About this site

Why it exists

In a dry climate a properly functioning direct evaporative cooler is an elegant piece of engineering. It delivers substantial cooling for a fraction of the electricity a compressor needs, uses no refrigerant, and supplies 100 % outside air. Most complaints about them turn out to be one of three things, and only the first is the cooler's fault:

• The unit is genuinely underperforming — dry pads, scale, a weak pump, a slipping belt.
• The unit is performing to spec and the house cannot exhaust the air it is being given.
• The weather is outside the machine's envelope, and nobody told the owner.

Telling those apart takes one measurement and a little arithmetic. That is the whole product.

How the numbers are calculated

Saturation vapour pressure follows the Hyland–Wexler formulation as published in the ASHRAE Handbook — Fundamentals, psychrometrics chapter, using the over-liquid-water coefficient set above 0 °C and the over-ice set below it.

Wet-bulb temperature is solved iteratively, by bisection against the ASHRAE humidity-ratio relation, to a tolerance far below the accuracy of any thermometer you will use. It is not approximated.

That distinction matters more than it sounds. The popular closed-form approximations for wet-bulb — Stull's in particular — are regression fits made at sea level. At 5,000 ft Stull's returns a wet-bulb about 2 °F too high for typical desert conditions, which understates available cooling by roughly 6 % and biases in exactly the wrong direction: it makes a healthy high-desert cooler look deficient.

So elevation is applied, not ignored. Barometric pressure comes from the weather station reading where available, and otherwise from the ASHRAE standard atmosphere. Here is the difference on one representative case:

Sea level

95 °F, 15 % RH, 101.33 kPa → wet-bulb 63.4 °F, depression 31.6 °F

5,000 ft

95 °F, 15 % RH, 84.31 kPa → wet-bulb 61.4 °F, depression 33.6 °F

The psychrometric module is pure, dependency-free, and covered by a test harness that checks it against published reference values to within 0.2 °F, along with assertions that the physics cannot be violated — wet-bulb never exceeds dry-bulb, dew point never exceeds wet-bulb, and lowering pressure always lowers wet-bulb. The same module computes every number on the site, including the chart, so no two figures can disagree.

Where the effectiveness bands come from

Saturation effectiveness values are drawn from manufacturer published data and the ASHRAE Handbook's treatment of direct evaporative equipment. They are ranges rather than constants because effectiveness genuinely varies with pad material, thickness, face velocity, and condition:

0.85 – 0.90

Rigid media, 8–12 in, well maintained

0.70 – 0.80

Rigid media at 4 in, or fresh aspen pads fully wetted

0.55 – 0.70

Aspen pads mid-season or partially scaled

The site reports a best case at 0.90 throughout — that figure is about the best a direct cooler can do, not about yours. The typical figure follows the pad type you select on the front page: 0.87 for 8–12 in rigid media, which is where it starts, 0.75 for a 4 in block, and 0.72 for aspen. The verdict threshold moves with it, because an aspen cooler at 0.68 is a working aspen cooler and calling that a fault would be exactly the misdiagnosis this site exists to prevent. You can also set ε yourself if you know your equipment.

These are honest engineering ranges, not precision claims, and a reading a few points either side of a boundary should be read as "near the boundary" rather than as a diagnosis.

Privacy

No advertising. No tracking. No analytics. No cookies beyond a single stored preference for whether you want °F or °C. Nothing about your visit is recorded or transmitted anywhere.

If you use the location button, your coordinates are rounded to two decimal places — roughly a kilometre — before being sent to Open-Meteo to fetch weather. That is far more precision than a weather lookup needs, and it avoids transmitting a precise home address. Coordinates are never written to persistent storage. Fetched conditions are cached in your browser's session storage for ten minutes and disappear when you close the tab.

The location button only fires when you press it. The site never requests your location on page load.

You never have to use it at all — manual entry is always available, and the calculator works with no network access whatsoever.

Sources and attribution

Psychrometrics

ASHRAE Handbook — Fundamentals, chapter on psychrometrics: Hyland–Wexler saturation pressure, humidity-ratio relations, and the standard atmosphere.

Weather data

Provided by Open-Meteo and licensed under CC BY 4.0. Used without an API key under their non-commercial terms — which is one reason this site carries no advertising.

Equipment data

Manufacturer published effectiveness and airflow figures for residential direct evaporative equipment.

Corrections and contact

If a number here is wrong, it should be fixed. The source code is not published, but the method is: every formulation used is named above with its reference, and the two worked cases in the table can be checked against any psychrometric chart or ASHRAE table you care to open. For a site whose entire claim is "here is the correct figure," being checkable against the handbook is the point.

Found a mathematical, physical, or engineering error? Please write to contact@ismycoolerworking.com. A bad coefficient, a sign error, a misapplied pressure correction, a citation that does not say what I claim it says — all genuinely welcome, and all will be fixed.

Less welcome is mail explaining that the laws of thermodynamics do not apply to your particular cooler. They do. If your machine is reliably producing air below the wet-bulb temperature, you have either a thermometer problem or a Nobel Prize, and the odds heavily favour the thermometer.