How to Measure Liquids with NIRONE Sensors?

For most liquids including beer, wine, spirits, water, urine, milk, fruit juices, and organic solvents, the best measurement method using near-infrared wavelengths is transmission mode. In transmission measurements, light passes through the sample. The thickness of the sample will affect the absorption of light. The thicker the sample, or larger the optical path length, the more the sample will absorb the light.

Depending on the wavelength range and the type of sample, the typical optical path lengths used in the range from 1350 to 2450 nm are typically between 0.5 and 5 mm.

Table 1. Suggested Optical Path Lengths for Transmission Measurements
Sample Type NIRONE S1.7 NIRONE S2.0 NIRONE S2.2
> 90% water 2 mm 1—2 mm 1 mm
Vegetable oils 2—5 mm 2—5 mm 2 mm
Gasoline, diesel 2—5 mm 2—5 mm 1—2 mm
Milk 1 mm 1 mm 0.5—1 mm
Beer, wine 2 mm 2 mm 1 mm
Organic solvents 2—5 mm 2—5 mm 2 mm
Orange juice 2 mm 1—2 mm 1 mm

The transmission setups are almost similar to the in-situ gas measurement setups, see Setup for In-situ Measurements. An external light source, which sends the light through the light sample towards the NIRONE sensor, is a sufficient test setup for most cases. A possible method is to use the SMA adapter, and use a typical laboratory cuvette holder to hold the standard optical liquid sample cell (the cuvette). The external light source can also be coupled to the cuvette holder through optical fibers.

The integrated light sources of the NIRONE Sensor can only be used in the setup, if the measurement method is transflectance, a combination of transmission and reflectance measurement. In this case, a reflecting element, such as a mirror or a Spectralon white target, would be used to reflect the light back through the sample, after the first transmission. Because of the NIRONE light-source-to-detector angle, the result of using such a method would need a substantial decrease in the optical path length, so in most cases the optical path length would be 1 mm or less.