[jsscripting] divide using quantity provides wrong results

[MikeTheTux]

When dividing Temperature Quantities (°C or °F) the result is unexpected:

My test code:

console.log("OK:  " + Quantity('13.57 W').divide('25 W'));
console.log("NOK: " + Quantity('13.57 °C').divide('25 °C'));
console.log("NOK: " + Quantity('13.57 °F').divide('25 °F'));
console.log("OK:  "  + Quantity('13.57 Wh').divide('25 Wh'));

Expected Behavior

4 times 0.5428

Current Behavior

2024-02-04 19:37:03.491 [INFO ] [hab.automation.script.file.energy.js] - OK:  0.5428
2024-02-04 19:37:03.495 [INFO ] [hab.automation.script.file.energy.js] - NOK: 0.96166359215160154284755995304376983552
2024-02-04 19:37:03.498 [INFO ] [hab.automation.script.file.energy.js] - NOK: 0.976416943487321270142571234035529257646688812319034944739032056175667696
2024-02-04 19:37:03.501 [INFO ] [hab.automation.script.file.energy.js] - OK:  0.5428

Your Environment

openHAB 4.1.1 Release Build

[mherwege]

This is not a bug. You cannot divide temperatures unless you are working in K. °C and F are not on an absolute scale.
QuantityTypes also allow dividing e.g. °C by F, and the result would become unpredictable.

Here is a PR with a discussion on this and related topics, that intended to fix unexpected behaviour: openhab/openhab-core#3792

[MikeTheTux]

Thanks for the info. It explains well the recognized behavior.

Quantity('13.57 K').divide('25 K') provides the expected result OK: 0.5428

In my examples, °F and °C have been implicitly transformed to K before doing the divide operation. This also explains the result >= 0,9.

Actually in my use case I'm dividing two Δ temperatures with each other - now I know, that I need to implement the entire formula in K.

const min = Quantity("40 °C");
const max = Quantity("66.5 °C");
let water_SOC_wrong = water.subtract(min).divide(max.subtract(min)).toUnit('%');
let water_SOC_right = water.toUnit("K").subtract(min.toUnit("K")).divide(max.toUnit("K").subtract(min.toUnit("K"))).toUnit('%');

I hope the discussion helps others who have the same issue.

closing the ticket.