Measuring the moisture

Written by: Tom Shelley | Published:

Tom Shelley reports on the latest technologies being employed to accurately measure moisture levels in manufactured products

Whether it is stopping bread going mouldy, ensuring the quality of encapsulated electronic devices or preventing the degradation of adhesives, new technologies are greatly enhancing the real time measurement of moisture and humidity in a number of industrial applications.

Near infrared and microwave measurements offer much greater reliability and precision than using altering capacitance. And it is much faster than traditional dew point and moisture content measurements.

In addition, there are new technologies under development including embedded optical fibres, measuring the capacitance of adhesive joints, magnetic resonance imaging, surface acoustic wave devices, and terahertz spectroscopy.

All were the subject of a seminar on Measurement and Control of Moisture in Materials, organised by the Thermal Measurement Awareness Network (TMAN) run by NPL in Teddington, Southwest London.

Production line compatible near infrared technology was described and demonstrated by Andrew Brunt, marketing manager of NDC Infrared Engineering located in Maldon, Essex. The company's 710e series NIR gauges use a quartz halogen lamp. The output is split into pulses of different wavelengths by a filter wheel rotating at 7,500rpm. The emitted light impinges on the products being monitored. The intensity of the backscattered light is then compared with that of the incident light and the moisture content, or concentration of other substances of interest, is calculated from the absorption loss.

Water vapour in the air path absorbs infrared light over a wide range of wavelengths but Brunt told us that the moisture measurement algorithms are designed so the 710e is desensitised to this effect. He also added that the company's NDC Optics Division manufactures the optical interference filters used in the gauges. He says: "We are users and customers of our own filters and also supply them to medical equipment and industrial instrumentation manufacturers as well as universities and research organisations."

Microwave moisture analysis was explained and demonstrated by NPL's Paul Carroll. He used a Sartorius supplied LMA300PA-000U Laboratory Moisture Analyser, which measured the moisture content of a bowl of breakfast oats.

It monitors both the shift in frequency and broadening of a microwave absorption peak, which varies with moisture content. "With calibration," Carroll says, "it is density independent."

Accuracy, he said, is ±0.05%. Measuring time is less than 1s with a measuring range of 0.01 to 60% moisture. NPL is able to measure, verify and calibrate air humidity measurements to 0.03%, and by weighing methods to 10µg absorbed moisture.

Looking to the future, Carroll said that his NPL research group is looking at surface acoustic wave techniques to measure moisture contents in coatings, and the laboratory's Dr Bill Broughton mentioned embedded optical fibres to measure swelling in composites and capacitance measurements to monitor the moisture content and quality of adhesive joints. The first method, he said, is being researched by Professor Gerard Fernando at the University of Birmingham and the second by Professor Dick Petherick at Strathclyde University.

Professor Peter McDonald from Surrey University then described how he had been using small, magnetic resonance imaging (MRI) machines to study tiny water filled pores in setting concrete. He said that they work in the same way as medical MRI machines, using a radio frequency excitation pulse to, 'kick the water molecule protons and start them precessing.' This is followed by emission of a decaying induced signal. However, whereas medical MRI scanners use very large superconducting electromagnets, these use much smaller commercially available permanent magnets.

Extracting meaningful information from the results is complicated, but it can be done. This is largely thanks to mathematical analysis methods developed by the oil and gas industry to assess the porosity of rock in oil wells. Relaxation times in free water are typically around 3s, around 10ms in the pores of cement, and just a few microseconds if frozen water.

But the newest idea to be studied for moisture measurement is terahertz spectroscopy, which was explained by NPL's Dr Mira Naftaly. Terahertz spectroscopy works in the far infrared, over the wavelength range of 100 to 1000µm. Until now, the main interest has been as a possible means of detecting weapons being carried by passengers about to board aircraft or trains.

However, within the 0.3 to 3THz frequency range, water vapour has sharp absorption lines while liquid water has a monotonously rising absorption edge. Interestingly, the relative magnitudes of the absorption lines seem to depend on the concentration in the air, or media, in which water molecules are found.

As effective is terahertz spectroscopy which has been demonstrated in studies of hydrated chemicals, paper, glass, acrylic plastic, pharmaceuticals, mixtures of oil and water and even chocolate coated biscuits.

In such cases, humidity should be maintained according to the past history of the article. One of the most dramatic pair of images that have been generated using terahertz imaging is that of a leaf from a plant that had been allowed to dry out, but was then able to take up water.

The images were produced by Daniel Mittleman at Rice University in the US using time domain spectroscopy. This opens up the possibility that such techniques may find applications in new medical diagnosis procedures. However, Dr Naftaly is doubtful. He says: "Medical diagnostic has been much talked about, hyped up in fact, and a lot of research has been done. It is now apparent that it is very unlikely that it will ever work, one of the many reasons being strong water absorption in living tissues. However, the equipment at this stage is quite unwieldy."

Current instruments tend to weigh around 500kg and cost £100,000. Sources typically can involve quantum cascade lasers, non-linear optical devices, novel gallium arsenide or indium phosphide diodes. But efficiencies are low and the detectors most common problem is cryogenic bolometer devices.

However, because developments in both lasers, solid state sources and devices tend to be rapid, we doubt if it will be very long before terahertz probe systems become available in at least desktop, if not hand held, sizes and at very acceptable prices.

Pointers

* New moisture content measuring technologies that are commercially available include near infrared reflection and microwave absorption peak shifting and broadening.

* Technologies under study and development include: use of embedded optical fibres, dielectric constant changes in adhesives, surface acoustic waves, MRI and terahertz imaging.


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