What a UV dose figure means — and why no number appears on our data sheet
A UV dose in mJ/cm² looks like a specification — a single number, the same unit on every manufacturer's sheet, apparently comparable. It is not one on its own. A dose describes a product together with the conditions it was measured under, so a bare figure does not travel: it does not tell you what the water was, what the flow was, or how the lamp was aged. This article sets out what a dose actually measures, the four variables that decide it, and the three questions to ask instead of asking for the number — which is also why this site supplies none for its own product.
This page deliberately publishes no dose figure. Dose depends on lamp output, sleeve condition, water transmittance and contact time at once — so a bare number would not be comparable.
What a dose is actually measuring
A UV dose — also written as fluence — is the total amount of ultraviolet energy delivered to a unit area of water as it passes through the treatment chamber. It is expressed in millijoules per square centimetre, mJ/cm². In its simplest form it is a product of two things: how much ultraviolet intensity the water sees, and how long the water is exposed to it.
dose ≈ intensity × time — and in a real treatment chamber, both terms vary from
point to point inside the chamber and from minute to minute across the life of the lamp. The
number a manufacturer publishes is therefore never a reading; it is a result calculated or
measured under a defined set of conditions.
That is the whole of the difficulty. The unit is standard, the arithmetic is simple, and the number is genuinely meaningful — conditional on the conditions. A dose figure quoted without those conditions is like a fuel consumption figure quoted without saying whether it was a motorway or a city centre: the number is real, and it does not describe your journey.
The four variables that decide the dose
Four things move the delivered dose. Two belong to the device, one belongs to the water, and one belongs to how the system is used — which is why the same unit on two different installations does not deliver the same dose, and why a number measured on a bench does not transfer to a cabinet under a sink.
Variables 1 and 2 · the device
Lamp output at 254 nm. An ultraviolet source does not emit a constant amount: output depends on the drive current, on the temperature of the junction or the lamp envelope, and on how long the source has been running. Path through the sleeve and the water. Light has to cross the sleeve and then the water before it reaches a microorganism. Anything deposited on the sleeve surface subtracts from the dose before the light ever enters the water — see quartz sleeve fouling.
Variables 3 and 4 · the installation
UV transmittance. Ultraviolet at 254 nm is absorbed by dissolved and suspended material. Turbidity, iron and dissolved organics all lower transmittance, and lower transmittance means less light arrives at a given depth. Contact time. Set by the flow rate against the chamber volume — the same unit at a higher flow gives every litre less time in front of the lamp. This is why the rated flow on a data sheet is not a marketing figure: it is one of the four inputs.
Why two identical numbers are not comparable
Two suppliers publish “40 mJ/cm²”. The units match, so the natural reading is that the products perform alike. Under the arithmetic above, that reading only holds if the four variables were the same in both cases — and the published figure usually does not say.
| The question behind the number | Why the answer changes the comparison |
|---|---|
| Was it calculated, or measured? | A calculated value follows from chamber geometry and a nominal lamp output. A measured value comes from a physical test. The two are not the same claim, and only one of them can be checked against a report. |
| At what transmittance? | A result at high transmittance and a result at low transmittance can carry the same headline figure. A water with visible turbidity or high iron involves a different calculation from a clear laboratory sample. |
| At the beginning or the end of lamp life? | Output falls as a source ages. A figure stated at initial output and a figure stated at end of life differ, and the second is the one that describes the unit late in its interval. |
| At what flow, and against what chamber? | Contact time is a ratio of flow to volume. A dose quoted at a low flow is not a dose at the rated flow, and the two cannot be interchanged. |
A dose figure can be true, published in good faith, and still not comparable with the one beside it. That is not an accusation about anyone's number — it is a property of what the number is. The comparison only becomes sound when the conditions travel with it, which is what a test report does and a table row does not.
Why this site publishes no dose for its own product
The straightforward reason is that no supporting report is held for one. The data sheet names four gaps of this kind, and dose is one of them.
What publishing a number would require
- A defined test water — its transmittance, turbidity and temperature stated.
- A defined flow, held stable for the duration, against a stated chamber volume.
- Lamp condition specified: initial, or aged to a stated point.
- A measuring method at 254 nm with the instrument and calibration stated.
- And, for any performance claim built on it, a biological result rather than a physical calculation.
What we publish instead
- The rated flow, the fitting size and the chamber's role in the line — the inputs a buyer can actually work with, and which are verifiable against the label and the manual.
- The feed-water limits that keep those inputs inside their range — turbidity, iron, hardness, transmittance-relevant parameters and temperature.
- A plain statement that no dose figure is asserted, so that nobody reads the absence as an oversight or fills it in from a competitor's sheet.
If a report is produced, it will be published with its test conditions attached — because a dose figure separated from its conditions would be a number this page has just spent four sections explaining the danger of. Until then, the answer to “what is your dose?” is “no figure is held, and here is what a figure would have to specify.”
The three questions to ask instead
These work on any supplier, including us, and they can be answered before a purchase rather than after a problem.
- “What were the test conditions?” Ask for the water, the flow, the lamp condition and the measurement method. A supplier who has a real result can answer all four. A supplier who has a table row cannot, and the honest answer — that it is a calculated nominal figure — is still a useful answer, because it tells you what you are comparing.
- “What is the maintenance interval, and what is it based on?” Dose at the start of an interval and dose at the end are different numbers. The interval is therefore part of the performance claim, not a separate service topic — and an interval based on your water rather than on a calendar is the one that holds.
- “What do you require upstream of the unit?” Transmittance is one of the four variables, and it is set by everything before the UV stage. The feed-water limits are therefore a performance document as much as a warranty document — they are what keeps the first three variables inside the range the fourth was calculated for.
The three questions converge on the same thing: a UV stage is specified by the conditions it operates under, not by a headline number. That is why the limits are on the feed water page and why the position on the line matters as much as the device — see why UV leaves no residual.
Dose figures and what they tell you
If two suppliers both state 40 mJ/cm², are they equivalent?
Only if the four variables behind the figure were the same — the lamp condition, the path through sleeve and water, the transmittance of the test water, and the flow against chamber volume. A published figure rarely states all four, so the safe assumption is that they are not directly comparable until the conditions are supplied. The unit being identical does not make the claims identical.
Does a higher stated dose always mean better treatment?
No, for two reasons. First, a higher stated figure may simply reflect more favourable test conditions rather than a different device. Second, dose is a physical quantity, not a biological result: what a given dose achieves against a given organism is a separate question that requires a microbiological test. A dose figure is an input to that question, not an answer to it.
Why not publish a calculated dose, since the geometry is known?
Because a calculated figure depends on assumptions — nominal lamp output, an assumed transmittance, a steady flow — and those assumptions would not be visible in a table row. On a page where every other value is traceable to a label, a manual or a certificate, a number that rests on unstated assumptions would be the one figure a reader could not check. The page is more useful with the gap named than with it filled in that way.
What should go in a purchase specification instead of a dose value?
Specify the conditions you can control and verify: rated flow against your peak draw, fitting size, the position on the line relative to any storage tank, the feed-water limits the unit requires, and the maintenance interval and what it is based on. Those are the variables the installed dose depends on, and unlike a headline figure they can be checked after commissioning rather than only on paper.
Does this apply to mercury lamps as well as UVC-LED?
Yes. The arithmetic is a property of ultraviolet treatment rather than of the light source. The variables differ in how they behave — a low-pressure mercury lamp and a UVC-LED array age differently and have different temperature characteristics — but the structure of the problem is the same, and a dose figure from either source needs the same conditions attached. The comparison between the two technologies is on the technology pages.
Ask about the conditions, not just the number
Send the line you are working on — the draw, whether there is a storage tank, what filters upstream, and the destination market. Those are the variables the installed result depends on, and they are also what any dose figure would have to be calculated against.
- Models: ZXSJ-UVC-1GPM (1 GPM / 4 LPM, 1/4") ｜ ZXSJ-UVC-3GPM (3 GPM / 11.4 LPM, 3/8")
- Available now: instruction manual · parameter table · CE certificate DPCW/01068/06/KA/26 · EPA Establishment No. 105777-CHN-1
- Not held: a UV dose figure, a microbiological report, and lifetime test evidence — this article sets out why, and what a figure would have to specify
Send an enquiry
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This form is not yet connected to a mail service — the enquiry endpoint will be wired up at launch. Until then, enquiries reach the manufacturer through the contact page.
Sources and scope
4 sources
- The relationship between delivered ultraviolet dose, intensity and exposure time, and the four-variable structure set out on this page, are general properties of ultraviolet disinfection as described in standard water-treatment references. No figure from any published source is reproduced here as a specification for our product.
- Feed-water parameters referenced in the closing sections: the feed-water limits table on this site, which follows the product parameter table and the instruction manual shipped with the unit, revision 01.03.
- Product statement: no UV dose in mJ/cm², no microbiological performance figure and no log-reduction figure is held or asserted for the ZXSJ-UVC series. This article explains the reasoning behind that position; it does not supply a value.
- Deliberately not published: any dose figure for any model. A value from another manufacturer's product is not reproduced as ours, and this article does not quote one even as an example.