Temperature measures along – Measuring without measurement error

Your EC meter does not compensate for temperature. The solution is at 10 °C instead of the 25 °C the scale refers to.
By how many percent is the reading off? Set your value – and say how sure you are.

0.00 %

And how sure are you?

Two vessels with an identical pattern of dots. On the right the dots jitter more, the thermometer is higher and the reading bar rises further. As it cools, the two bars meet.

The two-percent rule, said in full

The conductivity of a solution rises by about 2 percent per degree Celsius – referenced to 25 °C. That second part is part of the rule: without a reference point the same physics is ambiguous and yields 2.08 or 2.32 percent depending on how you divide.
Depending on the source, the coefficient lies between roughly 1.75 and 2.17 percent per degree. So „about 2" is an honest rule of thumb, not a constant of nature.
And that resolves the surprise from the first card: the widely quoted „under 2 percent" is the residual error of the linear approximation after conversion – not the temperature effect itself. The effect across 15 degrees is about 29 percent. A category error by a factor of 14, and the reason temperature is the single biggest error in an EC measurement.

Your meter says „ATC" – automatic temperature compensation.
What does that tell you?

Where do those 29 percent actually come from?

We now have a number, a standard and a correction table. That sounds like solid ground.
How solid is it?

The rule that needs no number

The standard contains one sentence that ends half the debate: whichever form of compensation is applied, the result will be less accurate than one actually measured at the reference temperature.
Every compensation is a calculation on top of an assumption. It is better than nothing, but never better than the original.
From this follows the rule this whole course is about, and it needs no number at all: always measure at the same temperature.
The reason is elegant: if two measurements share a temperature, the coefficient simply cancels out in the comparison – then it makes no difference whether it is 1.75 or 2.17, whether your meter computes for 20 or 25 °C, or whether the number was ever measured in a nutrient solution.
It is not accuracy that makes your values useful, it is comparability.

You measure 1.6 in the morning and 1.9 mS/cm in the evening in the same can. The room is cool in the morning.
What do you conclude?

A single point on an axis with a band growing around it. Then many more points fall in and stack into a hill – your own point stays visible inside it.

Why the grow diary shows you no average

When you see other growers' EC values in the grow diary, you get a distribution – never an average. That is not a design whim but the consequence of this lesson.
An average would claim the values can be arithmetically combined. They cannot: everyone measured at their own room temperature, with their own hard-wired coefficient, against their own reference temperature. A mean of such numbers looks precise and is invented.
A distribution instead states what is actually known: here is where the field lies, and this is how wide it is. Exactly the band you drew yourself on the first card of this lesson.
And the second reason comes from the neighbouring course: EC only ever sees what is dissolved right now – not what has precipitated or has yet to mineralise. Two reasons, one consequence: a single number is never the whole answer.

The key takeaways

✓ „About 2 % per °C" only holds with the addition „referenced to 25 °C"
✓ 15 degrees of difference cost about 29 percent, not 2 – a category error by a factor of 14
✓ What your meter compensates is in the manual, not in the standard
✓ For nutrient solutions the coefficient is transferred, not measured
✓ Any compensation is less accurate than a measurement at the reference temperature
✓ Same temperature beats exact number: then the error cancels out

🎯 Hands-on mission

Measure your next nutrient solution twice: once right after mixing, and once when it has reached room temperature. Write both values into the grow diary, with the time.
The difference is your personal temperature error – and from the moment you know it, you also know what your previous readings were worth.
After that: pick one moment in your daily routine and always measure then. That is the whole exercise.

To the grow diary →

✓

Lesson complete!

You now measure in a way that makes your values comparable – with themselves and with others. That is exactly why the grow diary shows community EC values as a distribution: as an honest band, never as a single number.

✓ I never say „about 2 % per °C" without „referenced to 25 °C"
✓ I know that 15 degrees make about 29 %, not 2
✓ I check the manual for what my meter compensates by default
✓ I know these numbers are transferred to nutrient solutions, not measured in them
✓ I always measure at the same temperature – then the error cancels out

To the grow diary → Back to the course overview

Something still unclear?

Your EC meter does not compensate for temperature. The solution is at 10 °C instead of the 25 °C the scale refers to.By how many percent is the reading off? Set your value – and say how sure you are.