Millimeter waves for water content monitoring in materials and media

Millimeter waves for water content monitoring in materials and media

Meriakri V.V., Chigrai E.E., Parkhomenko M.P.

Institute of Radioengineering and Electronics Russian Academy of Sciences,

ABSTRACT: Specific advantages of millimeter (MM) wave aquamerty are discussed. MM waves ensure better spatial resolution and better sensitivity to water than microwaves. MM waves are less sensitive to conducting impurities as compared with microwaves; in addition, they can be used for water testing in media that are opaque for optical and infrared radiation. In this paper, the water content measurement methods for some liquid and solid-state materials are described. Examples of devices for water control in crude oil and alcohol are considered. Usually, it is possible to measure water content in real time (including in flow) with uncertainty less than 0.1%.

Keywords: moisture monitoring, aquamerty, millimeter waves

1 Introduction

The microwave method shows the following advantages:

· Nondestructive measurement

· Using of frequency dependent material characteristic such as:

o high permittivity of water

o relaxation processes in polar liquids

o small influence of ionic conductivity

· Microwave radiation propagates through opaque dielectric media, such that the volume moisture content can be measured, in contrast to infrared radiation, which is absorbed near the surface

· Microwave methods are fast and do not ionosize the test material as do some nuclear methods

· They operate under rough environmental industrial conditions; dust and water vapor have no influence on the measurement results

· high accuracy of measurements, high sensitivity, high slew rate

· continuous and discontinuous measurements and simple adaptation to automation processes can be realised

· simple handling, servicing and high reliability

· calibration under processing conditions

The monitoring of the composition of materials (including materials containing water) is an important problem of applied spectroscopy using optical, infrared, and microwave wavelength bands.

The aim of this paper is to examine the peculiarity of application of the relatively new millimeter (MM) wavelength region to the water content determination in materials and substances.

There is now extensive literature devoted to the interaction between MM waves and different materials, including water [1] –[8]. The main conclusions in relation to the water content determination with help MM waves are as follows:

1. MM waves ensure better sensitivity to the water content and better spatial resolution than microwaves (absorption of MM waves in water a > 15 dB/mm is much greater than practically in all monitored host materials; as wavelength l decreases, the absorption in water increases more rapidly than the absorption in these host materials).

2. MM waves are less sensitive than microwaves to conducting impurities in water.

3. MM waves can be used for testing materials that are opaque for optical and infrared waves.

2 Measurement methods

For wavelengths greater than 6 - 8 mm, the effective waveguide and resonator technique for measuring the properties of low-loss materials was elaborated in [2], [5] On the other hand, for wavelengths smaller than 0.5 mm, very good Fourier transform and laser spectroscopy methods are available [1], [6]. However, there are some difficulties in carrying out material investigation in the wavelength interval from 5- 4 mm to 0.6- 0.5 mm. The reason is that the waveguide technique is ineffective due to a decrease in waveguide dimensions and a gap between a waveguide and sample walls; on the other hand, the optical technique is ineffective due to the diffraction affecting the field structure and preventing the use of geometrical optic approaches.

The best way for measuring the material properties is to use a quasi-optical lens beam waveguide [3] that transmits only the fundamental low-loss mode with small enough cross-section dimensions of a beam and large losses for high order modes. In this case, the wave incident on the plane-parallel specimen and the wave on the receiving aperture are of the Gaussian type. Therefore, it is possible to estimate the measurement errors due to a finite thickness of a plane-parallel specimen, inclination of its boundaries to the beam waveguide axis, as well as due to cross-section dimensions of the specimen and receiving aperture. The methods for the determination of complex permittivity e* =e¢-ie¢¢ are mainly based on measuring the dependencies of real and imaginary parts of transmission and reflection coefficients on frequency, specimen thickness, and polarization of the wave.The magnitude of errors in the real part of the transmission and reflection coefficients measured by the beam waveguide technique is less than 0.5% for e¢ ranging from 1.1 to 200 and e¢¢ ranging from 10^‑4 to 1,whereas, in conventional free space measurements, these values may be greater than 10%.

3 Some characteristics of substances of interest

Tables 1 and 2 present the complex permittivity of liquids with maximal loss in the near MM region [7]. These tables show that water has the highest value of e¢¢.

Table 2 gives the complex permittivity e =e¢- i e¢¢ and absorption a for water depending on temperature at MM waves [7], [9]. These results are in a good agreement with [10].

Table 2 shows that, for l= 10-8 mm, a is practically independent of temperature.

Investigations of aqueous solutions of salts [7] showed that the absorption coefficient a of NaCl solution in water changes little with concentration (e. g. from 57 dB/mm to 56.5 dB/mm for concentrations 1 and 3 moles/litre at frequency 320 GHz and for temperature T = 18C). The value of a for KCl solutions increases in this case from 60.4 to 66.1 dB/mm (negative hydration).

Table 1 Properties of lossy liquids. (T=20C)

Liquid	e¢	e¢¢	l, mm
H₂O	6.3	8.8	2.0
H₂O	5.1	5.1	1.0
H₂O	4.7	4.0	0.70
H₂O	4.5	3.5	0.6
DMCO	3.4	1.6	0.9
(CH₃)₂CO	3.0	3.2	1.0
CH₃COC₂H₅	2.7	2.2	1.0
CH₃NO₂	3.2	5.2	1.0
C₆H₁₀O	2.6	1.1	1.0

Table 2 Complex permittivity and absorption coefficient for water vs wavelength and temperature

l, mm

T, C

e¢

e¢¢

a, dB/mm

23.5

26.7

29.8

31.9

33.1

33.7

15.5

15.3

15.1

18.1

20.6

23.2

28.0

29.0

30.8

18.8

18.9

9.1

10.0

10.9

16.0

17.5

18.9

29.5

30.8

31.9

6.3

6.5

6.8

8.8

9.7

10.5

41.1

43.7

46.1

Also we have investigated solutions of sugar in water. Table 3 shows the results of this measurements.

Table 3 Dielectric characteristics of sugar (W%) in water solutions (T= 18C, f =42.2 GHz)

Number	W%	e¢	e²	tand	a, dB/mm
1	4.6	13.0	22.1	1.70	19.3
2	4.8	12.6	21.9	1.71	192.4
3	5.4	12.7	21.5	1.69	190.3
4	0	13.8	23.8	1.73	201.0

Here loss tangent tand= e¢/e¢¢.

Tables 4- 6 present the measured parameters materials of interest for aquametry [3], [4], [7] - [9], [11], [12].

Table 4 Properties of low- loss liquids. (T=20°C)

liquid	n	tand ´ 10³	l, mm
cyklohexane	1.424	0.50	0.63
octane	1.396	0.74	0.63
decane	1.407	0.83	0.63
nonane	1.405	0.93	0.63
carbon tetrachloride	1.490	3.5	1.0
cysdecalin	1.474	4.2	1.0
transdecalin	1.461	0.6	1.0
benzene	1.510	5.2	1.0
pentane	1.380	1.9	1.2
toluene	1.510	5.4	1.2
1,4 dioxan	1.343	5.0	1.0
cooling liquid FC-43	1.380	1.3	1.0
CS₂	1.343	3.0	1.0
crude oil	1.470-1.570	0.8-1.4	2.0

Table 5 Building and natural materials characteristics.(l=2 mm, T=20C).

Material	n	tand ´ 10²	l, r
brick (red)	1.78	3.5	r=1.5
brick (silic.)	1.82	4.2	r=1.8
concrete	2.40	5.5	r=1.7
asphalt	1.50	8.0	r=1.3
sand	1.55	2.5	r=1.8
soil, 18C soil, -39C	1.60 1.60	3.8 2.5	r=1.5
snow, -36C snow, -1C	1.12	0.65 2.6	r=0.23
pine tree wood	1.4	3.4/2.0	r=0.5
glass, window	1.45	5.0
organic glass	1.60	1.5
marble	1.50	1.0
ebonite	1.67	1.1	l=0.6mm
cardboard	1.80	6.0
cautchuck	1.66	30
glues	1.57-1.72	1.0-2.0	l=1.7 mm
phenolone	1.8	3.9	l=1.7mm
polymyde	1.66	1.8	l=1.7mm
veneer	1.5	10	l=7.6mm
plaster	1.7	0.7

In Tables 4 and 5 n is the real part of the complex refractive index n*=n-ik and the loss tangent tand=2nk/(n-k).

In Table 5, r is the density of a material (g/cm³). The lesser value of tand for pine-tree wood corresponds to electrical field perpendicular to wood fibers. The water content in wood is less than 7%

Frequency dependence of n in MM range is weak but tand varies considerably as frequency changes. For example tand of brick at l = 6.3 mm is only 0.01, tand of concrete is 0.005.

Table 6 gives the properties of cloths [8]. Hear l is cloth thickness, ½t½², ½r½² are transmission and reflection power coefficients

Table 6. Cloths materials properties. (l=1.6 mm, T=20C).

Material	½t½², %	½r ½², %	n- 1	l, mm
cloths for tents	82-94	£ 1.3	0.2- 0.3	0.3 - 0.5
cloths for coat, wool	77- 84	£ 3.0	0.1-0.2	2- 4
cloths for suits, wool	85- 98	£ 1.0	0.2- 0.3	0.5- 1.0
silk	89- 93	£ 1.0	0.28- 0.35	0.15- 0.25
leather, natural	79- 85	£ 5.0	0.22- 0.28	0.9- 1.5
leather, artificial	75- 89	£ 5.0	0.22- 0.28	0.7- 0.8
fur, artificial	75- 89	£ 3.0	0.04- 0.07	4.0- 12
astrakhan	71	£ 1.0	0.14	35
cloths for shirts	92- 95	£ 5.0	0.18- 0.23	0.2- 0.3

All materials in Tables 4 -6 have strong dependence of tand and êt ê² on moisture, e.g. dry cloths (Table 6) have êt ê²= 75-98%, while, for moisture 28%, ït ï²=25-80%.

Theresults of such investigations allow one to clear up the possibility of testing the water content in these materials, to estimate the accuracy of such control, and to find the most appropriate measurement setup.

The devices use a certain wave guiding structure connected to a material under test. The measuring parameters (losses or phase constants of the propagating wave) are changed due to the presence of this material.

Depending on e¢ and e¢¢ of dry material and the water content W, structures are used where a wave either propagates through the material, or is reflected from it, or some part of the power of the wave penetrates into the material under test [5], [9], [12]-[15].

We investigated water emulsions [7], [9]. Of the greatest interest for practice is the water content in a crude oil emulsion. In the MM wavelength range, crude oil has e¢ = 2.10 -2.20 and e¢¢ = 0.002 -0.003 depending on sort. Losses in crude oil are changed from a =0.05 -0.07 dB/cm at wavelengths l = 8-7 mm to a = 0.13 -0.18 dB/cm at wavelengths l = 4-3mm as compared with a = 200 dB/cm and a = 300 dB/cm for water, respectively.

The experimentally found characteristics of water in oil emulsions for different content of water W depending on wavelength are presented in Table 7, T = 20C.

Table 7 Characteristics of water in crude oil emulsion vs water concentration and wavelength

l, mm

W,%

e¢

e¢¢

a×10, dB/cm

0.5

1.0

2.16

2.19

1.5

2.5

2.7

4.6

0.5

1.0

2.16

2.19

1.5

2.7

3.6

6.1

0.5

1.0

2.15

2.18

1.6

2.8

7.4

12.8

0.5

1.0

2.15

2.17

1.5

2.5

13.8

23.4

The data in Table 7 are in a good agreement with the Lichtenecker formula at wavelengths greater than 5 -6 mm [9]

Here e and e are the permittivities of water and oil respectively, e is the permittivty of emulsion.

In the short-wave part of the MM range, these results are well described by the Maxwell-Garnett model [9].

The particle size of emulsified water was much smaller than the wavelength in the medium.

Table 7 shows that the sensitivity to water in crude oil at the MM range is very high. Therefore, it is possible to realize an aquameter for crude oil, capable of measuring the water content in- flow irrespective of the oil sort and in a wide interval of temperatures.

4 Devices for water monitoring in different media

Now we consider some examples of devices for water testing in different materials and substances.

One of such devices is described in [9], [13]. This device uses waves with l = 8-10 mm in standard steel oil pipe with an internal diameter of 50 mm and a central conductor. The device measures the absorption in a pipe filled by crude oil under test. The measurement uncertainty DW better than 0.08% was achieved over an interaction length of about 40 cm in the moisture content range W = 0.1-1 %.

A modified design [9] consists of rectangular exiting and receiving waveguides with longitudinal slots. These waveguides are inserted at an angle j into a pipe containing flowing oil. The slots are hermetically sealed by a dielectric. The angle j corresponds to the Brillouin angle for the mode in the waveguides. For a slot length of 30 mm, a spacing between the waveguides of 30 cm, and a pipe diameter of 50 mm, the moisture sensitivity at frequencies 32 -35 GHz was found to be 13 -15 dB per 1% of water.

A single slotted waveguide was used to monitor the presence of oil in water. In this case, the sensitivity to oil was about 4 -5 dB per 1% increase in the oil content for an oil concentration of W= 0.5 -2.0 %

The same slotted waveguide was used to measure the alcohol content in water [9].

Table 8 presents the real and imaginary parts of the permittivity of water-alcohol solutions.

Table 8. Permittivity of water-alcohol solution, (T = 20C, l = 9 mm)

W,%	0	10	22	40	94,5	96
e¢	19.6	12.4	8.6	6.4	4.2	4.1
e¢¢	29.0	21.2	14.6	7.9	0.9	0.8

When the alcohol concentration was W=8 -40%, the sensitivity at frequencies 35 -37 GHz was 0.30 -0.35 dB per 1% of alcohol. For W >40%, the sensitivity becomes lower.

Therefore, for low concentration of water in alcohol, a dielectric waveguide in contact with a cell containing alcohol-water solution was used [14]. Measurements were performed at frequencies of 25 -27 GHz. The dielectric waveguide was made of high-resistivity silicon (e¢= 3.42, tan d= e¢¤ e¢¢= 0.001). The mode was used in a waveguide with dimensions of 1.4 ´ 2.8 mm. The length of a polyethylene cell with a liquid under test was 25 mm, and the wall thickness was 1 mm. The losses introduced by the cell for water concentration in alcohol in the range W= 4-10 % were 0.5 -0.7 dB per 1% of water

The next object of interest is powders. These materials are widely used, for example, in metallurgy, and the water content in such powders essentially influences the quality of output.

The most interesting objects are metal-containing powders (e.g. iron concentrate, charge), connecting materials (for example bentonite), and some other materials used in the so-called powder metallurgy. For high-quality production, it is necessary to guarantee that the humidity of powder should lie within a narrow interval (no grater than fractions of a percent). In this and some other cases it is necessary to check water content in-flow or periodically in a few time intervals. Usial chemical methods using drying of material do not permit to realize this testing.

Some devices for the in-flow water content testing in powder media were elaborated in [15]. One of them is a quasi-optical interferometer for measuring the reflection coefficient ïr ï² depending on moisture.

Another device uses a waveguide supporting a slow wave. A part of power of this wave penetrates into the material under test with complex permittivity e*, and, as a result, the transmission coefficient changes depending on the material properties. Such type of moisture meters was realized at frequencies 26-38 GHz [15]. The accuracy of water content determination is about 0.2-0.1% or better.

In addition, a device using a dielectric resonator was elaborated. The resonance frequency and the quality depend on the material permittivity and moisture. The resonator may rotate if either the material or the measuring device move.

These measurements methods can also be used for the determination of water content in timber and paper.

The last device elaborated by us for the determination of water content in liquids is a quasi-optical instrument for crude oil flowing in pipes under pressure. This device uses short MM waves (W band) and allows one to measure the water content from 0 to 3% vol. with an accuracy of 0.05% in real time.

5 Conclusions

MM waves provide new possibilities for measuring the water content in media and materials. However, these possibilities have not been extensively used in practice. The reason is that the electrical properties of monitored objects were not adequately investigated at MM wavelengths and, secondly, MM waves were not widely used for moisture content monitoring despite the fact that such monitoring was technically possible. The above examples of successful application of MM waves to moisture content measurement suggest that we can look forward to the wide application of MM waves in aquametry.

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[14] Meriakri V.V., Parkhomenko M.P.: Application of dielectric waveguide for water control in alcohol

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Corresponding author Meriakri Viacheslav Viacheslavovich, Head of Laboratory of Millimeter and Submillimeter Waves Measurements, Institute of Radio Engineering and Electronics Russian Academy of Sciences, Vvedenski sq. 1, Fryazino Moscow Region, 141190, RUSSIA.

Tel: 07095-5269266 FAX: 07095-7029572

e-mail: meriakri@ms.ire.rssi.ru meriakri@ms.ire.rssi.ru

http:www.meriakri.de.vu