Hi - I think the sensing speed is affected by both:

• how well the thermistor is in contace with the water
• how much thermally conductive material is in contact with the thermistor

By the latter I mean that if the thermistor has a heat sink of any kind, it will have thermal inertia and will not change temperature quickly. So it may not be the sensor being slow but rather the mass it's attached to changing temperature slowly because it has too high a heat capacity.

Does that make sense? What was your setup?

Warren, Agreed. The thermister changes temperature when it is in your mouth, or submerged in water, faster than in air, because water is a better thermal conductor than air. The point here is that under the best circumstances, the thermister is not a very fast sensor, and some applications may need to show changes in temperature more quickly. For example, if a Thermal Bob is being towed across a body of water, it may have a speed of 10 meters per minute, or one meter every six seconds. Since it takes 20 seconds to show a thirty degree change in temp, then in the six seconds that it takes to move the sensor one meter, you would miss most of the temperature change if the bob were to pass in front of a hot water vent.

In a sense these sensors all have zero lag. As soon as the the materials that make up the sensor reach ambient temperature, the sensor will tell you what that temperature is. Sensors made from conductive materials will reach ambient temperature faster, and sensors made from very little material will reach ambient faster. So the smallest sensor might be the quickest, especially if it's made of metal. But any submersible sensor will have some thermal mass, so towing might have to be slower than you would like.

Another approach could be to increase the transfer of heat by using a high surface area like the heat dissipating fins on a computer's CPU:

You could attach the thermistor to a small one of those and the water would run over it, cooling or heating it quickly.

The thermister in use has a Thermal time constant <= 15 seconds, which is fine for some applications. However, to get a faster measurement, a different sensor is recommended, like a DS18B20 Digital temperature sensor , for which Query time is less than 750ms. That's about 20 times faster, and it's still under $10.