Turn up the heat

Adding to an already wide range of temperature sensors, NPL has developed what it believes to be the most accurate thermometer yet

Temperature sensing is a relatively mature technology, but it is not without its problems. For instance, a sensor’s characteristics can change over time this is known as drift so readings become less accurate. They can also break and give completely wrong readings, which can be catastrophic for product quality.

The device measures the speed of gas molecules
The device measures the speed of gas molecules

A large body of research has focused on improving the reliability of temperature sensors, while other developments have sought to make measurements in places or at times that would have previously proved impossible, such as monitoring the temperature of an article in transit.

Most thermometers use one of three types of ’contact’ sensing technology: thermocouples, platinum resistance thermometers (PRTs) or thermistors. But owing to an increased level of understanding and a lower cost, non-contact sensors, including thermal cameras, are now being used increasingly.

For most situations, the best sensor to use is a thermocouple. This sensor comprises two dissimilar metal wires joined at the tip and a readout unit, which measures the voltage generated by the sensor and uses it to calculate the temperature. Thermocouples are relatively cheap and versatile and typically have an accuracy of within a degree up to 100°C, with a widening uncertainty up to 1,000°C.

NPL’s Primary Acoustic Thermometer
NPL’s Primary Acoustic Thermometer

Thermocouple wires are generally packaged inside a metal tube and insulated from each other with plastic or glass fibre (for low temperatures) or ceramic (for high temperatures). The sensors are available packaged for specific measuring applications, such as determining the temperature of pipes, food or surfaces.

An alternative to the thermocouple is the electrical resistance sensor. A PRT sensor is a thin platinum wire whose electrical resistance varies approximately linearly with temperature. A thermistor sensor consists of a bead of metal oxide whose resistance varies in a highly non-linear manner.

In each case, the temperature is determined by a readout unit that measures the electrical resistance of the sensor. In general, PRTs offer better accuracy and stability than a thermocouple over the range -100°C to 500°C, while thermistors offer greater sensitivity but over a more limited temperature range.

Where objects are so hot they would destroy ’contact’ sensors, or where objects are moving, such as on a production line, a pyrometer is a good choice. This measures the thermal radiation emitted by an object to determine its surface temperature. They can work from -40°C to greater than 2,000°C, and can be as accurate as a contact sensor. The uncertainty of measurement depends on the surface finish of the object shiny objects are very difficult to measure. More recently, thermal cameras consisting of an array of thousands of pyrometer sensors have become available for less than £1,000 and open up a new way of viewing the world.

Thermal cameras are now available for less than £1,000 and open up a new way of viewing the world

All these sensors have been developed to solve particular measurement challenges. For example, button-like sensors can be placed in a package where they will record the temperature at preset intervals and provide a data readout on arrival. Sensors can even be built into food labels to ensure that a package, such as one containing raw meat, has not been warmed beyond a specified temperature for more than a preset time.

Temperature sensors come in all manner of shapes and sizes

Temperature sensors come in all manner of shapes and sizes

Temperature sensors are also used in networks to monitor changes over large areas. The biggest example is the network that monitors the Earth’s temperature, consisting of more than 10,000 sensors around the world held in Stephenson screens (slatted boxes standing 1m off the ground). Originally used for short-term weather prediction, their data has been re-analysed to help us understand the impact of climate change. The network analysis inter-compares sensors to find anomalous readings.

But how well can such measurements be trusted? This is a challenge the National Physical Laboratory (NPL) the UK’s national measurement institute has been addressing since its foundation in 1902.

NPL has produced thermocouple sensors that can ’self check’ by integrating a small sample of a pure metal

As all temperature sensors change over time, it is essential to check for drift, either by inter-comparing multiple sensor types or by regular calibration at a UK Accreditation Service (UKAS) laboratory. At NPL, thermometers are checked at ’fixed point’ temperatures those at which a pure substance freezes. NPL has also produced thermocouple sensors that can ’self check’ by integrating a small sample of a pure metal.

The thermocouple can be checked by heating it sufficiently to melt the sample and checking the reading matches the known melting point. At very high temperatures, pure metals rapidly react and become impure, so NPL uses combinations of pure metals and carbon to form what is known as a eutectic solution a substance that melts at a single temperature. When sealed in a graphite crucible, these devices create reliable fixed points up to 2,700°C.

NPL has now built what it believes to be the most accurate thermometer ever constructed. The NPL Primary Acoustic Thermometer measures the speed of sound in a gas inside a spherical copper resonator. The speed of sound depends on the speed of the gas molecules, which is linked directly to the fundamental definition of temperature. This is work that NPL hopes will underpin all the temperature measurements carried out in the UK by business and government every year.

what’s new – sensors

For hundreds more stories like these visit www.engineeringtalk.com

Cause and effect
Intelligent-sensor development company Product Innovation Limited (PIL) has unveiled its latest liquid-level sensor technology. The Hall effect sensor was designed and developed by PIL after a customer requested a sensor that would have good resolution and that would work at high temperatures and at very low power. The sensor works on the principle of a spherical float and a series of Hall effect transistors (instead of reed switches, which PIL claims can be unreliable) assembled on a long, very narrow printed circuit board. Using both software and hardware, the controller in the top of the unit is designed to avoid the inherent problems of Hall effect devices in this kind of application. www.productinnovation.com

Pipe dream
Confocal displacement sensors from Micro-Epsilon are being used to measure the diameter of round and oval stainless steel pipe destined for use as catalytic converters for passenger cars. According to the company, the optoNCDT IFS 2400-20 confocal sensor has a high aperture, large stand-off distance and the ability to measure reliably on a variety of different surfaces. The 2400 series devices are designed for distance and one-sided thickness measurement. The compact design, large tilt angle and large stand-off distance enable the sensor to be used in many different applications, but it is particularly suited to precision measurement against any diffuse or specular material, such as film, liquids, glass, metals and polymers. www.micro-epsilon.co.uk

Radiation response
Surface Technology International (STI) has been helping with the response to the recent nuclear emergency in Japan, by increasing supply of its electronic components designed for high reliability. Radiation detection instrumentation supplier Canberra UK has been using STI to supply parts for the iCAM Alpha/Beta Continuous Air Monitor. This unit acts as an alarm monitor for operators, while measuring airborne alpha and beta particulate activity in real time. STI said that it was able to refocus its production teams to meet Canberra’s extra requirements following the recent earthquake and major radiological incident at Fukushima. www.sti-limited.com

Hot metals
Optris has unveiled its portable and stationary infrared (IR) thermometers, which are designed for temperature measurement applications within the metal processing industry. The portable devices can be used for infrequent measurements, while the stationary devices are suitable for use in quality management and process control. Optris offers these short-wave measuring IR thermometers with the wavelength identifiers 1M, 2M and 3M, for the measurement of metal surfaces. The stationary 3M devices, with a starting temperature of 50°C, enable metal and composites to be monitored at close to room temperature. The 1M devices are calibrated for use at up to 1,800°C for high-temperature applications for example, welding or hardening. www.optris.com