GB2585629A - Ultra-violet transmission in water - Google Patents

Abstract

A method for determining the percentage transmissibility (%T) of UV radiation in a water sample 102, comprises; placing the sample between a first UV transmissive window 105 associated with a UV source 103 and a second UV transmissive window 108 associated with a UV detector 106. The photocurrent detected at the UV detector is measured at at least two separations d between the two windows. A value for %T is calculated from the difference between the window separations for any two such measurements and the ratio of the photocurrents detected in the two measurements. An apparatus includes a drive 111 adapted to cause relative movement between the windows to adjust the separation. The apparatus may be used in a UV sterilisation system for purifying water (Fig 6).

Description

ULTRA-VIOLET TRANSMISSION IN WATER

This disclosure is concerned with measuring the transmission of Ultra-Violet radiation ("UV") in water.

UV light, more particularly UVC light can be used to kill bacteria, viruses and other microorganisms. The light is believed to break molecular bonds in the organism's DNA, producing thymine dimers which can kill or disable the organism Typical UV sources used for disinfecting water for purification of drinking water or in swimming baths, aquaria, on-land fish farms, ornamental fishponds, and other situations where the use of chemical disinfectants is undesirable include: * Mercury-based lamps which emit UV light at the 253.7 nm line; * Light emitting diodes ("LEDs") emitting UV light at wavelengths between 255 and 280 nm; and * Pulsed Xenon lamps which emit UV light across the UV spectrum with a peak emission around 230 nm.

The water is passed through a chamber containing the UV lamps. The measure of the killing potential of the chamber is termed the "dose" and the mean dose is expressed in mJ/cm2 as the average energy an organism is exposed to when transiting the chamber. The measure of the UV energy intensity of the chamber is its irradiance, expressed in mW/cm2. Chambers are designed to achieve the maximum dose for the given flowrate and lamp power. An irradiance of 1 mW/m2 on a surface for 1 second will result in 1 mJ/cm2 on that surface.

Accordingly, knowledge of the mean irradiance of the chamber, together with the chamber volume, mixing efficiency, the percentage transmissibility ("%T") of the water sample over a path length of I cm, and the flowrate of water are all important for determining the dose. Of these, the %T is particularly significant, as it can vary widely for different samples of water or for water from the same source over time. A %T of 100% means that there is no absorption of UV. Even what appears to be relatively transparent water to visible light can have quite high attenuation of UV, resulting in lower irradiance and hence a lower dose within the chamber. For example, if a water sample has 70% transmission at lcm, then for a typical path length of 5 cm between lamp and water-borne, the transmission will be reduced to (0.7) = 16.8%. In other words around 73% of the lamp energy is being lost within 5 cm, and the situation is worse at greater distances from the UV source Thus, monitoring of the UV transmittance or %T of the water being treated is critical if the chamber is to achieve adequate sterilization Heretofore, the incorporation of hardware to measure %T in T_TV chambers for disinfecting water, has proved problematic. The hardware is expensive and requires calibration and maintenance over and above the routine maintenance of the chamber, as explained in more detail below with reference to Figs. 1 to 4 of the accompanying drawings.

Fig. 1 shows a UV lamp 1 behind a first window 2 and a UV detector 3 behind a second window 4 linked to a read-out device 5. The windows 2 and 4 have a fixed separation, for example 1 cm, and may conveniently be provided by a glass vial 6 in the water path of the water sample being tested. The detector 3 measures the irradiance of the UV after travelling through the water sample. Calibration with 100%T water is required.

In the illustrated system, small changes in the intensity of the lamp source 1 result in erroneous estimates of %T, which will be directly proportional to such changes Thus frequent calibration with 100%T water (water with perfect transmissibility for UV) is necessary. Despite this, a very stable UV source 1 is required with corresponding stability in the detector 3 and read-out device 5.

One enhancement for the system of Fig. 1 is the provision of a beam splitter 7, as shown in Fig. 2, positioned between the UV source 1 and its window 2. The separated portion of the UV radiation is diverted a second detector 8, with both the first and second detectors being linked to a processor 9, the detected signal at the second detector 8 serving as a reference signal, since with any variation in the source 1, both the detected signal at the first detector 3 and the reference signal should vary in the same way. Although the system of Fig. 2 may compensate for lamp variability, aging and build-up of contaminants on the windows 2 and 4 and on the beam splitter 7 will affect results, as will matching and stability of the two detectors 3 and 8.

The system of Fig. 3 avoids the use of a beam splitter and second detector, with the inaccuracies these introduce. Instead, glass vial 6 has two pairs of windows, namely windows 2 and 4 with first separation of say 1 cm in the plan views of Figs. 3a and 3b, and windows 10 and 11 with a different separation of say 2 cm. The vial 6 is rotated back and forth by motor 12 controlled by the processor 9 between a first orientation (Fig. 3a) in which windows 2 and 4 are in the path from UV source 1 to detector 3, and a second orientation (Fig. 3b) in which windows 10 and 11 are in the path from UV source to detector 3. %T is calculated by comparing the values for transmitted UV detected in the two orientations. Changes in lamp intensity and in the detector are only significant if they occur in the short time between measurements in the two orientations. However, if the differing windows for the respective measurements become contaminated or obscured to some degree, this will still affect the accuracy.

Changes in lamp intensity and in the stability of the detector in the time taken to rotate the vial 6 can be eliminated by taking both measurements at the same time, as in the system of Fig. 4, where there are two windows 4 and 13 on the detector side of the system, the windows 4 and 13 being associated with respective detectors 3 and 14 both linked to processor 9. There is risk of the windows becoming differently obscured, and the two detectors 3 and 14 must maintain stable gain matching.

The present disclosure has arisen from our work seeking to overcome or avoid the problems inherent in each of the above prior arrangements, and, in so doing, to provide an improved and more reliable method for measuring the transmissibility of UV in water.

In a first aspect of this disclosure, there is provided a method for determining the percentage transmissibility (%T) of UV radiation in a water sample, comprising the steps of: placing the water sample between a first UV transmissive window associated with a UV source and a second UV transmissive window associated with a UV detector; measuring the photocurrent detected at the UV detector at at least two separations between the first window and the second window; and calculating a value for %T from the difference between the separations of the first and second windows for ally two such measurements and the ratio of the photocurrents detected in the two such measurements.

The first and second windows need not be flat or parallel to each other, since all that matters is the difference in separations between the two measurements Indeed, the 30 UV source may be contained within a UV transmissive sleeve, which will suitably have a curved surface surrounding the UV source. The detector and its associated UV transmissive window may simply be moved towards or away from the sleeve between the two measurements. In one preferred arrangement, the UV source will be one and the same as the UV source used for disinfection, the detector and its associated window being located opposite the UV source in the path of water passing through a chamber in which the water is disinfected by the UV source. Other features which may be present in preferred embodiments include: The UV source provides collimated UV radiation. The predetermined separations are achieved by relative linear motion between the source and the detector. The predetermined separations are achieved by motion of one window relative to the other, with the source and detector remaining in fixed positions.

In a second and alternative aspect of this disclosure, we provide an apparatus for determining the percentage transmissibility of ultraviolet radiation in a water sample, the apparatus comprising: a UV source having a first UV transmissive window associated therewith; a UV detector having a second UV transmissive window associated therewith, the detector being adapted to detect UV radiation from the UV source that has passed through both said windows and said sample of water placed between them; and a drive adapted to cause relative movement between the first and second windowss to adjust the separation between them by a predetermined amount.

Reference may be made to the accompanying drawings, in which: Figs. I to 4 show generally schematic views illustrating prior systems for 20 determining %T; Fig. 5 is a generally schematic view of a first embodiment of apparatus, by way of example, in accordance with the teachings of the present disclosure, Fig. 6 is a generally schematic view of a second embodiment of apparatus, by way of example, in accordance with the teachings of the present disclosure, Fig 7 is a generally schematic view of a third embodiment of apparatus, by way of example, in accordance with the teachings of the present disclosure; and Fig. 8 is a generally schematic view of a fourth embodiment of apparatus, by way of example, in accordance with the teachings of the present disclosure.

Turning now to Fig. 5, an enclosure 101 contains water 102, the %T of which is to be tested. A UV lamp 103 powered by a power supply unit 104 to provide UV radiation at the wavelength at which %T is to be measured is positioned behind a first UV transmissive window 105 of the enclosure. A UV detector 106 sensitive to the radiation produced by lamp 103 in a liner fashion is mounted within a detector housing 107 behind a second UV transmissive window 108 opposite window 105, with window 108 parallel to window 105 and a separation d between the windows. UV detector 106 is coupled electrically to processor 109, also coupled electrically to a read-out device 110. Processor 109 acts as a controller for a linear actuator 111 adapted to move the detector housing 107 towards and away from window 105 to vary the separation d by a predetermined amount, which is accurately determined by linear actuator 111. The photo-current from the detector 106 is measured by the processor 109.

In order to carry out a measurement, the following steps are performed: * The processor 109 as controller commands the actuator 111 to move the detector housing 107 to a first pre-set separation dl between windows 105 and 108.

* The photocurrent ii at separation dl is measured.

* The processor as controller commands the actuator 111 to move the detector housing 107 to a second pre-set separation d2 between windows 105 and 108.

* The photo-current i2 is measured again at separation d2.

Let us assume that: * The output of lamp 103 is Id.

* The first window 105 associated with lamp 103 has a transmission coefficient of kw I. * The second window 108 associated with the detector 106 has a transmission coefficient of lew2 * The transmission coefficient of UV in the water sample 102 at distance d from window 105 is %T(d).

* The current conversion coefficient of the detector 106 is kdek.

* The Lambertian detector coefficient (the falloff in detector output due to the distance between the source 103 and the detector 106) at distance d from window 105 is kdl(d 11 and 12 will then be given by the following equations: II = krkw I *kw 2*°,'OT(d1)*kdet*kdl(d1) and 12 = krkw I *kw 2*%1('612)*kdet*kd1412).

If i 2 is divided by i, kl, kwl, loti2 and kdet all cancel out and we get: 12, = [%T(d2)1%T(d1)]* [MI(12) kdad1)] Equation 1 If the measurement process is carried out with water having as near 100% transmissibility as possible, %T(d]) and %T(d2) will both be unity, so that, in this case, /2/1 approximately equals [kdl(d2) kdl(dI)].

Thus, the relative Lambertian coefficient for separations d2 and di, namely [kd1412pkdIldIA, can be determined for the particular source/detector set up.

The approximation is due to there being some dependency of kdl(d) on %T(d) due to differential attenuation of diverging rays due to unequal path length.

Having established a value for the relative Lambertian coefficient for separations d2 and dl, this value can be inserted into Equation 1 Since [°10T(12)1%T(d1)] equals [%T(d2-d/)], if d2 is exactly lcm larger than di, we get %T = 112, K Equation 2, where K equals the calculated value of [kdI(12) kdl(d1)].

Although the difference between the two measurement positions is preferably exactly 1 cm, this is not essential, provided that the separation is accurately known In the more general case, for an arbitrary, but accurately known, difference dt, given by d# -d2-dl, if the transmissivity of water over lcm is %T, and the transmissivity over the distance di% is %T(d,#), these are related by the Equation: T(d2)1?,-OT(d1)]= [%-r(d2-d1)]= %T(tki) = Rill) Equation 3 Thus, provided that the second of the two measurements at separations dl and d2 can be carried out quickly following the first, the normal causes of measurement error (lamp output drift, window fouling and detector drift) are all avoided, and %T can be determined from the ratio i2 H. Depending on the extent by which the rays are not parallel, the calculated values for %T will differ slightly from the true value, but the results obtained will be consistent and repeatable.

The minor approximating factor could be compensated for by using a look-up table based on a number of values for i2 il obtained using water samples with known %T, and corresponding calculated %I derived from the measured values for i2 The advantages of the system described above are as follows: * There is no necessity for use of a lamp with long term stability, allowing use of less expensive lamps and power supplies therefor, since lamp age is no longer an issue.

* Optical surfaces, although they obviously still need to be kept unobscured, need not be constantly cleaned to preserve the accuracy of the results.

* Once the initial relative Lambertian coefficient for separations d2 and dl, namely [kdl(d2) kdl(dI)] has been determined, there is no further need for reference water samples or repeated calibration steps.

* The only requirement for accuracy and care in repeatability, is in the physical features of the apparatus, namely the difference between the predetermined separations dl and d2, which must be precisely known.

In alternative arrangements: The UV lamp source need not be Lambertian, it may produce collimated radiation so that kdl(d2) and kdl(d1) are both equal to one and cancel out in Equation 1, above, and K equals 1 in Equation 2. The only moveable component may be window 105, with all other components of the apparatus in fixed positions. The only moveable component may be window 108 of detector housing 107, the detector 106 itself and all other components being fixed in position. The detector 106 may have a field of view that only accepts parallel (that is: collimated) rays from the source 103. The detector 106 may not be directly electrically connected to the processor 109, but instead is sealed within its housing 107 together with a built-in amplifier, and communicates with and is powered externally by an inductive coupling system connected to processor 109 Fig. 6 shows a first preferred embodiment of apparatus for performing methods in accordance with the present teaching. Sterilisation chamber 200 has a water inlet 201 and a water outlet 202. The same source 203 used for sterilisation is here also employed for measurement of %T. The source 203 comprises a linear low pressure mercury lamp 204 suitably of 100W contained within a quartz glass sleeve 205 and powered via a ballast 206. A detector 207 together with an amplifier 208 is contained within a detector housing 209 provided with UV transmissive window 210, suitably of quartz glass, positioned at a separation d from sleeve 205. Water within the chamber 200 will have a pressure typically of 5 bar. The detector housing 209 is mounted on a small diameter shaft 211 passing through a wall 212 of chamber 200 via a sliding 0-ring seal 213. Shaft 211 is reciprocated by an actuator 214 to vary the separation between window 210 and sleeve 205 between two separations dl and d2 that differ by a known distance, at which the photocurrents /I and i2 are measured. The shaft 211 has a small diameter so that the loading due to swept volume against the chamber pressure is minimised. As illustrated actuator 214 comprises a motor 215 adapted to rotate back and forth under control of processor 216 to drive a rotary-to-linear-motion converter 217 to cause the detector housing 209 to reciprocate linearly towards and away from sleeve 205. The individual separations dl and d2 do not need to be known. What matters is the precision of the controlled drive to housing 209 so that it moves between two separations from sleeve 205 that differ by a precisely known distance.

By employing the UV source employed for sterilisation for the measurements, or alternatively, one of them, if several sources are employed, in which case, the field of view of the detector will need to exclude all sources but one, there is a substantial reduction in cost. The detector employed may also serve other instrumentation functions, and so be a detector that would be present in any case. The only added instrumentation costs may therefore lie solely in providing a mechanism for accurately moving (say) the detector between two measurements. Thus the present disclosure represents a low cost solution to achieving accurate %T values As an alternative to use of the sterilising UV source for the measurement of %T, as in the arrangement of Fig. 6, Fig. 7 illustrates a smaller low cost measurement instrument that can be provided either as a stand-alone portable instrument or mounted in line to the sterilising chamber. A low pressure mercury lamp 300 is mounted within a quartz glass sleeve 301 which is then mounted within a chamber 302 provided with entry 303 and exit 304 ports for a portion of the water passing through the sterilising chamber. Instead of being connected into the sterilising system, chamber 302 may be provided as a portable open-topped vial, omitting the ports 303, 304, the water to be tested being inserted via the open top of the vial. A UV detector 305 is mounted within a housing 306 having a UV transmissive window 307, in the same fashion as for the embodiment of Fig. 6, and the separation between window 307 and sleeve 301 is adjusted in exactly the same fashion as for Fig. 6. A controller 308 may serve both as processor and display. Both the lamp 300, via a ballast 309, and the controller 308 may be powered by a battery 310. When configured as a portable measurement device, care must be taken to ensure that a user is shielded from the UV radiation.

Fig. 8 illustrates a higher cost but more accurate variation of the measurement device of Fig. 7, which can also be made smaller than the device of Fig. 7. In place of the mercury lamp, it employs an UV LED 400 emitting collimated UV radiation at a wavelength of around 254 nm mounted within a chamber 401. A water sample 402 is introduced into chamber 401 through an opening (not shown). A UV detector 403 is mounted within a housing 404 having a UV transmissive window 405, in the same fashion as for the embodiments of Figs. 6 and 7, and the separation between window 405 and LED 400 is adjusted in exactly the same fashion as for Fig. 7. Alternatively, or additionally, the LED source is mounted on a shaft 406 for movement towards and away from window 405. A controller 407 may serve both as processor and display. Both the LED source 400, and the controller 407 may be powered by a battery 408 When configured as a portable measurement device, care must be taken to ensure that a user is shielded from the UV radiation.

Besides giving an indication of %T as an output in its own right, this value can be used within an instrumentation algorithm to give a precise and reliable indication of the dose provided by a sterilisation chamber at little or no added cost. Moreover, the systems described above can give reliable results for the dose over all practical %T values likely to be encountered Previously sterilisation chambers had to be sold depending on the quality of water they were designed for The present disclosure allows the design of sterilisation chambers that can confidently cope with a range of water qualities likely to be encountered, and give an accurately known dose of UV radiation in all cases As a secondary considerations, by monitoring %T over a period of time for the same sterilisation chamber, a fall-off in %T will indicate a fall-off in effective irradiance by the source, indicative of failing lamp irradiance or an alert that the optical components of the system need cleaning. L1.

Claims (13)

1. Claims 1 A method for determining the percentage transmissibility (%T) of UV radiation in a water sample, comprising the steps of placing the water sample between a first UV transmissive window associated with a UV source and a second UV transmissive window associated with a UV detector; measuring the photocurrent detected at the UV detector at at least two separations between the first window and the second window; and calculating a value for %T from the difference between the separations of the first and second windows for any two such measurements and the ratio of the photocurrents detected in the two such measurements.
2. 2. A method according to Claim 1, wherein the UV source provides collimated UV radiation.
3. 3. A method according to Claim I, wherein the UV source is contained within a UV transmissive sleeve with a curved surface surrounding the UV source, and wherein the detector and its associated UV transmissive window is moved towards or away from the C\I 15 sleeve between the two measurements. LtD
4. 4 A method according to Claim I or Claim 2, wherein the predetermined separations CO are achieved by relative linear motion between the source and the detector.
5. 5. A method according to Claim 1 or Claim 2, wherein the predetermined separations are achieved by motion of one window relative to the other, with the source and detector remaining in fixed positions.
6. 6. A method according to any preceding Claim, wherein the UV source is one and the same as a UV source used for disinfection of water passing through a chamber, the detector and its associated window being located opposite the UV source in the path of water passing through said chamber where it is disinfected by the UV source.
7. 7. A method according to any of the preceding Claims other than Claim 2, wherein the UV source is Lambertian, wherein a preliminary test using the same relative separations is first performed using 100%T perfect transmissibility water to determine an approximate relative Lambertian coefficient for the two separations in the specific source/detector set up from the ratio of the photocurrents detected at the two separations.
8. 8. An apparatus for determining the percentage transmissibility of ultraviolet radiation in a water sample, the apparatus comprising: a UV source having a first UV transmissive window associated therewith; a UV detector having a second UV transmissive window associated therewith, the detector being adapted to detect UV radiation from the UV source that has passed through both said windows and said sample of water placed between them; and a drive adapted to cause relative movement between the first and second windows to adjust the separation between them by a predetermined amount.
9. 9. Apparatus according to Claim 8, wherein he UV source is adapted to provide collimated UV radiation.
10. 10. Apparatus according to Claim 8, wherein the UV source is contained within a UV transmissive sleeve with a curved surface surrounding the UV source, and wherein the drive is arranged to move the detector and its associated UV transmissive window towards o or away from the sleeve between the two measurements.
11. C\Iis 11.
12. Apparatus according to Claim 8 or Claim 9, wherein the drive is adapted to provide relative linear motion between the source and the detector to provide theCDpredetermined separations.
13. 13. Apparatus according to any of Claims 8 to 12, wherein the UV source is one and the same as a UV source used for disinfection of water passing through a chamber, the detector and its associated window being located opposite the UV source in a path through said chamber provided for water to pass therethrough to be disinfected by the UV source.CO12. Apparatus according to Claim 8 or Claim 9, wherein the drive is adapted to create motion of one window relative to the other, with the source and detector remaining in fixed positions, to provide the predetermined separations.