Every water-diluted Cortec product, whether it is a VpCI-41x cleaner in a parts washer, a VpCI-377 or BioCorr rust-preventative dip tank, or a VpCI-649 hydrotest additive, only performs at the concentration it was designed for. Cortec Laboratories publishes a refractometer (Brix) correlation chart for 18 of these products and a pH correlation chart for 16 of them. This page collects every formula from those charts into one table, adds a quick target-Brix lookup, explains when pH is useful and when it is not, reproduces Cortec’s seven-step procedure for building a chart on your own process water, and gives you a simple Brix-to-percent calculator. Each product also has its own anchor further down the page so you can bookmark the one chart you use. If you are still choosing a product, start with the VpCI product selection guide.
Why concentration control matters
A working bath never stays where you mixed it. Water evaporates from a heated cleaner tank and the concentration climbs; parts carry solution out (drag-out) and water top-ups dilute what is left; oil, fines and dissolved soils accumulate and change how the bath reads. A rust-preventative dip tank drifts more slowly, but Cortec’s BioCorr guide is explicit that refractometer readings fall over time and the tank must be topped up with product to hold them.
Under-dosing is the expensive failure: a VpCI-416 bath that has drifted from 10 percent to 4 percent still cleans but no longer leaves enough inhibitor to stop flash rust before the next operation, and a VpCI-377 tank below its 5 percent target quietly shortens protection from years to months. Over-dosing wastes concentrate, leaves heavier residue or tackiness than intended (Cortec’s VpCI-316 and 377 guides note tackiness rising above 10 percent), and in cleaners raises the alkalinity your rinse has to handle.
Bio-based products add a third failure mode. Cortec’s BioCorr guide gives a working pH window of 8.4 to 9.1 and warns that a reading below it invites biological growth and fouling of the emulsion. For BioCorr, pH is a bath-health alarm rather than a concentration check, and the response to a low reading is to look at contamination and tank hygiene, not simply to add product.
Concentration control therefore means a scheduled reading (daily on heated cleaner tanks, weekly on dip tanks is a reasonable start) against a written target with an action limit on either side. The refractometer is the primary instrument; pH is a secondary check for some products and a health indicator for others.
How a refractometer works and why Brix
A refractometer measures how much a liquid bends light. Dissolved solids, whether sugar, salt, glycol or a corrosion-inhibitor package, raise the refractive index in proportion to how much is dissolved. Brix (°Bx) was defined for grams of sucrose per 100 grams of solution, but almost every industrial hand refractometer carries a 0 to 32 °Bx scale, so Cortec standardized its correlation charts on it; its chart-making note says refractometry “is most easily measured using the Brix scale.” A Brix reading on a Cortec product is not a sugar content, just a repeatable number Cortec’s laboratory has plotted against known concentrations. A few drops on the prism, a look through the eyepiece, a wipe, and you have a reading in under a minute; digital instruments read the same scale without the shadow-line guesswork. Two cautions apply: temperature (let sample and prism equalize, or use automatic temperature compensation) and contamination (emulsified oil, fines and hardness all shift the reading, which is why the site-chart procedure below matters).
Master refractometer formula table
Every entry below is taken from a Cortec Laboratories Brix reading correlation chart. The relationship is linear across the charted range, and Cortec reports a coefficient of determination (R²) of 1.00 on all but two charts. Use the by-volume column if you mix by volume (the usual shop practice) and the by-weight column if you batch by mass. The chart range is the span of concentrations Cortec actually plotted; readings that convert to a value outside that range are extrapolations and should be confirmed by dilution and re-reading. Products stocked by SRS are linked; the others are listed for reference.
| Product | By volume: Vol % = | By weight: Wt % = | R² | Chart range |
|---|---|---|---|---|
| BioCorr (chart labelled BioCorr SC) | 1.07 × Brix + 0.39 | 1.00 × Brix + 0.39 | 1.00 | 0–20 % |
| M-640L | 1.23 × Brix + 0.01 | 1.37 × Brix + 0.01 | 1.00 | 0–10 % |
| VpCI-316 | 2.04 × Brix − 0.32 | 2.08 × Brix − 0.32 | 1.00 | 0–30 % |
| VpCI-344 | 1.59 × Brix + 0.10 | 1.69 × Brix + 0.10 | 1.00 | 0–10 % |
| VpCI-345 | 2.22 × Brix − 0.05 | 2.38 × Brix − 0.05 | 1.00 | 0–30 % |
| VpCI-347 | 0.93 × Brix + 0.31 | 0.96 × Brix + 0.31 | 1.00 | 0–20 % |
| VpCI-377 | 1.64 × Brix − 0.13 | 1.79 × Brix − 0.13 | 1.00 | 0–30 % |
| VpCI-408 | 7.14 × Brix + 0.02 | 7.69 × Brix + 0.02 | 1.00 | 0–20 % vol / 0–25 % wt |
| VpCI-414 | 3.85 × Brix + 0.09 | 4.00 × Brix + 0.09 | 1.00 | 0–25 % |
| VpCI-415 | 6.67 × Brix − 0.02 | 6.67 × Brix − 0.02 | 1.00 | 0–25 % |
| VpCI-416 | 3.85 × Brix + 0.10 | 4.00 × Brix + 0.10 | 1.00 | 0–25 % |
| VpCI-418L | 5.00 × Brix + 0.04 | 5.26 × Brix + 0.04 | 1.00 | 0–20 % vol / 0–25 % wt |
| VpCI-418LM | 6.25 × Brix + 0.06 | 6.67 × Brix + 0.06 | 1.00 | 0–25 % |
| VpCI-426 | 3.45 × Brix − 0.09 | 3.57 × Brix − 0.09 | 0.99 | 0–20 % |
| VpCI-609 powder | no volume chart (powder) | 0.67 × Brix − 0.12 | 1.00 | 0–14 % wt |
| VpCI-611 | 3.45 × Brix + 0.04 | 3.57 × Brix + 0.04 | 1.00 | 0–20 % |
| VpCI-645 | 2.00 × Brix + 0.01 | 2.50 × Brix + 0.01 | 1.00 | 0–20 % |
| VpCI-649 and VpCI-649 BD | 1.47 × Brix − 0.10 | 1.69 × Brix − 0.10 | 0.99 | 0–5 % vol / 0–6 % wt |
A useful sanity check: the BioCorr application guide gives a working target of 4.4 to 4.8 °Bx. Putting 4.6 °Bx into the BioCorr formula gives 1.07 × 4.6 + 0.39, or roughly 5.3 percent by volume, which agrees with the 1:19 (5 percent) dilution Cortec specifies for BioCorr SC concentrate.
Read the slope as sensitivity. A steep slope such as VpCI-408 (7.14) or VpCI-415 (6.67) means a full percentage point of concentration moves the reading only about 0.15 °Bx, so a hand refractometer resolves these cleaners to roughly plus or minus one percent. A shallow slope such as VpCI-347 (0.93) or BioCorr (1.07) means each degree Brix is close to one percent and the instrument is comparatively precise. For low-dose water treatment (VpCI-649 at 0.5 to 3 percent) Cortec’s chart spans only 0 to 5 percent, and the laboratory also publishes molybdate tracer charts for VpCI-649, 649 BD and S-69: those products carry a molybdate tracer that a standard colorimetric test kit reads against a log-log curve, which is more sensitive than refractometry at those doses. Ask us for the tracer charts if you run a closed loop or hydrotest program.
Quick target-Brix lookup
This table inverts the by-volume formulas above to answer the question people actually ask at the tank: “what should the refractometer read if the bath is right?” Values are rounded to the nearest 0.1 °Bx, which is the practical resolution of a hand instrument. A dash means the target is above the top of Cortec’s chart.
| Product | 2 % | 5 % | 10 % | 20 % |
|---|---|---|---|---|
| VpCI-316 | 1.1 | 2.6 | 5.1 | 10.0 |
| VpCI-377 | 1.3 | 3.1 | 6.2 | 12.3 |
| VpCI-414 | 0.5 | 1.3 | 2.6 | 5.2 |
| VpCI-415 | 0.3 | 0.75 | 1.5 | 3.0 |
| VpCI-416 | 0.5 | 1.3 | 2.6 | 5.2 |
| VpCI-418L | 0.4 | 1.0 | 2.0 | 4.0 |
| VpCI-418LM | 0.3 | 0.8 | 1.6 | 3.2 |
| VpCI-408 | 0.3 | 0.7 | 1.4 | 2.8 |
| VpCI-426 | 0.6 | 1.5 | 2.9 | 5.8 |
| VpCI-611 | 0.6 | 1.4 | 2.9 | 5.8 |
| VpCI-645 | 1.0 | 2.5 | 5.0 | 10.0 |
| M-640L | 1.6 | 4.1 | 8.1 | — |
| VpCI-344 | 1.2 | 3.1 | 6.2 | — |
| VpCI-345 | 0.9 | 2.3 | 4.5 | 9.0 |
| VpCI-347 | 1.8 | 5.0 | 10.4 | — |
| BioCorr SC | 1.5 | 4.3 | 9.0 | — |
| VpCI-649 / 649 BD (at 1 %, 2 %, 3 %, 5 %) | 0.75 | 1.4 | 2.1 | 3.5 |
Notice how tightly the cleaners bunch. Between 5 and 10 percent, which is where Cortec suggests starting most VpCI-41x baths, the difference in reading is only about 1.3 °Bx for VpCI-414 and 416 and under 1 °Bx for 415, 418L, 418LM and 408. That is workable with a clean sample and a temperature-compensated instrument, but it is why Cortec’s foaming-cleaner guide lists titration alongside refractometry as an acceptable monitoring method, and why you should read the same sample three times and average.
pH windows and when to use pH
Cortec also publishes a pH correlation chart for most of the same products. Unlike the Brix charts, these carry no equation; each shows the plotted readings with a logarithmic trend line, meaning pH rises quickly over the first few percent and then flattens. That shape is the whole story of pH as a concentration tool: it is sensitive at the bottom of the range and nearly useless at the top. The table gives the pH window printed on each chart’s axis and the concentration range plotted.
| Product | pH window on chart | Concentration range (vol / wt) | Character |
|---|---|---|---|
| VpCI-316 | 7.0–8.0 | 0–30 % / 0–30 % | Near neutral and flat; pH is a weak indicator, use the refractometer |
| VpCI-344 | 7.2–8.2 | 0–10 % / 0–12 % | Near neutral |
| VpCI-345 | 8.0–9.0 | 0–30 % / 0–35 % | Mildly alkaline |
| VpCI-377 | 7.2–8.2 | 0–30 % / 0–35 % | Near neutral; concentrate pH 7.5–9.5 per the application guide |
| VpCI-408 | 8.5–9.2 | 0–20 % / 0–25 % | Low-pH cleaner for pH-restricted sites |
| VpCI-414 | 10.0–11.4 | 0–25 % / 0–25 % | Alkaline cleaner; usable secondary check |
| VpCI-415 | 7.0–8.4 | 0–25 % / 0–25 % | Near-neutral cleaner |
| VpCI-416 | 10.4–12.2 | 0–25 % / 0–25 % | Strongly alkaline; steepest useful pH curve of the cleaners |
| VpCI-418L | 11.0–13.0 | 0–20 % / 0–25 % | Very alkaline, silicated |
| VpCI-418LM | 10.0–11.4 | 0–25 % / 0–25 % | Alkaline; usable secondary check |
| VpCI-426 | 1.5–2.5 | 0–20 % / 0–20 % | Acidic rust and scale remover |
| VpCI-609 powder | 2.0–3.0 as printed | wt only, 0–16 % | Treat with caution; verify against the current data sheet |
| VpCI-611 | 9.0–10.0 vol / 9.0–9.8 wt | 0–12 % / 0–12 % | Alkaline |
| VpCI-645 | 10.0–13.0 | 0–14 % / 0–14 % | Very alkaline; wide, useful swing |
| VpCI-649 | 7.5–8.9 | 0–5 % / 0–6 % | Low-dose water treatment |
| VpCI-649 BD | 8.0–9.4 | 0–5 % / 0–6 % | Low-dose water treatment |
Our guidance, following Cortec’s chart set: use the refractometer as the primary control for every product on this page. Use pH as a secondary check only where the window spans more than about one pH unit across the working range, which in practice means VpCI-414, 416, 418L, 418LM, 645 and the acidic VpCI-426. For the near-neutral rust preventatives (316, 377, 344) a pH meter will tell you very little about concentration. For BioCorr, use pH as a bath-health indicator: Cortec’s working range is 8.4 to 9.1, and a drift below 8.4 is a flag for biological growth rather than a dosing instruction. A pH reading that disagrees with the refractometer is also a useful contamination alarm; acidic drag-in from a rust-remover stage, for example, will pull a cleaner’s pH down while leaving the Brix reading roughly unchanged.
Build your own reference chart on your process water
Cortec’s laboratory charts are all generated in deionized water with nothing else present. Your tank is not. Hardness, biocides, scale inhibitors, rinse-water carry-over and dissolved soils all shift the refractive index, so Cortec recommends that any site running a controlled bath build its own correlation chart. The procedure, from Cortec’s “How to Make a Reference Chart” note, takes an afternoon:
- Collect a sample of the actual process water you will dilute with (DI, softened, tap), including every other chemical that will be present in operation, such as biocides or scale inhibitors.
- On a small scale, mix that water with the Cortec product at your intended operating concentration.
- Make at least two standards above the target. For a 5 percent target, mix 7 and 10 percent; for a 10 percent target, mix 15 and 20 percent.
- Make at least two standards below the target. For 5 percent, mix 1 and 3 percent; for 10 percent, mix 1 and 5 percent.
- Read each standard on the refractometer (and pH meter, if you will use pH) at least three times. If the readings differ slightly, average them.
- Plot concentration on the x-axis and the reading on the y-axis.
- Fit the curve. Refractometer readings should fall on a straight line; pH readings should follow a logarithmic curve. If a point sits well off the line, remix that standard and read again.
Keep the chart at the tank with the date, the water source and the product lot, and rebuild it if the water source or any co-additive changes. Once you have a site chart, the Cortec formulas on this page become your cross-check rather than your primary reference.
Brix to percent calculator
Pick the product, choose whether you batch by volume or by weight, enter the refractometer reading, and the calculator applies the Cortec formula from the table above. It flags any result that falls outside the range Cortec plotted. If the calculator does not display, the formulas in the master table give the same answer by hand.
Enter a reading to see the concentration.
Per-product refractometer formulas
One line per product for bookmarking. All formulas are Cortec Laboratories by-volume correlations unless noted; the by-weight versions are in the master table above.
BioCorr refractometer chart
Vol % = 1.07 × Brix + 0.39 (0–20 %). Working target 4.4–4.8 °Bx for the standard 5 percent BioCorr SC dilution; pH 8.4–9.1. BioCorr listing.
M-640L refractometer chart
Vol % = 1.23 × Brix + 0.01 (0–10 %). Oil and hydraulic additive; no pH chart published.
VpCI-316 refractometer chart
Vol % = 2.04 × Brix − 0.32 (0–30 %). Target 2.6 °Bx at 5 percent, 5.1 °Bx at 10 percent. pH 7.0–8.0, too flat to use for control. VpCI-316 listing.
VpCI-344 refractometer chart
Vol % = 1.59 × Brix + 0.10 (0–10 %). pH window 7.2–8.2.
VpCI-345 refractometer chart
Vol % = 2.22 × Brix − 0.05 (0–30 %). pH window 8.0–9.0.
VpCI-347 refractometer chart
Vol % = 0.93 × Brix + 0.31 (0–20 %). The shallowest slope in the set, so the most precise on a hand instrument.
VpCI-377 refractometer chart
Vol % = 1.64 × Brix − 0.13 (0–30 %). Target 3.1 °Bx at 5 percent, 6.2 °Bx at 10 percent. pH 7.2–8.2 across the range; use the refractometer. VpCI-377 listing.
VpCI-408 refractometer chart
Vol % = 7.14 × Brix + 0.02 (0–20 %). Very steep slope: 0.7 °Bx at 5 percent, 1.4 °Bx at 10 percent, so read carefully. pH 8.5–9.2. VpCI-408 listing.
VpCI-414 refractometer chart
Vol % = 3.85 × Brix + 0.09 (0–25 %). Target 1.3 °Bx at 5 percent, 2.6 °Bx at 10 percent, 5.2 °Bx at 20 percent (coating removal). pH 10.0–11.4, usable as a secondary check. VpCI-414 listing.
VpCI-415 refractometer chart
Vol % = 6.67 × Brix − 0.02 (0–25 %); the by-weight formula is identical. Target 0.75 °Bx at 5 percent, 1.5 °Bx at 10 percent. pH 7.0–8.4. VpCI-415 listing.
VpCI-416 refractometer chart
Vol % = 3.85 × Brix + 0.10 (0–25 %). Target 1.3 °Bx at 5 percent, 2.6 °Bx at 10 percent. pH 10.4–12.2, the most useful pH curve among the cleaners. VpCI-416 listing.
VpCI-418L refractometer chart
Vol % = 5.00 × Brix + 0.04 (0–20 %). Target 1.0 °Bx at 5 percent, 2.0 °Bx at 10 percent. pH 11.0–13.0. VpCI-418L listing.
VpCI-418LM refractometer chart
Vol % = 6.25 × Brix + 0.06 (0–25 %). Target 0.8 °Bx at 5 percent, 1.6 °Bx at 10 percent. pH 10.0–11.4. VpCI-418LM listing.
VpCI-426 refractometer chart
Vol % = 3.45 × Brix − 0.09 (0–20 %, R² 0.99). Acidic: pH 1.5–2.5. VpCI-426 listing.
VpCI-609 refractometer chart
Wt % = 0.67 × Brix − 0.12 (0–14 % by weight; powder, so no volume chart). VpCI-609 listing.
VpCI-611 refractometer chart
Vol % = 3.45 × Brix + 0.04 (0–20 %). pH 9.0–10.0.
VpCI-645 refractometer chart
Vol % = 2.00 × Brix + 0.01 (0–20 %). pH 10.0–13.0, a wide and useful pH swing.
VpCI-649 and VpCI-649 BD refractometer chart
Vol % = 1.47 × Brix − 0.10 (0–5 %, R² 0.99). At typical 0.5 to 3 percent doses, molybdate tracer testing is the more sensitive method; ask us for the tracer charts. pH 7.5–8.9 (649) and 8.0–9.4 (649 BD).
Checking parts-washer cleaner concentration
The aqueous Build-All and Magido parts washers we sell are heated spray cabinets and immersion units, and they are the most common place our customers run a Cortec cleaner. The non-foaming products, VpCI-418LM (silicate free), VpCI-418L (silicated, aluminum safe) and VpCI-408 (for pH-restricted sites), are the natural fits for spray cabinets; the foaming VpCI-414 and VpCI-416 belong in immersion and soak tanks. All are in the cleaners and degreasers category.
A heated washer is the hardest case for concentration control: evaporation from a 120 to 160 °F reservoir concentrates the bath every shift, automatic make-up water then dilutes it, and the oil load builds until the emulsion starts to affect the refractometer. A practical routine: at the start of each shift, draw a sample from the reservoir (not the spray return), let it cool, take three readings, and compare against your site chart or the quick-lookup values above. Top up with concentrate below the low action limit; add water above the high limit. Once a week take a pH reading as well: for VpCI-416 or 418LM a pH that has fallen while the Brix has held points to acidic contamination or a spent builder, and a Brix reading that keeps climbing after the oil has been skimmed points to dissolved soil rather than product. Cortec’s cleaner guide adds that pre-wiping heavy soils extends bath life, that the tank should be covered when idle, and that starting at 5 to 10 percent with a longer dwell beats starting strong.
The reason to care is what happens after the wash. Cortec’s cleaners leave a thin flash-rust inhibiting film, and that film is concentration dependent. Hold the bath at target and parts come out of a Magido or Build-All cabinet protected for days to weeks; let it drift and they rust in the rack. Our article on clean-then-protect parts washing covers the full sequence, and the compatibility charts list which seal materials each cleaner has been tested against.
Frequently asked questions
Which refractometer do I need for Cortec products?
Any hand or digital refractometer with a 0 to 32 °Brix scale. All of Cortec’s correlation charts are built on the Brix scale, and a low-range instrument reads the cleaners, which sit between 0.3 and 6 °Bx at working strength, more precisely than a wide-range one. Automatic temperature compensation is worth having on a heated washer.
Why does my reading not match the Cortec chart?
Cortec’s charts were generated in deionized water with no other chemistry present. Hardness, biocides, scale inhibitors, dissolved oil and fines all shift the refractive index. Build a site chart on your own process water using the seven-step procedure above and use the Cortec formula as a cross-check.
Can I use pH alone to control a VpCI-377 or VpCI-316 tank?
No. Both products sit between pH 7 and 8.2 across their entire working range, so a pH meter cannot distinguish 3 percent from 15 percent. Use the refractometer.
What does a low pH mean in a BioCorr tank?
Cortec’s working range is 8.4 to 9.1. A reading below 8.4 is a warning that biological growth may be fouling the emulsion. Check for contamination, keep the tank covered, agitate daily, and consult Cortec before simply adding product.
How often should I test concentration?
Daily on heated cleaner baths and spray washers, where evaporation and make-up water move the concentration every shift. Weekly is a reasonable start on unheated rust-preventative dip tanks. In both cases set a written high and low action limit around your target.
Is by-volume or by-weight the right formula?
Use whichever matches how you mix. Most shops measure concentrate and water by volume, so the by-volume formula and the quick-lookup table apply. If you batch on a scale, use the by-weight formula. The difference is a few percent for most products and larger for the denser cleaners.
Where do I get the original Cortec charts?
Cortec’s laboratory supplies its refractometer and pH charts on request. Contact us with the product name and we will send the chart PDF, or the molybdate tracer charts for VpCI-649, 649 BD and S-69.
Need help setting a target, choosing an instrument, or matching a cleaner to a Build-All or Magido washer? Contact Solvent Recycling Systems and we will walk through it with you. Related guides: Cortec coatings application guides, how to shrink wrap equipment with MilCorr, and the full Cortec product category.
Data source: compiled from Cortec application guides and technical literature (Cortec Laboratories refractometer and pH correlation charts and the “How to Make a Reference Chart” note, 2013–2017 vintage); confirm against the current product data sheet before specifying.