Thermochrons are small standalone temperature recorders about the size of 5 dimes stacked together. They have a stainless steel case and contain a battery, temperature sensor, real-time clock, microcontroller and oscillator. They can record temperatures to a resolution of 0.9 deg F at intervals from once/minute to once/255 minutes. 2048 readings can be stored.
Hygro iButtons look like Thermochrons with 8 holes drilled in the top. They can measure temperature with about a 0.06 deg F resolution along with relative humidity. Hygro iButtons are not recorders like the Thermochrons and must be hooked to a computer or external microcontroller to operate.
This page shows some random data taken with these iButtons.
More information on the iButtons is available from the manufacturer, Dallas Semiconductor
The first Thermochron plot shows the temperature inside a couple of cars on a hot day. The cars were a silver Audi A4 and a dark green BMW 318ti. The Audi had its Thermochron hanging from the passenger-side visor out of direct sunlight. The BMW had two Thermochrons, one on the visor in the same location as on the Audi, the other taped to the center of the dashboard. The cars were in a parking lot near each other away from trees and other sources of shade. Both faced the same direction, roughly SSW.
The day started and ended clear but clouds formed during the midday resulting in the temperature fluctuations seen in the plot. The ambient temperature in the shade was measured with a fourth Thermochron about four miles away.
Thermochrons have an upper temperature limit of 185 deg F so the dashboard curve is clipped where it exceeded this temperature.
In this test, the thermochron was placed in a padded 4"x7" envelope and mailed at a standard blue curb-side mail box. It was mailed Sunday morning before the Columbus Day holiday and thus wasn't picked up for two days. The destination was across town. The highest temperature occurred on the last day and is either inside the postal carrier's vehicle or the outside mailbox it was delivered to.
This plot shows the outdoor temperature and relative humidity as measured with a Hygro iButton. Comparison data includes temperature as recorded by a Thermochron iButton plus temperature and humidity measurements from a local television station and a school. The school is about 10 miles away. I'm now sure where the television station's readings are taken.
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