Selecting the right temperature sensor

Chris Jones, Managing Director at Micro-Epsilon (UK) Ltd, discusses two key parameters in selecting the most suitable infrared temperature sensor.

Plant and maintenance engineers use infrared temperature measurement devices, both handheld and fixed/online versions, as non-contact, relatively low cost, preventive maintenance tools. These devices accurately monitor, control and manage process temperatures and help to locate 'hot spots' on critical process plant, machinery and electrical connections - without having to interrupt production.
 
Using an online infrared sensor is beneficial in applications where the temperature of an object, material, surface or liquid is critical to the production process. When selecting the most suitable temperature measurement device for the application, engineers need to carefully consider their measurement requirements.

Benefits of non-contact infrared thermometers
Infrared thermometers measure the temperature of an object without touching it. It is therefore possible to perform fast, reliable temperature measurements of moving, hot or difficult-to-access objects. While contact temperature sensors or probes can influence the temperature of the target object, sometimes even damage the product itself, the non-contact method ensures precision measurements without damaging the target object. Infrared sensors can also measure very high temperatures, whereas a contact sensor would either be destroyed or would have a short service life.
 
Not only are infrared devices now relatively inexpensive, they also offer a raft of technical benefits and a variety of options for users, including handheld or inline process control, open connectivity to fieldbus systems and options for hazardous environments.
 
For accurate temperature measurement using infrared sensors, users must carefully consider two key parameters: emissivity and wavelength.
 
Emissivity
All bodies above absolute Kelvin (-273°C) emit infrared radiation in three ways, via a combination of emitted radiation, radiation reflected from the surroundings, and by transmitting the radiation through itself. How these factors interact depends on the material of the measurement object. However, for non-contact infrared temperature measurements, only the emitted radiation element is important.
 
The relationship of the emission types to each other is best described in the following way. If it is considered that at any given temperature, the sum of the radiation of the three emission types is equal to one, and it is assumed that solid bodies transmit negligible radiation, the transmitted element can be treated as zero. Therefore, the heat energy coming from an object only comprises emitted and reflected radiation.
 
It is now easier to understand why objects such as polished and shiny metals can only have a low emission or emissivity, as radiation from the surrounding environment is strongly reflected (and so proportionally high) by these surfaces.
 
For example, the typical emissivity for freshly milled steel at 20 deg C is 0.2 (reflected energy would be 0.8). This means 80% of the emitted heat energy from the object would be reflected 'heat energy' from surrounding objects! However, at a much higher temperature of 1,100°C, the same material will have a typical emissivity of 0.6.
 
In contrast, objects such as textiles or matt black surfaces reflect very little and therefore emit a high proportion of the heat energy. Emissivity of a black, matt paint at 100°C is typically 0.97 and so is much more suited to non-contact temperature measurement.
 
Many low cost devices have fixed emissivity correction of 0.95, which makes them unusable for almost all accurate temperature measurement tasks. All Micro-Epsilon temperature sensors have adjustable emissivity correction.

Wavelength
The previous description of emissivity is rather simplistic in order to explain the relationship between the three radiated energy components. However, it should be noted that the emissivity of an object will vary when monitoring the radiated heat energy at different wavelengths. Therefore, developing sensors that measure temperature at specific wavelengths can significantly increase measurement stability.
 
Put simply, material groups can be used to describe the optimum wavelengths for highest object emissivities and therefore the most stable results. For metals, 0.8 to 2.3µm, glass 5µm, textiles and most matt surfaces 8-14µm. Plastics are more complex, requiring specific wavelength sensors to be developed for polyethylene, polypropylene, Nylon and polystyrene (3.43µm). Polyester, polyurethane, Teflon, FEP and polyamide require 7.9µm. Thicker, pigmented films require 8-14µm.

To summarise, when selecting an infrared temperature sensor, it is crucial that the wavelength band over which the sensor measures is known, and that the correct wavelength band is used for the object to be measured. In addition, the object emissivity values over this wavelength and the temperature range to be measured must also be known or calculated.
 
A free guide to 'The Basics of Non-Contact Temperature Measurement', can be downloaded from website: www.micro-epsilon.co.uk   Refer to page 180

Impress Sensors' new pressure, flow and level sensors   

Sensor specialist Impress Sensors & Systems' now offer a new range of silicon pressure sensor modules for OEMs, a new differential pressure transmitter for level and flow measurement, as well as a new range of process indicators and panel meters, including a new compact, multi-channel controller with touch-screen display.
 
Impress Sensors' DSP range of silicon pressure sensor modules, have been specifically developed for OEMs to integrate into their own equipment or manifolds. The sensors are also designed to help solve the historical problems often associated with oil-filled pressure sensors.
 
Targeted at OEMs, the DSP range of piezoresistive silicon pressure sensor modules are suitable for general industrial use, environmental engineering, level measurement, process monitoring and laboratory/calibration equipment.
 
The stainless steel, 18mm diameter pressure sensor modules are available in two main versions: a direct contact type, where the media (air, non-aggressive gases and liquids) comes into direct contact with the silicon chip; and media separation via an oil-filled diaphragm.

The DSP series offers a wide range of operating pressures, from 20 mbar up to 1000 bar. The sensors also provide excellent thermal and long term stability, high overload protection and a high output signal. OEMs can choose between a thermally compensated, a non-compensated or a fully signal conditioned version of the sensor with ratiometric output.

As well as silicon pressure sensors, Impress Sensors are offering a brand new range of smart differential pressure transmitters. The XMD range is ideal for level measurement of closed, pressurised tanks, pump and filter control, or for closed pipe flow measurement when used in combination with an orifice plate, Venturi or nozzle.

The XMD pressure transmitters can switch the output from a standard linear one to a square root extraction where flow rate can be output, making the sensor suitable for a wide range of industry sectors, including chemicals, food and dairy, paper and pulp, petrochemicals, energy, power and pharmaceuticals. Pressure range is from 75 mbar up to 2 bar.

Pictured left to right: The SRP 147, the DSP 410Z and the XMD instruments.

Impress Sensors' complete range of process indicators, panel meters and controllers, including the CMC-99, a new compact, multi-channel controller with touch-screen display, which is ideal for the simultaneous measurement and control of a wide variety of process applications, can be found at: www.impress-sensors.co.uk
 
The CMC-99 controller offers an enormous variety of input and output combinations to suit every measurement task. Three card slots on the reverse of the unit each comprise 16 I/O pin options, which enable user to select, for example, up to 48 analogue current or voltage inputs; up to 16 relay/SSR outputs; 24 thermocouple inputs; or 12 RTD inputs; or a combination of these, depending on the application. Triggers and relays can also be set up via the controller's 24V DC digital input.
 
The SRP 147, a new wall mountable process indicator/display meter with extra large digital display, (pictured above), is also available now.
 
With more than 25 years' experience in the instrumentation industry, Impress Sensors is a rapidly expanding UK manufacturer and distributor of instrumentation products. Based in Aldermaston, Berkshire, the company is renowned for providing expertise in pressure measurement and for its knowledge of general instrumentation.
 
Impress Sensors' range of products includes sensors for pressure, temperature, level and distance measurement; process controllers; process indicators; data logging and recording systems; calibration equipment; as well as SIL2-approved and ATEX-approved products. 
 
For further information, view website: www.impress-sensors.co.uk  or  e-mail: info@impress-sensors.co.uk  Refer to page 191