METHOD FOR CALIBRATING A RADIOMETRIC DENSITY MEASURING APPARATUS

Abstract

The invention relates to a method for calibrating a radiometric apparatus for determining and/or monitoring density of a medium (6) located in a container (1). The method includes method steps as follows: determining the mass attenuation coefficient .sub.C of the empty container (1) with application of the half value thickness N/N.sub.0=0.5 of the radioactive radiation upon passage through the empty container (1) according to the formula: N/N.sub.0I/I.sub.0=e.sup..sup.C.sup.1D, with .sub.C: mass attenuation coefficient, 1: density of the material of the wall of the container, D: distance traveled by the radiation, or inner diameter of the container (1), I: intensity the measured radiation, I.sub.0 intensity of the transmitted radiation, N measured counting rate, N.sub.0 counting rate of the transmitted radiation, determining the mass attenuation coefficient (.sub.M) based on the measured intensity, or the counting rate, of the radioactive radiation after passage through the container (1), when a calibration medium of known density (2) is located in the container (1), ascertaining the dependence of the linear absorption coefficient () on the geometric dimensions of the container (1) based on the two mass attenuation coefficients, calculating a calibration curve, which shows the dependence of the density of the medium on the count of measured radiation intensity after passage through the container (1).

Claims

1. Method for calibrating a radiometric apparatus for determining and/or monitoring density of a medium (6) located in a container (1), wherein a transmitting unit (3) and a receiving unit (4) are provided, wherein the transmitting unit (3) transmits radioactive radiation of a predetermined intensity and wherein the receiving unit (4) receives radioactive radiation transmitted by the transmitting unit (3) after passage through the medium (6), and wherein a control/evaluation unit (7) is provided, which determines density of the medium (6) located in the container (1) based on intensity measured by the receiving unit (4), wherein the method comprises method steps as follows: determining the mass attenuation coefficient .sub.C of the empty container (1) with application of the half value thickness N/N.sub.0=0.5 of the radioactive radiation upon passage through the empty container (1) according to the formula: N/N.sub.0I/I.sub.0=e.sup..sup.C.sup..Math.1D, with .sub.C: mass attenuation coefficient, 1: density of the material of the wall of the container, D: distance traveled by the radiation, or inner diameter of the container (1), I: intensity of the measured radiation, I.sub.0 intensity of the transmitted radiation, N measured counting rate, N.sub.0 counting rate of the transmitted radiation, determining the mass attenuation coefficient (.sub.M) based on the measured intensity, or the counting rate, of the radioactive radiation after passage through the container (1), when a calibration medium of known density (2) is located in the container (1), ascertaining the dependence of the linear absorption coefficient (p) on the geometric dimensions of the container (1) based on the two mass attenuation coefficients, calculating a calibration curve, which shows the dependence of the density of the medium on the count of measured radiation intensity after passage through the container (1).

2. Method as claimed in claim 1, wherein the mass attenuation coefficient of the container (1) is calculated according to the following formula: .sub.C=0.693/1 D, wherein 0.693=ln 0.5.

3. Method as claimed in claim 1 or 2, wherein water is used as calibration medium.

4. Method as claimed in claim 1, 2 or 3, wherein the transmitting unit (3) and the receiving unit (4) are so positioned relative to one another that the container (1) is irradiated perpendicularly to the longitudinal axis of the container (1), inclined to the longitudinal axis of the container (1) or in parallel with the longitudinal axis of the container (1).

5. Method as claimed in at least one of the preceding claims, wherein a pipeline is used as container (1), and wherein the transmitting unit (3) and the receiving unit (4) are secured on opposite surface regions of the pipeline.

6. Method as claimed in claim 4 or 5, wherein the receiving unit (4) is so embodied and positioned that the sensitive components (5) of the receiving unit (4) are struck by the radiation.

Description

[0027] The invention will now be explained in greater detail based on the appended drawing, the figures of which show as follows:

[0028] FIG. 1a a schematic view of an arrangement for radiometrically determining the density of a medium,

[0029] FIG. 1b a schematic view of a graph, which shows dependence of counting rate on density,

[0030] FIG. 2 a graph of the density-calibration curve in the case of a one point calibration,

[0031] FIG. 3 a graph of the density-calibration curve in the case of a two point calibration,

[0032] FIG. 4 a graph of a number of curves, which show the dependence of the mass attenuation constant on the diameter of a container, when different radiation sources are utilized and when media with different densities are placed in the container, and

[0033] FIG. 5 schematic view of the distance traveled by the gamma radiation through a tubular container.

[0034] FIG. 1a shows a schematic view of an arrangement for radiometrically determining the density of a medium 3 located in a container 1, here a pipeline. The transmitting unit 3 with the gamma source and the receiving unit 4 are arranged on opposite surface regions of the pipeline 1. Both components 3, 4 are secured externally on the pipeline 1 via a clamping mechanism (not shown).

[0035] The gamma source is housed such that the gamma radiation escapes from the transmitting unit 3 only in the region of the exit area A. The gamma radiation irradiates the container 1 with the medium 6 located therein, whose density is to be determined, on the indicated radiation path RP. The gamma radiation attenuated as a result of the Compton effect is received by the receiving unit 4. Based on the intensity, i.e. based on the counting rate of the receiving unit 4, the evaluation unit 7 determines the density of the medium 6 located in the container 1. Corresponding radiometric, density measuring arrangements are manufactured and sold by the applicant.

[0036] As already mentioned above, the arrangements of transmitting unit 3 and receiving unit 4 relative to the container 1 can be differently embodied.

[0037] FIG. 1b shows a schematic view of a graph of the counting rate N as a function of the density of the medium 6. The absorption F of the gamma radiation on the radiation path RP through a medium 6 can be described via the Lambert-Beer law in the form of an e-function. The absorption F corresponds to the ratio of counting rate of the gamma radiation after passage through the medium 6 to counting rate No of the gamma radiation transmitted from the gamma source at the exit opening A. The counting rate N is given in counts (number of events) per second (c/sec). The ratio of these two counting rates is proportional to the dose rate H, which is given in Sv/h. Contained in the e-function are the absorption- or attenuation coefficient, the density of the medium [kg/m.sup.3] and D [m] the distance traveled on the radiation path RP through the medium 6. In the illustrated case, the distance traveled on the radiation path RP corresponds to the inner diameter D of the pipeline 1.

[0038] FIG. 1 b shows the difference Fs between the maximum arising attenuation and the minimum arising attenuation, which can be described by the ratio of maximum counting rate N.sub.max to minimum counting rate N.sub.min of the gamma radiation as a function of the density p. In the case of maximum density .sub.max, the counting rate N/No is approximately zero, while in the case of minimum density .sub.min, the counting rate essentially equals the counting rate No of the gamma radiation transmitted from the gamma source. The following mathematical relationships hold:

[00003]$.Math.F.Math.s=\frac{N_{\max }}{N_{\min }}=e^{.Math.D.Math.\left({}_{\max }-{}_{\min }\right)}.Math..Math.\mathrm{and}$$\frac{N_{\min }}{N_{\max }}=e^{-.Math.D.Math.\left({}_{\min }-{}_{\max }\right)}$

[0039] In order to be able to provide reliable radiometric, density measurements, the radiometric measuring arrangement must be calibrated. Shown in FIG. 2 are a one point calibration and the calibration- and subsequent measurement error associated therewith. The calibration error results from the fact that in the case of a one point calibration the slope of the exponential function is not defined. A reliable measurement in the case of the one point calibration is only assured, when the density measured value to be determined for the medium 6 lies as near as possible to the calibration point. For calculating the calibration point, the standard absorption coefficient was used. This has a constant value of 7.7 mm.sup.2/g.

[0040] FIG. 3 shows the attenuation curve (counting rate as a function of the density of the irradiated medium) in the case of a two point calibration. As a result of the two relatively widely separated calibration points, the slope of the attenuation curve is defined. Therefore, in the case of a two point calibration, a high accuracy of measurement is assured over the total density range.

[0041] It was stated above that the the mass attenuation constant under ideal conditions is (theoretically) independent of the density and the character of the medium and only dependent on the energy of the incoming gamma radiation. This assumption is not quite correct. Shown registered in FIG. 4 are a number of curves, which reflect the dependence of the mass attenuation coefficient on the diameter of a container 1. Constant in the case of all shown values is only the above mentioned standard mass attenuation constant. All other mass attenuation constants show a dependence on the diameter of the container 1.

[0042] It is clearly evident that, in the case of the same density measuring device type (here FMG 60), the curves for equal density of the medium 6 are similar but shifted relative to one another (compare curves 1 and 3 and curves 2 and 4). In the case of application of the same radiation source, the curves are shifted parallel relative to one another as a function of the density of the medium after a diameter of about 300 mm. In the region of smaller diameter of the container 1, or the pipeline, the attenuation coefficient falls relatively rapidly, while from a diameter of greater than 300 mm it is dependent essentially on the intensity of the radiation source of the transmitting unit 3 and the density of the medium 6 located in the container 1. Nevertheless, the curves also show above a diameter of 300 mm athough smalllinear dependence on the diameter of the container and, thus, on the irradiated medium.

[0043] In actual applications, there is another influencing variable: In most cases of application, the medium, whose density is to be measured, is composed of a mixture of different components (i.e., the medium is a slurry). Therefore, the mass attenuation constant of a medium has no constant value, but, instead, it assumes a value, which is composed of a weighted sum of different components of the medium. In most cases of application, the mass attenuation constant is, consequently, not available and it is difficult to establish a constant value for a mixture. Often, one uses the fallback option of the above mentioned, standard mass attenuation constant.

LIST OF REFERENCE CHARACTERS

[0044] 1 container [0045] 2 wall of the container [0046] 3 transmitting unit with gamma source [0047] 4 receiving unit [0048] 5 sensitive component [0049] 6 medium [0050] 7 evaluation unit