PISONICS
Selection note
Free Alkali + Available Chlorine
Three technical routes · chosen by how many numbers you need
What an “Ultrasonic Dual-Parameter Concentration Meter” Actually Means
—— Not a category with an agreed definition — settle which route you want before it goes on a specification ——
【Key measurements: free alkali NaOH wt% / available chlorine wt%】
Where the Term Comes From
“Ultrasonic dual-parameter concentration meter” turns up in the item-name field of enquiry sheets, with the medium given as “NaOH + sodium hypochlorite solution”. What the writer wants is clear enough: two numbers — free alkali and available chlorine, because those are what the liquor is accepted on.
But there is no product category of that name with an agreed definition. Going through what is publicly documented, at least three different technical routes answer to the words “dual parameter”, and they differ widely in price, capability and where they stop working. Choosing wrong does not cost a little accuracy — it either spends money for nothing or buys something that cannot produce the second number at all.
Three Routes
| Route | Where the second quantity comes from | Numbers out | Publicly documented |
|---|---|---|---|
| ① Single parameter, along the reaction path | There is no second quantity. Stoichiometry pins the three solutes to one dimension — Cl₂ + 2 NaOH → NaOCl + NaCl + H₂O, so every 2 mol of caustic consumed makes exactly 1 mol of hypochlorite and the composition is a one-parameter family | One progress number (residual alkali and available chlorine are equivalent) | Our PS7020 sound-velocity approach; see the tail-gas scrubbing page |
| ② Sound velocity + a second physical quantity | Add an independent sensor, typically conductivity; two independent measurements solve for two unknown concentrations | Two components | SensoTech (LiquiSonic, Germany) document this method for NaOH/NaCl mixtures in chlor-alkali electrolysis and list gas-scrubber duties including chlorine, phosgene and NOx scrubbers |
| ③ Spectroscopy | A full spectrum is collected at once; the spectrum is itself multi-dimensional and a multivariate model separates the components | Two or more | Our PS7100; in China, Chuyi (楚一测控) offer models aimed at chlorine absorber circulating liquor giving free alkali and available chlorine together |
One term worth separating out: when instrument vendors say “binary system” they mean a solution of one solute in water, which a single measurement resolves. That describes the medium, not the number of measuring parameters in the instrument. An enquiry saying “dual parameter” usually means the latter.
How Many Numbers Do You Actually Need
This decides everything downstream, and it has only two answers.
| What the liquor is | Numbers | Why | Route |
|---|---|---|---|
| A consumable the caustic is there to absorb; when it is spent it is discharged | One | All you need to know is how much absorbing capacity is left. Residual alkali and available chlorine are the same progress number under two names; report either | ① cheapest |
| A product the outlet is sold as hypochlorite, or has to enter storage on specification | Two | Free alkali and available chlorine are independent acceptance figures and the buyer takes delivery on both. One scalar cannot separate them — our own PS7020 page says so in its exclusion list | ② or ③ |
The first row also needs three conditions, and losing any one of them sends you back to the second: fixed make-up strength (always the same caustic, not 20 % one week and 32 % the next), a closed water balance (no separate water addition, no heavy evaporation), and chlorine only in the gas (HCl or a lot of CO₂ from air runs a path of different slope). The magnitudes are worked out on the tail-gas scrubbing page under “three ways it drifts off the path”.
Why “A Second Number Cannot Hurt” Is Wrong
A two-dimensional solution carries three costs that never appear on the quotation:
- Modelling needs samples. A spectroscopic method trains its multivariate model on real liquid. Where those samples come from, who takes them, and whether they cover the whole operating envelope are invisible at contract signature and very visible six months after commissioning.
- A changed process means a rebuilt model. New caustic source, different gas composition, a new product line — the model is redone. Who does it, how often, and whether it is inside the service scope belong in the contract.
- Opportunity cost. Two single-parameter meters covering two points on one tower (make-up and circulating liquor) are often worth more than one multi-component meter covering a single point.
The converse holds equally: fit a single-parameter meter where two numbers are genuinely required and the saving comes back the first time a shipment is rejected. The method follows the requirement, not the budget.
What We Have and What We Do Not
| Route | Us |
|---|---|
| ① Single parameter, sound velocity | Yes — PS7020; method and limits on the tail-gas scrubbing page |
| ③ Spectroscopic multi-component | Yes — PS7100; comparable duties at available chlorine + free alkali and multi-component baths |
| ② Sound velocity + conductivity in one instrument | No. |
That third row is the main reason this page exists. Someone writing “ultrasonic dual-parameter concentration meter” on an enquiry most likely wants route ②, and we do not have it. Better said while you are still drafting the specification than explained after an award. What can be discussed from here: whether the two numbers you want can be met by route ① instead (on most tail-gas towers they can); if not, whether route ③ is acceptable (capable, more expensive); and if neither fits, that we cannot do this point — which we will say plainly.
What We Will Ask You
- Is this liquor discharged, reused, or sold as product?
- Is acceptance on one figure or two, and at what thresholds?
- Is make-up caustic a purchased fixed strength, or made or recovered on site?
- Gas composition: chlorine only, or with HCl / CO₂ / organics? Is the ratio stable?
- Is the tower continuously loaded or on emergency standby? Any separate water addition?
- Temperature range, pressure, bore, hazardous-area classification, wetted-material requirement.
- Any laboratory results from field samples (free alkali + available chlorine + temperature at the time) — with those we can work out which route stands up before quoting.
What this page is: a method and selection note, not a case study. There are no unit counts, saving percentages or payback periods here — we do not invent those. Where the page says what the industry does, it cites something you can check for yourself (published vendor literature, granted patents) rather than implying we have installed base there. Send us the process conditions and you get a selection opinion you can check.
