Then the temperature will be at the mercy of the liquid.
What are you trying to predict or design?
It's perhaps not a bad way to make a level switch. You should be able to predict the steady-state temperature with and without the liquid being present for a given current or series resistor (given a constant and (let's say) equal temperature for the liquid and air). Then play with that single variable (the current or resistance) and see what limits you run afoul of, and then you can play with variations in ambient temperature.
Best regards, Spehro Pefhany
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"it\'s the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Can anyone point me to a GOOD ref for running a thermistor in self heating mode? How do you figure out how much power you need to be putting through it??
All the instrumentation texts describe this, but not at a real practical level. I think I'm sneaking up on the current from the low side, but I want to avoid smoking these things.
Are you talking about a PTC or NTC thermistor? PTCs typically have a snap characteristic, almost like a thermostat, where their resistance increases about 3 orders of magnitude at some temperature between about
40C and 300C. That makes them useful as heaters as long as that's the temperature you want, but they're not very settable, their accuracy is quite poor, and the repeated thermal shock tends to be very hard on the leads, if any. On the plus side, apart from the thermal shock issue, they're almost completely insensitive to supply voltage variations. That makes them good for stuff like keeping condensation off windows.
NTCs have a resistance that decreases about 3-4% per kelvin, which makes them more settable but much more of a puzzle to drive. Constant-current drive will always be stable, because an increase in temperature will always result in a decrease in dissipation--P=I**2 R, so dP/dT = I**2 R**2 dR/dT, which is always negative. On the other hand, constant voltage drive will be unstable above some current--P=V**2/R and dP/dT = -V**2/R**2 dR/dT, which is positive. Heat loss Q-dot tends to be proportional to delta-T, as does the dissipation in the thermistor, so when you crank up the voltage past the point where dP/dT + d(Q-dot)/dT = 0, it'll run away.
Soooo, if I really had to build a temperature control loop with an NTC thermistor in a self-heating mode, I'd start with a constant-current drive and put a very slow P-I feedback loop around it. Figuring out what the set point should be is left as an exercise for the reader. ;)
Good control results when P = Q-dot and dP/dT is large compared with the thermal conductance G_th=d(Q-dot)/dT. (At short times, you also have to contend with the fact that G_th isn't frequency-independent and that the thermistor itself has some thermal mass.) You can't achieve both of those things at the same time with a barefoot NTC unless your insulation is really really amazing--the temperature coefficient is just too low.
I like running in self heating a PTC, well I guess it does not matter. I like them in water baths. When the water runs dry, it auto shuts down. You can get an excellant self heating differential by dipping in water and comparing results with the air.
Spehro Pefhany wrote in news: snipped-for-privacy@4ax.com:
At the moment, just trying to understand thermistor self-heating through experience. No urgent practical use, but lets just call it measuring liquid flow at constant temp. It's not going to be in a control loop. I know its not the only, or best way to do it, but I'd like to understand the method.
I have a feeling I'll just end up finding out why its not particularly easy or reliable, but I'd like to know.
I have this in my "things to do file when I am smarter".
I attended a seminar at the EAA Convention in Oshkosh in 1982(?) where there was a presentation by a scientist from Langley AFB who built an aircraft autopilot based on the principal. Two thermistors were in a bridge with an air jet (tiny) that drove an RC servo on a trim tab on one aileron. There was a professor from Ohio State University (Walt(?)) who had it working on his Thorpe.
Apparently it worked very well but Gyro's won out in that application.
John Ferrell W8CCW Beware of the dopeler effect (pronounced dope-ler). That's where bad ideas seem good if they come at you fast enough.
Jim Thompson wrote in news: snipped-for-privacy@4ax.com:
Thanks. I'm trying to put together a classroom demo or lab exercise for a sensors and instrumentation course. I had a nice thermodilution lab, but a self heating setup just never came to be, so the class never saw one beyond reading about it.
Jim Thompson wrote in news: snipped-for-privacy@4ax.com:
Thermodilution is the method currently used to measure cardiac output, which isn't necessarily easy to do. A catheter is inserted into the right side of the heart, and a balloon-like thing is used to float a thermistor into the pulmonary artery. A cold water bolus is injected into the heart, and the integral of the temp change is proportional to flow
My thesis was based on two thermistors on the end of a catheter, one with current flow to heat it, one simply to measure blood temperature. This was back in the days of Germanium transistors, so I built my own chopper-stabilized amplifier ;-)
...Jim Thompson
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| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
I was already married, had a typewriter, but decided the PDP-8 was ideal for generating a correctable copy, and I could spit out an additional copy at will. (I think I still have the punched paper tape around here somewhere ;-)
...Jim Thompson
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| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
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