Choosing the Right Thermometer

Before choosing a thermometer, it’s important to consider the following questions:

• Do you need a fixed device for process monitoring or a portable device for on the spot or remote measurements?
• Will the thermometer make physical contact with the target?
• What temperature range will you be measuring? What are the minimum and maximum temperatures?
• Is it possible that the temperature being measured could vary rapidly (response time)?
• Where will the thermometer be installed? Will readings need to be carried out remotely?
• How will the thermometer be installed? Can the thermometer be mounted and if so how?
• In what sort of environment will the thermometer be used (atmospheric or vibrational conditions, does it need to be compatible with abrasive fluids)?
• Do you need the measurements to be recorded? Does there need to be an output signal in order to regulate a process?

There are two main types of thermometers: contact thermometers, which require physical contact with the target they are measuring, and non-contact thermometers, which take measurements at a distance.

Contact thermometers

These thermometers have different types of sensors depending on the temperature measured, and it is actually the temperature of the sensor that they measure. To measure the temperature of the target, the sensor, which is usually integrated into a probe, needs to reach thermal equilibrium (the same temperature) with the target. For this to occur, good contact between the probe and the target is essential, and this contact must last long enough to allow them to reach thermal equilibrium.

There are several different types of contact thermometers:

• Glass
• Liquid expansion
• Gas expansion
• Bimetallic
• Electronic (thermocouple, thermistor, resistor) 

Advantages: Depending on the configuration of the thermometer and probe, you can measure:

• Surface temperature (make sure the probe and surface make good contact)
• Internal temperature, with a probe able to penetrate the material
• The temperature of a liquid or gas

Disadvantages:

• Response time is fairly slow because you need to wait for the thermometer to reach thermal equilibrium with the target.
• Measurement accuracy depends on the quality of the contact between the probe and the target.
• The maximum temperature that can be measured depends on the resistance of the materials.

Non-contact thermometers

Non-contact thermometers allow you to measure temperatures from a distance, without making physical contact with the target. This type of thermometer uses infrared technology. 

Advantages: 

• Measurements are instantaneous.
• Non-contact thermometers are compatible with very high temperatures.
• They are ideal for taking measurements when contact with the target is impossible, such as when measuring the temperature of corrosive liquids or rotating cylinders.

Disadvantages:

• This type of thermometer only measures surface temperatures, not internal temperatures.
• Non-contact thermometers cannot measure gas temperatures.

SIKA glass thermometer

Glass thermometers consist of a liquid-filled reservoir and a capillary tube, all enclosed in a glass tube.

How it works

Glass thermometers use the volumetric expansion of liquids caused by an increase in temperature to make measurements. Since the diameter of the capillary tube is very small, slight changes in the volume of the liquid result in visible differences in the height of the liquid. This height corresponds to a given temperature, indicated by a scale engraved on the tube.

Types of liquids used:

For a long time, mercury was the most widely used liquid as it was considered to work the best. It does not stick to the glass, its conductivity allows it to reach thermal equilibrium quickly, and it allows a wide range of temperatures to be measured.

However, because of its toxicity, mercury has been replaced in most applications by other liquids, such as colored alcohol.

Applications:

This type of thermometer is still widely used in laboratories and the food industry. It is available in both portable and fixed versions. In the fixed industrial version, the glass tube is encased in a metal body for added strength.

Advantages:

• This type of thermometer operates without a power supply.
• It offers precise measurements.
• The measurement range depends on the liquid used, varying between -200°C and 1000°C.
• It is not affected by vibrations.

Disadvantages:

• The glass tube is very fragile.
• Response time is slow because the liquid has to reach thermal equilibrium with the target.

JUMO gas expansion thermometer

Mechanical gas expansion or liquid expansion thermometers are dial analog thermometers.

They consist of a measuring tube with a reservoir inside and a Bourdon tube, which form a sealed pressurized system filled with a liquid or neutral gas.

It is possible to place a capillary tube between the measuring tube and the Bourdon tube in order to move the dial away from the measuring point and take readings from several meters away.

As the temperature rises, the internal pressure of the system changes, causing the Bourdon tube to deform. This movement is transmitted to a needle via a mechanical system.

Advantages:

• These thermometers do not require a power supply as their operation is completely mechanical.
• They give precise measurements.
• The measuring range depends on the liquid or gas used (from -40°C to 400°C for liquids, and from -200°C to 700°C for gases).
• The response time is fast.
• The dial can be mounted at a distance from the measurement point (up to several meters) by using a capillary tube.

Disadvantages:

• The mechanism in these thermometers is sensitive to vibrations. As with Bourdon pressure gauges, which have the same operating principle, manufacturers offer thermometers whose casing is filled with shock-absorbing liquid.
• Ambient temperature may affect measurements.

AMETEK bimetallic thermometer

There are two types of solid expansion thermometers: bimetallic thermometers and rod thermometers. Both are designed on the basis of the thermal expansion properties of metals.

Rod thermometers

The sensor is a metal bar that expands (lengthwise) in proportion to the temperature. The variations in length are transmitted to the needle by a mechanical system.

Bimetallic thermometers

The sensor consists of two strips of different metals that are permanently welded or bonded together. This bimetal is wound in the shape of a spiral or helical spring. 

Since the two metals have different thermal expansion coefficients, the two strips do not expand in the same way with temperature changes. This results in bimetal deformation (bending or coiling of the spiral or spring). A mechanical system transmits the deformations of the bimetallic strip to the needle that indicates the measurement on the scale. 

These thermometers are available in a fixed version, where they are integrated into process installations, as well as a portable version for on the spot measurements.

Advantages:

• These thermometers do not require a power supply as their operation is completely mechanical.
• They are less expensive than gas or liquid expansion thermometers.
• Ambient temperature does not affect measurements.

Disadvantages:

• Response time is slow because the bimetal reacts slowly to temperature changes.
• It is not possible to separate the display dial from the thermometer sensor in order to take readings at a distance, as with gas or liquid expansion thermometers with capillary tubes.
• These thermometers are sensitive to vibrations.

CHAUVIN ARNOUX portable thermometer

Electronic contact thermometers have a sensor and an electronic circuit that converts the electrical signal from the sensor into a temperature reading on a display.

There are three types of electronic thermometers:

Resistance thermometers

The electrical resistance of a metal conductor varies with temperature.  Resistance thermometer sensors are made with a coiled metal wire whose electrical resistance is known, and temperature measurements are determined by measuring variations observed in its resistance.

The metals most commonly used for sensors are copper, nickel and platinum. Each of these metals can measure a certain temperature range. The most popular are PT100 and PT1000 platinum sensors, which have a resistance of 100 and 1000 Ohms respectively at 0°C.

Advantages:

• These thermometers are highly accurate, and are used as reference thermometers.
• Resistance thermometer sensors provide a more linear signal than other electronic sensors.
• They offer a wide measurement range (from -250°C to 1100°C for platinum sensors).

Disadvantages:

• Response time is slow compared to thermocouples.
• They are more expensive.
• They are bulkier.

Thermistor thermometers

A thermistor is a semiconductor (made of sintered metal oxides) whose electrical resistance varies greatly with temperature (10 times more than a platinum sensor).

There are two types of thermistors:

• NTCs (Negative Temperature Coefficients), whose resistance decreases with increasing temperature, are used in temperature ranges of -200°C to 1000°C.
• PTCs (Positive Temperature Coefficients), whose resistance increases with increasing temperature, are used in temperature ranges of 0°C to 100°C.

Advantages:

• Thermistor thermometers are more sensitive than resistance thermometers.
• They are more compact than resistance thermometers.

Thermocouple thermometers

A thermocouple works by using the Seebeck effect, and it consists of two different metal wires welded together at their ends. In short, a thermocouple measures the temperature at the soldering point of the two wires.

There are several types of thermocouples, each identifiable by a letter corresponding to the different metals they are made with. They differ in terms of sensitivity and measurement range.

Advantages:

• Thermocouple thermometers have a fast response time.
• The various types of thermocouples cover a wide measurement range from -270°C to 2000°C.
• They are affordable.

One of the main advantages of electronic thermometers is the wide variety of configurations available: fixed thermometers for continuous process monitoring, portable inspection devices, sensors able to be moved for remote readings, etc.

FLUKE infrared thermometer

Infrared thermometers are a recent technology, but they are becoming increasingly prevalent in many sectors.

All surfaces emit energy in the form of infrared radiation. The higher the surface temperature, the greater the radiated energy. Infrared thermometers, also known as pyrometers, use a lens to focus the radiation emitting from a surface onto an infrared detector, whose output signal is converted into a measurement that can be read on the thermometer’s display.

The main advantage of infrared thermometers is that measurements are carried out at a distance, without the need for physical contact. This technology is therefore particularly useful when other types of thermometers cannot be used because it is impossible to make physical contact with the target, such as with moving surfaces (e.g. rotating cylinders), very high temperatures, aggressive environments or hard to reach places.

Infrared measurement technology is fast and reliable, and has become quite affordable in price. Nowadays, it’s easy to find low-cost thermometers for basic applications.

Use of infrared technology is now very widespread, and there are many models and configurations available for every application. These include fixed thermometers for process monitoring and portable or pocket thermometers for spot checks and measurements.

This technology is very simple to use, simply aim the thermometer at the target and its temperature will appear on the screen. There are, however, limitations to this technology that need to be taken into account:

• The infrared detector in these thermometers operates at a fixed wavelength. However, for certain applications, a particular wavelength may be required depending on the type of target (e.g. glass, flames, etc.) or the composition of the atmosphere, which could interfere with the measurement (presence of water vapor, etc.). Manufacturers therefore offer thermometers that operate at different wavelengths, so be sure to choose the right model for your needs.
• Infrared measurement is directly influenced by the emissivity of the target surface. This quantity, between 0 and 1, characterizes the way in which the surface radiates its infrared energy; it is specific to each surface and depends on material, color, surface condition, etc. To obtain an accurate measurement, it is therefore necessary to set the target’s emissivity correctly, or to choose a bichromatic pyrometer that measures at two different wavelengths, therefore eliminating this parameter.
• An infrared thermometer is an optical device with a certain field of view. It does not take the temperature at a precise point, but measures the average temperature of all surfaces in its field of view. To obtain an accurate measurement, the target surface must occupy the thermometer’s entire field of view. Otherwise, the thermometer will detect surfaces around the target with different temperatures and the measurement will be incorrect. To avoid this, infrared thermometers are generally equipped with an optical or laser sighting device.

Advantages:

• Measurements do not require physical contact and can be taken at a distance, making it possible to measure the temperatures of targets that move, are inaccessible or in aggressive environments.
• Measurements are possible at very high temperatures (>2000°C).
• Measurements are fast and reliable.
• They have become affordable in price.

Disadvantages:

• They only provide surface temperatures.
• Measurements may be affected by the atmosphere between the target and the thermometer (dust, water vapor, etc.). 
• The emissivity of the target surface must be taken into account.