Casio Watch Thermometer Comparison

Update November 2005: I recently purchased a Casio GW-400j Silencer watch with vibrating alarm, solar recharge, and wave ceptor time calibration. This model has what appears to be the same temperature calibration feature as the DW-9400, but I have not yet tested it. Unlike the DW-9400, which has C° to F° switch, the GW400j displays temperatures only in Celsius. The GW-400j uses module 2931 (pdf). See "What? Your watch only tells time?" by Michelle Slatalla. New York Times, 17 July 2005.

In 2003, I conducted a comparison test of three Casio watches with built-in thermometers and found that there was significant variation from watch to watch. In a closed room with no ventilation or direct sunlight, I placed the watches side-by-side on a table and came back about an hour later. Here are the results:

Casio TS-1200, module 515 (early 1980s) in off-wrist thermometer mode: 81.5° F.

Casio TS-100, module 815 (1991) in off-wrist thermometer mode: 80.2° F.

Casio G-Shock Thermometer Watch DW-9400B-2V, module QW-1680 Casio DW-9400B-2V, module QW-1680 (1998) in thermometer mode set to factory default: 79.0° F. [English version of Casio DW-9400B-2v webpage without graphics | Spanish version of Casio webpage with graphics | Module QW-1680 user manual (pdf)]

Old, round Honeywell thermostat on a nearby wall: approximately 80.0° F.

This point of this test was not to determine which of the three watches is the most accurate, but to show that there is significant watch-to-watch and/or model-to-model variation in thermometer accuracy among Casio watches when used off the wrist. Newer models, such as module 1680, allow the user to calibrate the temperature within a range of ±18.0° F (±9.9° C), but the older watches are fixed at a factory default.

The older watches have an on-wrist temperature setting, but it has proven to be useless because it cannot be calibrated to the individual. Because of the calibration feature of module 1680, it may be possible to calibrate the temperature to work on-wrist within some reasonable margin of error.

Thus, my tentative recommendations would be to

Avoid Casio thermometer watches that cannot be calibrated;
Calibrate your watch off-wrist using an accurate control thermometer and remember the calibration factor; and
Calibrate your watch on wrist under a variety of conditions and check for approximate accuracy.

[If you own a Casio watch but have lost the manual, Casio US will mail you an operation chart if you call them at 973-328-1670.]