To avoid the effects of temperature and humidity fluctuations in the production and process environments, measurement is critical, reports Greg Pitcher.
If you’ve ever travelled to the Tropics, or the Arctic Circle, you will know that very moist or very dry air can have quite an impact on the human body.
So it should come as no surprise that fluctuations in relative humidity – the amount of water vapour in the air as a proportion of the volume of water that air can hold – can be damaging for the manufacturing process.
“Just about all processes and production techniques are affected in some way by humidity levels,” says Chris Fox, sales manager at the UK arm of Swiss-headquartered manufacturing products firm Rotronic Instruments.
“Low humidity can lead to the possibility of electrostatic build up, while high humidity can lead to deterioration of product because it gets too moist.”
Fox says that many products are designed to contain a degree of moisture as binding material, but when this level creeps up it can reduce shelf life.
Low humidity can lead to the possibility of electrostatic build up, while high humidity can lead to deterioration of product because it gets too moist
Chris Fox, sales manager at Rotronic Instruments
At the other end of the scale, a build-up of static electricity can damage components and in extreme circumstances potentially lead to explosions. So measuring humidity in factories is critical.
Rotronic sells a range of instruments for measuring relative humidity in manufacturing environments, including probes and handheld devices.
“We look at the humidity and temperature of the air within the manufacturing process,” says Fox.
“We look within the process – that could be in the exhaust air but usually within the machinery, which could include ovens, climate chambers and dryers. It could be an area the size of a room right down to small pieces of kit – we have 4mm diameter sensors.”
The key is to get the right piece of kit for the application. Rotronic guides customers through this process, first looking at what temperatures the measuring devices will be working at.
“This could range from -100°C to +200°C,” says Fox. “You need very different sensors to be able to withstand the different operating conditions.”
Bespoke measurement
Temperature is also an important factor in humidity levels.
“It is very important to measure temperature and humidity, as humidity varies greatly according to temperature,” says Fox. “The greater the heat, the lower the humidity. A 1°C difference in temperature can lead to a 6% change in relative humidity.”
Production engineers should work out tolerance levels for temperature and humidity at each part of their processes, Fox urges.
Another factor in deciding which kit to use is the physical environment.
“If there is a pilot hole in a duct then we can suggest a suitable sensor for this. A customer might want a handheld or wireless sensor.
“You might have some control kit and an engineer may also use a handheld instrument to make double checks.”
The sensors typically provide analogue output that is translated into a reading for manufacturers. However, it remains for the production firm to treat the information the right way.
“If customers don’t act on alarms, and allow humidity and temperature to go outside tolerances, products may have to be destroyed as they don’t meet quality standards,” warns Fox.
Different sectors want different things from their humidity readings.
“The pharmaceutical industry worldwide needs very high accuracy, whereas all manufacturers need reliability and repeatability,” says Fox.
“Brick and tar manufacturers want to know their probes will give similar measurements today as in a year’s time.”
There may soon be the chance to read humidity and temperature remotely on your phone
Stuart Mooney, test engineer at TÜV SÜD
Measurement certainty is a huge topic in itself. If an instrument is checking levels of humidity, what is checking the accuracy of the instrument?
The United Kingdom Accreditation Service (UKAS) plays a huge role here, assessing bodies that provide certification, testing, inspection and calibration services.
“We have UKAS accreditation for temperature, humidity and dew point laboratory calibration,” says Fox. “That means we can calibrate sensors and produce a UKAS certificate to support performance of instruments.”
Rotronic offers a range of calibration devices to check the performance of customer’ sensors.
“If it turns out that the sensors are not as accurate as before, then a customer can add that tolerance on,” says Fox. “In pharmaceuticals, some companies may calibrate every three months. Elsewhere it may be every two years.
“If you do the calibration, you will find you are measuring ‘X’ amount below or above and you can make an arithmetic adjustment.”
Sensing change
Testing and certification firm TÜV SÜD uses sensors to monitor humidity conditions.
“We have sensors installed in all of our humidity-capable chambers for the purposes of humidity control and monitoring; there are numerous systems monitoring the lab ambient conditions in select locations throughout the building; and we also use handheld instruments as required,” says TÜV SÜD environmental test engineer Stuart Mooney.
The firm has humidity chambers of various sizes that customers can use to test kit out in to ensure it can stand up to different conditions.
“We do have pieces of manufacturing kit that go into the chambers,” says Mooney.
“We go from -65°C to +250°C, and control humidity within the range +5°C and +85°C, from between near 0% relative humidity (RH) to 97% RH.”
When providing such a test environment, it is critical that the firm is measuring humidity accurately. “Hygrometers are the measuring device in every chamber,” says Mooney.
“When we tell a customer something was tested at 40C, 80% RH, they can ask how we know that and, if we’re relying on a sensor, then we have to prove the sensor itself is accurate.”
Again, TÜV SÜD is a UKAS-accredited lab. “Mobile measuring devices are sent to our calibration lab which uses gold standard equipment to calibrate our devices,” says Mooney.
“They will try many different levels of conditions and see what our day-to-day equipment is saying.”
Instrumental drift
The process works in reverse when it comes to assessing the accuracy of the sensors built into the chambers.
“We can put [the calibration] temperature monitoring gear in the chamber and set the chamber to 50°C. Does the gold standard equipment agree? If not, we can apply offsets.”
It is rarely good practice to tweak the sensors when inaccuracies are found – more commonly an adjustment is made to the readings.
We have sensors installed in all of our humidity-capable chambers for the purposes of humidity control and monitoring
Stuart Mooney, test engineer at TÜV SÜD
“If you hear back from the lab that your sensor is reading 1% point below true humidity levels, you can add that on so you know a reading of 49% RH represents a true 50% RH,” says Mooney.
“Sensors drift – you will never have each chamber running exactly the same. Each will have quirks and there will be wear and tear of the sensors.”
Sometimes humidity-measuring kit will become particularly inaccurate at certain levels of humidity or temperature, and the firm will stop using these devices in those conditions. Other times a sensor will get to the end of its useful life and be replaced. But often adjustments are just recorded as a matter of course to ensure accuracy.
Technology has also improved the measurement of humidity in labs and the future could bring more advances.
“There may soon be the chance to read humidity and temperature remotely on your phone,” says Mooney. “Apps are versatile and anyone who is savvy could almost certainly write a piece of software that shows what is going on in a chamber – but it needs someone interested enough to do it.”
Fox forecasts an increased demand for humidity measurement. “It is led by regulation,” he says.
“We have seen an uptake in the use of humidity sensors in the manufacturing of pet foods, for example. This leads to an increase in the product quality. We expect more regulation and more demand.”
