sensortemperature
I'm making a Sous Vide using Arduino with a DS18B20 temperature sensor. My problem is that the DS18B20 reacts too slowly to temperature changes. For example, if you take it from ice water (1°C) to boiling water (100°C), DS18B20 takes a full minute to reach 95°C. Worse, the temperature show asymptotic convergence; near final temp, DS18B20 approach at like 7sec/°C.
So, what is the fastest temperature sensor available today (and cheap) ?
Diodes are not as good as diode-connected transistors in this kind of multi-current \$\Delta V_{BE}\$ temperature measurement circuit. I note that the part number you specify is actually a BJT, not a diode.
Recall the ideality factor that is close to 1 for a diode-connected transistor and more like 2 for a diode.
The manufacturer also has detailed recommendations for the type of transistor to be used, and recommends that large power transistors not be used.
You could try a small power transistor such as a BD159STU (That one is in the convenient TO-126 through-hole package), or simply make a small PCB with the SMT part on one side and no tracks or vias on the other, and attach that to the panel (that's probably what I'd do- it could be connectorized, and made as a snap-off bit attached to some other board).
Simple algebra. Plug one of them into the other and simplify:
mv = code * 5000/1024
temp = (mv - 500) / 10
temp*10 = mv - 500
temp*10 = code * 5000/1024 - 500
temp*10 = (code * 5000) / 1024 - 500
(add divisor/2 before performing an integer division for rounding)
temp*10 = (code * 5000 + 512) / 1024 - 500
temp*10 = ((code * 5000 + 512) >> 10) - 500
One multiply, one shift right by 10, one addition, and one subtraction. Then convert to BCD and display it.
However, if you have an 8-bit ADC, you're probably going to want to divide by 256 instead of 1024 (shift right by 8 instead of 10) and add 128 instead of 512.
And here is an algorithm for converting binary to BCD: http://www.eng.utah.edu/~nmcdonal/Tutorials/BCDTutorial/BCDConversion.html
Just run that, split into groups of 4 bits, and drive each digit in the display with one group (after decoding to the proper 7 segment signals, of course).
Best Answer
1 minute to reach 95% is incredibly slow. Something must be wrong with your sensor. It's just not possible for a TO-92 alone to have such a slow response.
In this Texas Instruments Datasheet for an LM34 (or LM35) in a TO92 package, they give a graph of thermal response in a stirred oil bath, which should be quite similar to your situation:
So in a liquid it will reach 95% in less than ten seconds.
Perhaps you're using a waterproof DS18B20, and it's not thermally connected to the walls of the stainless steel tube. The shiny tube, and thin layer of air, will form quite a good insulator.
They say
So the problem isn't the sensor itself, it's the packaging, which is something you have to solve no matter what sensor you use. The DS18B20 is a great little sensor, mainly because it is reasonably accurately calibrated at the factory. Any more analogue sensor - thermocouples and thermistors, will need special care to calibrate to better than a couple of degrees C.
Try this again with a bare sensor, in a heatshrink tube or a blob of epoxy, or take it apart and fill the tube with oil or glue to improve the thermal conduction to the device.