Microwave and NIR. Complementary, not competitive, technologies

There is a common misconception that, where moisture measurement is required, there is a choice between NIR and microwave sensors.  Both technologies can measure moisture, but each has its advantages and disadvantages in different applications that must be considered.

This article compares both NIR and microwave systems and explains that, when it comes to measuring and controlling moisture, these technologies do not compete against each other but are complementary solutions. Each system should be selected based on what they do best.

NIR sensors can, in addition to moisture, measure other constituents like fat and protein content. They do not require contact with the actual substance being measured; and as they measure only the surface layer are able to measure small amounts and static material.

As they only measure the surface of the material NIR sensors will not give a representative measurement if the surface layer is a different moisture to deeper layers.  This is common in materials that have been heated by dryers or strong sunlight as the surface layer dries more quickly.

NIR sensors, whilst designed to minimise the effects, are still affected by variations in dust, light, and changes in the material colouration.

NIR Sensors are considerably more expensive than microwave sensors to purchase and set up and require ongoing maintenance contracts which are not required for microwave sensors.

Digital microwave sensors, use a penetrative measurement technique to measure deeper into the material. they are not affected by changes in dust, light and material colouration. They can use a highly wear resistant ceramic that is designed to withstand material contact rather than delicate lenses. This enables microwave sensors to withstand high wear, dusty industrialenvironments.

Components used for measurement do not degrade over time and therefore require no maintenance.  This resistance to wear and tear, coupled with their ability to perform non-destructive, inline measurement, lowers the total cost of ownership by virtue of the devices’ physical longevity and low maintenance requirements.

The main drawbacks of microwave sensors are that they can only measure moisture, must be in contact with the material and require a consistent material flow.

Because of the huge range of potential applications, and the wide range of requirements within each of those applications, NIR and digital microwave sensors each have their place. The key is to understand what type of sensor to choose for each application, how many to install, where to place them and where they would benefit from being paired with a counterpart, whether NIR or digital microwave. They are not mutually exclusive.

There are many processes that only affect the level of moisture in material, so there is no need to measure the other properties such as fat and protein that are typically the province of NIR sensors.

If many sensors are required, it makes more sense to invest first in microwave sensors that are less expensive, easy to install and have much lower long-term maintenance requirements, all with no compromise in performance.

Such a choice would be particularly applicable to measure moisture levels associated with raw material intakes, conditioning, milling systems, pelletiser and dryer inputs/outputs as well as storage inputs, for example. Where measurements for protein, fats or other components are required an NIR sensor is the obvious choice.

The relative lower costs of digital microwave moisture sensors when compared to NIR make it cost-effective to install a sensor on both the input and output of a process.

The sensor on the input to the process can be used to calculate parameters to control the process, and the output sensor can then be used to determine the remaining error and used as a process variable to calculate correction factors for the process.

In such cases, digital microwave moisture sensors are the clear and most cost-effective choice.

One common example is drying applications where controlling the moisture will affect the amount of protein denaturisation and degradation that occurs. Digital microwave sensors can be used before and after the process to measure the moisture content of the material and determine control variables for the dryer.

Using two NIR sensors would be costly whereas digital microwave sensors will be unhindered by the effects of the surface layer drying at the output and ensure more material is measured due to the penetrative measurement technique.

An NIR sensor can be used to monitor the effects on the proteins after the material has been dried, and this information can be used to refine the drying process target and optimise the process. Many modern systems are now using AI to automatically optimise process control, and require sensors that deliver highly accurate, repeatable, and precise measurements in as many locations as possible. Both types of sensors have these qualities.

Sensors should be used for their relevant strengths and specific purpose, for example, NIR to measure fat and protein measurement and/or where small amounts or static material must be measured. Digital microwave sensors where only moisture measurement in online dynamic processes is required. This can result in a comprehensive, cost-effective solution for a much wider range of process steps, and better overall process control.

Hydronix has a wide range of moisture sensors for use in everything from high wear, high temperature and food safe applications to explosive atmospheres. The Hydronix XT sensor range, perfectly complements NIR sensors.

Although differing in their measurement techniques and approach, this alignment of purpose enables any user to choose sensors tailored for their specific need in a way that minimises capital outlay and ongoing maintenance costs without compromising the number of measurement locations and quality, whether it be solely to measure moisture, fats and proteins, or all three.