Measuring and testing – Specific gravity or density of liquid or solid
Reexamination Certificate
1999-10-01
2001-07-17
Moller, Richard A. (Department: 2856)
Measuring and testing
Specific gravity or density of liquid or solid
Reexamination Certificate
active
06260406
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a densitometer for measuring the density value of a to-be-measured substance such as a solid or suspension matter in a to-be-measured liquid using microwaves.
A densitometer using microwaves measures density by measuring the phase delay of microwaves on the basis of the fact that the microwaves have a phase delay almost proportional to the density value of a to-be-measured substance in a to-be-measured liquid as a medium.
A densitometer of this type using microwaves comprises transmission and reception applicators
62
and
64
arranged in a tube
63
as microwave antennas, a microwave circuit
79
as an electronic circuit, and a calculating section
81
, as shown in FIG.
1
A. An oscillator
55
generates microwave signals
56
and
57
having a frequency
f
. The microwave signal
56
is amplified by an amplifier
58
. When switches
59
and
60
are in the states shown in
FIG. 1A
, a transmission signal
61
is sent to the transmission applicator
62
in the tube
63
and then into the tube
63
in which a liquid
63
A to be measured is passed. The signal is received by the reception applicator
64
which is set in the tube
63
to oppose the transmission applicator
62
.
A reference oscillator
65
generates reference signals
66
and
67
having a frequency f+&Dgr;f slightly different from the frequency
f
of the microwave signals
56
and
57
from the oscillator
55
. The microwave signal
57
and reference signal
66
are mixed by a mixer
68
to obtain a reference-side heterodyne output
69
as a difference frequency &Dgr;f. The output
69
is converted into a reference-side digital signal &thgr;REF
71
through a low-pass filter
69
A and a comparator
70
whose threshold value is 0V, and sent to a phase difference measuring section
72
.
A reception signal
73
received by the reception applicator
64
is amplified by an amplifier
74
. The amplified reception signal
73
and the reference signal
67
are mixed by a mixer
75
to obtain a measurement-side heterodyne output
76
as a difference frequency &Dgr;f. The output
76
is supplied to a comparator
77
via a low-pass filter
76
A, converted into a measurement-side digital signal &thgr;FB
78
, and sent to the phase difference measuring section
72
.
The phase difference measuring section
72
obtains a phase difference &PHgr;V between the two digital outputs &thgr;REF
71
and &thgr;FB
78
. As shown in
FIG. 1B
, the difference between the leading edges of the signals &thgr;REF and &thgr;FB is obtained as the phase difference &PHgr;V.
In the microwave circuit
79
indicated by the dotted line, the phase changes due to, e.g., a change in temperature in the circuit to generate an error. To compensate for the error, the switches
59
and
60
are connected to the sides opposite to those in
FIG. 1A
to measure a phase difference &PHgr;R through a fixed reference
80
, and the phase difference &PHgr;R is subtracted from the phase difference &PHgr;V.
The fixed reference
80
uses an attenuator to drop the signal level of the microwave to the same level as that of the signal received by the applicator
64
.
A phase difference &PHgr; is given by &PHgr;=&PHgr;V−&PHgr;R.
When data (calibration curve data) associated with the reference density is obtained in advance, a calculating section
81
can calculate, on the basis of the data, the density value of the to-be-measured substance in the to-be-measured liquid
63
A from the obtained phase difference &PHgr;.
Let D be the density value. The relationship between the density value D and the phase difference is essentially described by linear equation (1). Values a and b can be determined by measuring phase differences while changing the density value and performing regression analysis.
D=a&PHgr;+b . . . (1)
In water as a conductive fluid to be measured, the following relationship holds between the attenuation and phase delay of a microwave, and a conductivity a, permittivity, and temperature t of the fluid to be measured:
α
=
z
0
2
σ
+
ωϵ
0
ϵ
r
″
1
+
1
+
(
σ
ωϵ
r
′
ϵ
0
+
ϵ
r
″
ϵ
r
′
)
2
(
2
)
β
=
ω
μ
r
ϵ
r
′
c
0
2
1
+
1
+
(
σ
ωϵ
r
′
ϵ
0
+
ϵ
r
″
ϵ
r
′
)
2
(
3
)
where &sgr; is the conductivity of the fluid to be measured, and &egr;r′ and &egr;r″ are respectively the real part and imaginary part of the complex relative permittivity of the fluid to be measured.
As is well known, when the density of sludge or pulp as a substance to be measured changes, the effective permittivity changes accordingly. Especially, the permittivity real portion and the density value have high correlation.
When
σ
ωϵ
r
′
ϵ
0
+
ϵ
r
″
ϵ
r
′
<<
1
in equations (2) and (3), the permittivity imaginary portion is small. When the conductivity is also low, we obtain
α
=
z
0
2
(
σ
+
ωϵ
0
ϵ
r
″
)
(
4
)
β
=
ω
μ
r
ϵ
r
′
c
0
2
(
5
)
The attenuation amount and phase delay are obtained from &agr; and &bgr; obtained on the basis of equations (4) and (5). Let P
0
be the transmission power, and P be the microwave power traveling in the z direction. Then,
P=P
0
exp (−
2
&agr;z) (6)
The attenuation amount is 20 &agr;z/ln10 dB.
The phase delay is &bgr;z rad.
In the above-described scheme, the density value is obtained by obtaining the phase delay. As shown in equation (5), &egr;r′ is in proportion to &bgr; in the small change range of &egr;r′. For this reason, the density value is obtained from &bgr;z. Since &agr; has lower correlation than &bgr;, &agr; is not directly used for density measurement.
The above-described conventional densitometer using microwaves has the following problems.
(a) When the temperature or conductivity of the liquid to be measured changes, the amount of attenuation of a microwave due to the liquid to be measured greatly changes. When the microwave attenuates to decrease the amplitude of the measurement-side heterodyne output
76
, the switching time for phase difference measurement changes due to the influence of noise or drift in digitizing the reception signal with the comparator
77
, resulting in a measurement error.
(b) Due to the same reason as in (a), when power of the reception signal
73
changes, the phase changes due to the non-linearity of the electronic circuit, resulting in a measurement error.
(c) The influence of temperature drift in the electronic circuit is compensated for by the fixed reference attenuator. When the signal level of the measurement-side heterodyne output
76
changes, the influence of temperature changes and therefore cannot be completely compensated for.
(d) The density value is obtained on the basis of a change in phase. For this reason, when the phase of the reception signal
73
exceeds 360°, the density value cannot be accurately obtained.
More specifically, when the tube diameter is large, or the substance to be measured has a high density, the phase changes by 360° or more, and the density value cannot be uniquely determined from the change in phase. In continuous measurement, the number of cycles can be estimated from previous
ext measurements, as disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 8-82606. However, when the tube empties and is filled with the liquid to be measured again, no accurate density value can be measured.
(e) The microwave is received through portions other than the liquid to be measured because of runaround or induction from the wiring pattern of the circuit, resulting in a measurement error.
(f) When bubbles are present in the liquid to be measured, the microwave traveling path becomes long, or the microwave is re
Shimokawa Yoshiyuki
Suzuki Masuo
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Moller Richard A.
LandOfFree
Densitometer using microwaves does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Densitometer using microwaves, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Densitometer using microwaves will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2544134