My application is pressurized boilers, such as those used in small "prosumer" espresso machines (typically <600mL), so the response time required would have to be pretty fast. While I agree that the 6mm probe would be far too slow, it gives me some hope that I will be able to find a IC based temperature sensor (which addresses most of my off the shelf accuracy needs without calibration) that can be potted into a thinner probe.

I was originally looking at thermistors for their fast response time, however if I develop a precision temperature controller based on thermistors, I would need to calibrate every unit I build.

I can see where you will have some problems. Doing what you want to do is relatively simple, the biggest problem is making it inexpensive. The problem I see with IC based solutions is whichever of them you choose it needs to be placed in a probe with a compression fitting to mount in the tank. So while the IC solutions are cheap once in a stainless steel probe they become slow responding and expensive.

A Google of "thermistor probes" will bring up dozens of hits, none of which are really inexpensive. Again, thermistors are cheap right till we embed a tiny fast responding one in a probe, then they become expensive.

Finally you get to thermocouples which are relatively inexpensive and can be placed in a small diameter stainless sheath or probe with a compression fitting and used in a tank. They will have a pretty fast response, not sure how fast without looking it up.

You can do a Google of "Thermowells" and see about rolling your own. That comes down to buying blank tubes and stuffing them with whatever you choose and potting them. Use a stainless tube and use compression fittings to mount in the tanks.

Ron

 

My application is pressurized boilers...

For what it's worth, if you have steam and water in equilibrium, the pressure of the steam is directly related to temperature. Maybe you could just use a pressure sensor.

 

I like thermistors, and for what you're trying to do I'm with team lookup table. The temperatures of interest add up to 130 values in a table, easy enough to type in. BUT, getting 0.1F repeatability is hard, and really hard up at the high temp values where the thermistor response curve slope is so low. You'll need a ratiometric input plus a very accurate and stable excitation voltage.

ak

 

For what it's worth, if you have steam and water in equilibrium, the pressure of the steam is directly related to temperature. Maybe you could just use a pressure sensor.

Given the temperature range listed, I believe this system is running with no head space, below boiling, with other equipment (check valves, opv, etc.) keeping pressure controlled, but with no relation to temperature. In a steam boiler you'd be right, but I think this is different (and perhaps not actually a "boiler")

 

I'm beginning to question the reasoning behind the precision level required. It's very hard to achieve uniform temperature in any real system that has thermal mass and inflows and outflows of heat and/or mass. If you can measure precisely and quickly, you'll see streams of hotter and cooler water going past the sensor. The results of imperfect mixing will cause "noise". You can integrate over time to get an average, but there is no guarantee that the probe is seeing the bulk average. It could be located where it sees a "cold" stream more often, for instance.

My point is, the precision and accuracy of the measurement may not be dictated by the probe and electronics, it may be a result of the system itself.