Bluelab Calibration fluid PH 4.01 20ml

A sealed sachet of 20 ml Bluelab pH 4.01 buffer. The second calibration point, correcting the slope of your scale.

The temperature of your buffer counts

This is the calibration error almost nobody mentions, and it can shift your readings noticeably.

A buffer solution's pH is not one fixed number under all conditions: it is defined at a reference temperature, usually 25 °C. If the liquid is colder or warmer, its actual value changes slightly. Good packaging often prints a small table of value against temperature for exactly this reason.

Why it goes wrong in practice: your buffer lives in a shed, a garage, or a grow room that cools overnight. You take the sachet, pour it, calibrate straight away, and set your meter against a value that does not match what the meter thinks it is seeing. You then measure your nutrient solution, which is at room temperature, and your whole scale is off.

The fix costs you five minutes and nothing else: let the sachet come to room temperature before you open it. Put it beside your meter while you prepare your reservoir, then calibrate.

The same applies to your 7.01. Calibrate both points at the same temperature, and measure at that temperature too. Then your meter works the way it was designed to.

Technical Specifications:

• Value: pH 4.01
• Volume: 20 ml, sealed sachet
• Brand: Bluelab
• Use: second calibration point, after 7.01
• Handling: single use

Frequently asked questions

How much does temperature really matter?
A little, but not nothing, and it adds to other small errors. Calibrating at room temperature is free, so there is no reason not to.

Can I warm the sachet in hot water?
Better to let it come up on its own. Warming it quickly leaves the liquid at an uneven temperature.

In what order do I calibrate?
7.01 first, then 4.01, rinsing with clean water in between.