cheap sensors

Sometimes you need to be able to measure some basic information, but don't need those measurements to be particularly accurate

sensors present in smart phones

Many Android phones have four sensors that can be read by apps, however not all sensors are present in all phones:

An ambient light sensor, usually used only to automatically adjust the screen brightness.

• Lux Light Meter Pro, Android
• The sensor used on my Pixel 6 is the TMD3719. Its datasheet claims the lux reading is 85 to 115% accurate.

An ambient humidity sensor.

An ambient pressure sensor, used to supplement the phone's altitude measurements.

• Barometer & Altimeter

An ambient temperature sensor.

• Pixel Temperature (sensor isn't present on the Pixel 6)

Reads all available sensors:

• Sensors Toolbox
• Sensors Multitool
• Sensor Data
• Sensor Box

temperature

If you have a meat thermometer, many of those are capable of working as an ambient temperature thermometer. Just check its specified temperature range to ensure it can also read lower temps. Optional: crumple up a small aluminum foil ball over its tip.

You could keep a few thermometer strips on hand, as they're cheap and very compact.

airflow, indoors (anemometer)

To measure the direction and magnitude of airflow, you'll need a smoke generator. Then, observe which direction the smoke drifts, and roughly how quickly it drifts away.

• incense stick
• a smudge stick, if you need a lot more smoke
  • You don't actually need dried leaves for this, just make one out of loosely-rolled pieces of paper.
  • There's lots of videos out there about how to light and make a smudge stick.
  • A smudge stick can generate a lot of smoke, so be sure to disable any fire alarms.

length (smart phone) — TODO: trim this section, after evaluating these

Apps that guesstimate length using the camera:

• AR Ruler
• AR Plan 3D Tape
• CamToPlan
• magicplan
• Floor Plan Creator

Apps that will display a mostly accurate ruler on-screen:

• Ruler by Nikola Kosev
• Ruler by NixGame
• Ruler by Xalpha Lab
• Millimeter - screen ruler

Similar measurements:

• Ruler - Bubble level
• Bubble Level - Spirit Level
• Google Measure

length (physical)

There are lots of things that are near you every day that you can use as ad-hoc length gauges, since they come in standard sizes:

• credit cards, letter-size paper, dollar bills
• coins, both the radius, but especially the thicknesses allow for precise small measurements
• the width and height of a USB-C connector, or the USB standard (Type-A) connector

But for me, the most useful one is the size of a playing card, since I always have them on-hand.

• The outsides measure 2½ x 3½" (63 mm x 88 mm).
• For the 9 or 10 of diamonds, for the standard Bicycle design: (other designs may vary a bit)

• If you have a card that you don't mind folding in half, then the half-sizes are 1¼ x 1¾".