained. Within these 19,000 points, the instrument may perform differently because of internal linearity.
  • Electronic components: The internal electronics have a drift that depends on the ambient temperature. For this reason, the accuracy of the instrument is stated at a specific temperature of 20 or 25°C, and the drift has to be specified for each degree of variation with respect to the reference temperature.
  • LCD: Liquid crystals have an operating limitation which is a function of temperature. Their normal range is between 0 and 50°C, but there are components capable of performing between -20 and 70°C.
  • Batteries: Instrument battery power supply also has limitations of use.
  • Temperature sensor: This is a separate accuracy, which is to be added to the instrument’s error.

With all the possible forces influencing accuracy, calibration verification is essential. Hanna’s thermometers with CAL Check™ can verify an accurate calibration quickly and easily.

Importance of Accuracy

Up to a few years ago, accuracy was not a very critical aspect and tolerances of a few degrees did not jeopardize a process. From the time that hazard analysis and critical control points (HACCP) programs became a necessity in the food industry, measurement accuracy has become a discriminating factor. Due to health risk factors, now an error of a few tenths of a degree can decide whether food can still be kept or must be discarded. In 1990, Hanna began to produce thermometers for our customers’ HACCP programs to comply with new governmental regulations. Soon after, Hanna became the market leader in Europe as a result of the technological solutions offered to our users.

Hanna CAL Check™ Calibration Feature

As previously described, the electronic components of an instrument shift with time. Hanna has made it possible for users, with the simple touch of a button, to verify whether the response of the instrument is within the tolerance limit of ±0.02oC.

The CAL Check system acts by substituting the sensor with an internal resistor which corresponds to 0oC; thus simulates the response that the temperature probe would have at 0oC.

Hanna thermometers can be divided into four main technologies: thermistor, thermocouple, Pt100, and infrared.

 

Thermistor Thermometers

The thermistor is a semi-conductor device whose resistivity (r) varies as a function of temperature (T):

  • R = Ro [1 + a (T-To)] where,
  • R = resistance of temp. at T
  • T = temp at the end of measurement
  • Ro = resistance of temp. at To
  • To = temp at the beginning of measurement

Temperature resistance coefficient is the parameter that determines if the resistivity variation is positive (as with the Positive Temperature Coefficient, or PTC sensors) or negative (as with the Negative Temperature Coefficient, or NTC thermistors). It is possible to determine the temperature by applying a potential difference and measuring the resistance.

Thermistor sensors are suitable for a temperature range of -50 to 150oC (-58 to 302oF). Higher temperatures may damage the semi-conductor sensor. Accurate temperature measurements are possible (tenths of degree) due to the high sensitivity of the sensor.

Portable Thermistor Thermometers

Below are the portable thermistor thermometers include ones made for Foodcare. Also included are version with a calibration feature that allows the user to calibrate the meter and probe in an ice bath at 0 oC.

 

 

Testers

A variety of thermistor based testers are available. They include versions with a folding probe, reinforced handle for insertion into semi-solids, and ones that can mount on a refrigerator.

 

 

Monitors

A wall mounted thermometer is available that allow for continuous measurements to be taken.

 

 

Data Loggers

Multiple temperature data loggers are available. These include versions with built-in and external thermistor probes.

 

 

Thermocouple Thermometers

The thermocouple consists of the junction of two wires of different metals. At a given temperature, a potential difference results at the opposite extremes of the two wires (Seebeck effect), with the respective variations linearly related within small intervals. It is therefore possible to determine the temperature given the potential difference and characteristics of the two metals. The measurement end of the thermocouple probe is called the hot junction, while the connection of the thermocouple to the meter is the cold junction. An error is introduced as the cold junction is exposed to the ambient temperature. This error can be eliminated by physically putting the cold junction into an ice bath and forcing a reference temperature of 0oC, or by electronically compensating for the cold junction temperature effect. There are various types of thermocouples, identified by an ANSI code using a letter of the alphabet. The K type is the most commonly used thermocouple.

 

 

Thermocouple Thermometers with Calibration Feature

Although quite fast, thermocouple thermometers read with a response time much slower than other sensors and technologies. Unfortunately, the measurement of the thermocouple emf (electromotive force) loses accuracy because of the measuring system itself, based on the emf generated by the temperature difference between cold and hot junctions. The same emf may be generated under different conditions, for example:

Hot junction at 100oC; cold junction at 20oC; difference: 80oC or Hot junction at 90oC; cold junction at 10oC; difference: 80oC.

A temperature difference of 80oC is obtained with two different temperatures of the sample. It is, therefore, very important to determine the cold junction temperature very precisely. To solve the problem, Hanna offers thermocouple thermometers with a user calibration feature that allows the user to calibrate the measuring system in an ice bath at 0oC.

Thanks to this solution, it is now possible to use thermocouple thermometers for HACCP controls with an accuracy of ±0.3oC, which is the same performance of our Pt100 or NTC thermometers, but with a faster response time.

 

 

K-Type Thermocouple Tester

A thermocouple tester is available. It is supplied with general-purpose probe. Other probes are available including air/gas and surface measurements.

 

 

Pt100 Thermometers

The operating principle of resistance thermometers is based on the increase of electric resistance of metal conductors (RTD: Resistance Temperature Detectors) with temperature.

This physical phenomenon was discovered by Sir Humphry Davy in 1821.In 1871, Sir William Siemens described the application of this property using platinum, thereby introducing an innovation in the manufacturing of temperature sensors. Platinum resistance thermometers have been used as an international standard for measuring temperatures between hydrogen triple point at 13.81 K and the freezing point of antimony at 630.75°C (1167.26°F).

Among the various metals to be used in the construction of resistance thermometers, platinum (Pt), a noble metal, is the one that can measure temperatures throughout a wide range; from -251°C (-419.8°F) to 899°C (1650.2°F), with a linear behavior.

Platinum RTD thermometers were common in the seventies but have now been replaced with thermistor sensors because of their smaller dimensions and faster response to temperature changes. The most common RTD sensor using platinum is the Pt100, which means a resistance of 100Ω at 0°C with a temperature coefficient of 0.00385Ω per degree Celsius. For a higher price one can buy platinum sensors with 250, 500 or 1000⁄ (Pt1000).

The main disadvantage of RTD probes is the resistance of the connection cable. This resistance prevents the use of standard two-wire cables for lengths over a few meters, since it affects the accuracy of the reading. For this reason, to obtain high levels of accuracy in industrial and laboratory applications, the use of a three or four-wire system is recommended.

For all its Pt100 thermometers and probes, Hanna has chosen the multiple-wire technology for higher accuracy.

 

 

Infrared Thermometers

All objects emit a radiant energy in the infrared (IR) spectrum that falls between visible light and radio waves.

The origins of IR measurements can be traced back to Sir Isaac Newton’s prism and the separation of sunlight into colors and electromagnetic energy. In 1800, the relative energy of each color was measured, but it was not until early 20th century that IR energy was quantified. It was then discovered that this energy is proportional to the 4th power of the object’s temperature.

IR instrumentation using this formula has been around for over 50 years. They almost exclusively use an optic device that detects the heat energy generated by the object that the sensor is aimed at. This is then amplified, linearized and converted into an electronic signal which in turn shows the surface temperature in Celsius or Fahrenheit degrees.

Infrared measurements are particularly suitable for areas where it is difficult or undesirable to take surface measurements using conventional contact sensors. Applications for IR meters include non-destructive testing of foodstuffs, moving machinery, and high temperature surfaces.

An ideal surface for IR measurements is a black body or radiator with an emissivity of 1.0.Emissivity is the ratio of the energy radiated by an object at a certain temperature to that emitted by a perfect radiator at the same temperature.

The shinier or more polished the surface, the less accurate the measurements. For example, the emissivity of most organic material and rough or painted surfaces is in the 0.95 region and hence, suitable for IR measurements.

On the other hand, surfaces of highly polished or shiny material, such as mirrors or aluminum, may not be appropriate for this application without using some form of filtration. This is due to other factors, namely, reflectivity and transmissivity. The former is a measure of an object’s ability to reflect infrared energy while the latter is its ability to transmit it.

Another important and practical concern with IR measurements is the field of view. Infrared meters measure the average temperature of all objects in their field of view. To obtain an accurate result, it is important that the object completely fills the instrument’s field of view and there are no obstacles between the meter and the object. The distance-to-target ratio, or the optic coefficient, is therefore an important consideration.

 

 

Temperature Probes

Available are K-type, T-type, thermistor and pt100 temperature probes. These probes include many different styles from penetration, liquids, air/gas, and wire probes.

 

 

 

 

 

 

 

 

 

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