Choosing an Infrared Pyrometer for Plastics – 8 Important Things to Know.

Infrared pyrometers are ideally suited to plastics industry applications. As a non-contact method of measurement, the sensor can be mounted at a safe distance from the often arduous environment of the process. There are however some careful considerations when selecting a sensor. This guide will help you but remember, we are here to help with your sensor selection. Why not contact us now to discuss your application.

1. The material you want to measure and its surface finish.

Knowing which material you wish to measure is extremely important to be able to take a reliable temperature measurement. This will have a direct impact on the sensor chosen and the emissivity setting you use. Fortunately, most plastics are easily measured using non-contact infrared methods and have a high emissivity. This enables measurement using low-cost sensors in many cases.

It is important to recognise however that metallic parts are less easy to measure and if your product combines both or you wish to measure the temperature of mould tools, care is required. For metal parts, we offer various sensors sensitive to short-wavelength infrared radiation. Using the shortest wavelength possible for your measured temperature will help to avoid most issues associated with low, unknown, or changing emissivity.

2. The size of the object that you are measuring.

When using an infrared pyrometer the size of the object being measured needs to be known so the correct sensor can be selected. The measuring spot for the sensor needs to be smaller than the part being measured and needs to be positioned completely on the part. We can offer plenty of choices when it comes to selecting a sensor with a variety of optical resolutions available including spot sizes down to less than 1mm in some cases.

For plastics applications where small parts are being measured, we offer a variety of high-quality optics which give small measuring spots at a larger distance and use laser aiming to ensure correct positioning of the sensor.

 

3. Ambient temperature.

Is the sensor you have chosen suitable for the operating environment it will be working in? If not, it is possible to cool or heat the sensor to ensure it meets the operating parameters. Often this is achieved by fitting air or water cooling.

Thermal shock is another phenomenon that can cause issues with infrared temperature measurement and needs to be avoided. This is a potential problem for smaller sensing heads and the addition of thermal mass, usually a larger housing can overcome the issue.

plastic products - bottles containers packaging

4. Dust, dirt, and humidity.

Is there a high possibility of debris or moisture settling on the lens of your sensor? Infrared temperature sensors work on their ability to be able to “see” the object you wish to measure. If there is dirt, debris, or condensation on the lens of the sensor the ability of the sensor to measure properly is diminished resulting in inaccurate measurements. Usually fitting an air purge collar to the sensing head is sufficient to prevent this.

In some plastics applications, there is a potential for smoke, steam and dust to be present in the environment and again this can prevent an accurate measurement from being taken.

 

5. How will you aim the sensor?

For many applications, it is easy to aim the sensor as the parts are large enough that it is almost impossible to miss. However, when considering smaller parts and when measuring through gaps in machinery aiming is extremely important.

Laser aiming is the simplest method and is commonly used. The Optris system of dual laser aiming shows you exactly where the sensor is aiming and also the size of the measuring spot.

6. How thick is your material?

For most plastic parts the thickness of the material is not an issue and can be disregarded. However, in applications where thin-film plastics are being used, it is extremely important to consider.

Where materials are sufficiently thin, it is perfectly possible for an infrared sensor to be able to “see through” the material and have its measurement influenced by the objects behind the material being measured giving false readings. This phenomenon is only an issue where materials are thinner than 0.4mm. In these cases it is important to choose a sensor that has been specifically designed for the purpose and is sensitive to a specially selected wavelength which does not pass through the plastic film.

Optris offer such designs for a variety of thin-film materials and processes.

 

7. Sensor output signal.

A sensor is no use if you cannot integrate it with your process measurement and control equipment. Of course, there are the industry standard mA and V signals and these are utilised extensively across the Optris range. However, if you have been using a different type of sensor or intend to use a communication system, what can you do?

In some cases Optris sensors are able to provide a thermocouple output so if you replace an existing thermocouple sensor, simply wire up your new IR sensor and no other configuration is required.

Many Optris sensors have the capability to fit an interface card which upgrades the capabilities of the sensor. This can be USB or Ethernet for communication with a PC, or ProfiBus or CANBus for industrial communication.

8. Speed of response.

By their very nature infrared temperature sensors are massively quicker than contact probes such as thermocouples or RTDs. However, in some applications speed of response is extremely important. Typically, an Optris sensor will have a response time of less than 100 milliseconds but we offer products with response times as fast as 1 millisecond and with our new CT 4ML sensor the response time is now as low as 90µs.

 

This article is intended as a guide and does not replace our own free technical assistance which is always readily available.

If you would like additional assistance, we can help you. Please contact us and we would be glad to discuss your application and help with your product selection.

T: 01628 778688

E: sales@processparameters.co.uk

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