GB2229269A

GB2229269A - Surface moisture detection

Info

Publication number: GB2229269A
Application number: GB9003148A
Authority: GB
Inventors: Ian Robert Fothergill
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1989-02-16
Filing date: 1990-02-12
Publication date: 1990-09-19
Anticipated expiration: 2010-02-12
Also published as: GB2229269B; GB8903556D0; GB9003148D0

Abstract

Surface moisture is detected by directing a beam of electromagnetic radiation (13) onto or through a surface (16), detecting radiation (10) reflected from or transmitted through the surface, and deriving from the detected radiation information concerning the surface moisture e.g. whether a surface is dry enough to paint or that a leak has occurred. A laser beam may be scanned over the surface and may travel down fibre optics. <IMAGE>

Description

Surface moisture detection The present invention concerns the remote detection of surface moisture.

According to the present invention a method of detecting surface moisture comprises directing a localised beam of electromagnetic radiation onto a surface, detecting reflections from or transmissions through the surface, and noting changes in the reflected or transmitted radiation resulting from moisture on the surface.

The beam can be pulsed or continuous and a preferred form of beam is a continuous wave laser. The laser beam can be directed onto a fixed or constant position on the surface or alternatively the laser beam can scan over an area of the surface. The reflected or transmitted radiation can be detected and a signal obtained from the detected radiation and its characteristic displayed on an oscilloscope.

The invention can be performed in various ways and some specific embodiments with possible modifications will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of one arrangement; and Figure 2 is a graph.

Referring to Figure 1, when a laser beam 10 from a laser 11 is directed onto a dry surface 12 a fraction of the incident light is reflected as at 13 and, if the reflectivity characteristics are stable, the fraction of the reflected power received by a remotely located detector 14 will be substantially constant. The detector 14 may provide an output signal in line 15. However, if moisture is present on the surface (as at 16) stability no longer exists due to such effects as motion of liquid molecules, surface tension and evaporation. These effects generate random changes in localised reflectivity coefficients. Consequently an incident laser beam exhibits corresponding random changes in reflected signals (see Figure 2) until the surface has completely dried out. The existence of changes in the reflected signal is therefore an indication that moisture is present on the surface.

In the above arrangement the laser beam 11 of electromagnetic radiation is directed onto a fixed or constant position on the surface 12. An alternative approach for detecting the presence of moisture over a large surface area is to employ a scanning laser. Under dry conditions this will generate a random scattered signal waveform characteristic on an oscilloscope detector screen. The present of any surface moisture will generate temporal instabilities in the characteristics. If the moisture is confined to a part of the surface being scanned then only the corresponding part of the characteristic will be affected, the remainder staying unchanged. In this way it is possible to rapidly monitor a surface for the presence of moisture.

It is anticipated that the method can be developed for the remote detection and location of leaks in, for example, vessels, conduits and flanges in addition to detecting spillage. Such remote detection and location can be useful for monitoring surfaces in plant for radioactive liquids. Additionally, the method can be useful for detecting the state of dryness of surfaces prior to painting or other operations, and the drying characteristics of surface preparations such as paint.

An alternative procedure for monitoring relatively large areas for moisture is to stop the laser beam by means, for example, of a computer-controlled orthogonal mirror arrangement. In this way a two-dimensional matrix type pattern of inspection locations can be produced, the laser image spot residing at each location for a fraction of a second. The distribution of moisture over the selected area can thus be quantified and displayed on a video display unit. Signals may be displayed above some selected threshold value to eliminate low level noise or spurious signals; or it is possible to confine displayed 'moisture' signals to levels above some selected minimum value.

In circumstances where optical line of sight conditions cannot be achieved or are inappropriate or the environment is hostile (thus precluding location of sensitive optical equipment close to the surface) optical signals can be delivered to and transmitted from the surface using fibre optics cables.

For fluid leak detection applications, involving insulated pipes or vessels, leaked fluid will eventually diffuse to the outer surface of the insulation. The highly unstable signals associated with moisture result from the vector summation at the detector of randomly varying scattered light signals. If too many signals are involved then averaging effects become dominant and the detector fluctuating signal output amplitude will decrease. Such a situation can exist with semi-porous surfaces such as mineral wool insulation and in order to compensate for this, small semi-permeable membranes can be located at the inspection points on the outer region of the insulation surface to limit penetration of light into the insulation and hence the number of scattered light signals at the detector. In this way signal to noise ratios can be significantly increased.Another way of increasing signal to noise ratios is to focus the incident laser beam. This will reduce the number of individual scattering events and also increase their scattered light intensity.

If light (incoherent or coherent) is directed onto a semitransparent substance with optical discontinuities arising, for example, from an open cellular structure or the fibres of mineral wool insulation, then significant scattering from the primary penetrating beam will result causing attenuation proportional to the depth of penetration.

This scattering and hence attenuation of a transmitted light beam will be reduced if the air spaces within the fibres or cells are filled with a transparent fluid such as water.

By directing a beam of light through a given thickness of the above material, a measure of its water content can thus be made by monitoring the amplitude of the transmitted signal. In this way the occurrence of any leaks in a pipe or vessel surrounded by this type of insulation can be detected and to some extent quantified the location of such a leak being deduced by comparing signals from different inspection sites.

Claims

1. A method of detecting surface moisture comprising direction a localised beam of electromagnetic radiation onto a surface, detecting reflections from or transmissions through the surface, and noting changes in the reflected or transmitted radiation resulting from moisture on the surface.

A A method as claimed in Claim 1, in which the beam is pulsed.

3. A method as claimed in Claim 1, in which the beam is continuous.

4. A method as claimed in any preceding Claim, in which the beam is scanned over the surface.

5. A method as claimed in any preceding Claim, including detecting reflected or transmitted radiation, and obtaining from the detected radiation a signal indicative of characteristics of the detected radiation.

6. A method as claimed in any preceding Claim, in which the beam passes through a semi-permeable membrane before reaching the surface.

7. A method of detecting surface moisture substantially as hereinbefore described.