APAX LAB TDS Meter

APAX LAB TDS Meter

APAX LAB TDS Meter

249 NOK

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Product details

A pen-style digital TDS meter for checking the total dissolved solids in your brew water. It measures electrical conductivity and converts that to a ppm reading, which tells you how much mineral content is in the water in front of you.

Useful in two places: confirming your base water is close to zero before you build a recipe, and verifying that the recipe you mixed came out where you intended.

  • Ranges: 0 to 999 ppm (1 ppm resolution), 1,000 to 9,990 ppm (10 ppm resolution)
  • Range switching: automatic
  • Functions: single-button operation, reading hold
  • Auto power off: after 10 minutes idle
  • Battery: LR44

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What it actually measures

A TDS meter passes a small current between two electrodes and measures how easily the water conducts it. Dissolved minerals carry charge, so more minerals means higher conductivity, and the meter converts that into a parts per million figure.

The important consequence is that it tells you how much is dissolved, not what. It cannot distinguish calcium from sodium, or hardness from alkalinity. Two waters reading 80 ppm can taste completely different in coffee if one is mostly bicarbonate and the other mostly magnesium. This is a quantity check, not a composition analysis.

Where it earns its place in a water workflow

If you build brew water from mineral concentrates, the meter answers the two questions that actually cause problems. First, is my base water really at zero? Filtered or distilled water drifts as a filter approaches the end of its life, and a base sitting at 15 ppm instead of 000 quietly shifts every recipe you mix on top of it. Second, did I dose what I think I dosed? A recipe targeting a known ppm gives you a number to check against, so a mis-measured dose shows up before you brew rather than after.

It is also the simplest way to find out what your tap water is doing, which is worth knowing even if you have no intention of building water from scratch.

Not a refractometer

This is a common mix-up worth being clear about. A TDS meter measures dissolved solids in water, before brewing. A refractometer measures dissolved coffee solids in a finished brew, which is what you use to calculate extraction yield. They sound similar and do entirely different jobs. If you are trying to work out your extraction percentage, this is not the tool.

Coffee’s dissolved solids are mostly uncharged. What’s actually in the cup is largely melanoidins, polysaccharides, caffeine, lipids and other organic compounds, none of which carry a charge in solution. They contribute essentially nothing to conductivity. The parts of coffee that do conduct are the mineral ions, potassium above all, plus the partially dissociated organic acids like chlorogenic and citric. That’s a real but small and variable fraction of the total. So the meter is blind to most of the mass it’s supposed to be measuring, and the ratio between the ionic fraction and the total shifts with roast level, origin, processing and brew method. There’s no stable conversion to fall back on.

The conversion factor is calibrated for something else entirely. TDS pens derive ppm from conductivity using a factor based on a reference salt solution, typically sodium chloride or the “442” mix. That’s a reasonable model for tap water. It bears no relationship to the chemistry of brewed coffee, so even the ionic portion gets converted using the wrong assumption.

The range problem is more immediate. Filter coffee lands around 1.35 percent TDS, which is roughly 13,500 ppm. The Apax meter tops out at 9,990. Espresso runs somewhere around 8 to 12 percent, so 80,000 to 120,000 ppm. You’d be off the scale before any of the chemistry arguments mattered.

And temperature. Conductivity is strongly temperature dependent, and these pens are calibrated for water near 21 °C. Coffee is hot, and if you cool it to measure you’ve changed nothing about the other three problems.

A refractometer works instead because refractive index responds to dissolved mass regardless of charge. It sees the melanoidins and sugars that the conductivity meter cannot, which is why coffee refractometers are built around a Brix scale correlated to coffee solids rather than around conductivity.

Worth adding that the idea isn’t absurd, which is probably why it comes up so often. Conductivity does correlate loosely with coffee strength, and it has been used as a rough proxy in some research settings. It just isn’t reliable enough across different coffees to give you a number you’d trust for dialling in, and the consumer pens aren’t built for it anyway.

Getting reliable readings

Measure at room temperature. These meters are calibrated for water around 21 °C, and readings drift high in hot water and low in cold, so testing a kettle that has just boiled will not give you a usable number. Let the sample settle to room temperature first. The cap doubles as a small sample cup if you would rather not dip the probe into your main container.

One other thing to know: converting conductivity into ppm requires a conversion factor, and manufacturers do not all use the same one. Two different meters can therefore give slightly different readings on the same water. This does not matter for tracking your own consistency, since you are comparing against yourself, but it does mean a reading from this meter and one from another brand are not always directly comparable.

Specifications

Type Digital pen-style TDS meter, conductivity based
Low range 0 to 999 ppm, 1 ppm resolution
High range 1,000 to 9,990 ppm, 10 ppm resolution
Range selection Automatic
Controls Single button
Hold function Yes, freezes the reading on screen
Auto shut-off 10 minutes of inactivity
Battery LR44
Calibration temperature Approx. 21 °C

Good to know

Rinse and dry the probe after use, particularly if you have measured mineral-heavy water, and store it with the cap on. Do not submerge the meter past the probe section.

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