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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
IamSmooth wrote ...
They seem nice. I don't want to take on a secondary project. Would I be able to simply put an Ohm meter across the leads, measure the resistance and just compare it to a chart to get the temperature? This would give me a solution that only costs a few dollars.
That's basically the solution I settled for in the end with the thermistors. I'm using ammeters, and a fixed voltage, so basically just measuring the resistance (Ohm's law), but I don't need great accuracy. What accuracy do you require?
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I don't need great accuracy. I just want to know if I'm getting closer to really cold temperatures within a few degrees. To me, there is not much difference between -190C and -191C. Both feel cold.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Well, the datasheet for those RTD's (which I assume are basically thermistors) is a bit confusing, but seems to say 0.1% from -40 to 125 C, and 2% from -75 to 540 C, so its anybodies guess how accurate they are at -200 C (2% would be 4 degrees C, but the error seems likely to be much greater). Datasheet here:
I don't know if better figures are available elsewhere.
I suspect that type 'K' or type 'T' TC's will be a lot more accurate, as there is plenty of documentation available for them. All you'd need would be a DMM (or digital voltmeter) and thermometer at the reference junction for accurate results.
In my project I wanted to use the four moving coil ammeters, with FSD of ~200uA, and internal resistance of 750 Ohms, without great accuracy, at ~100 C, so it made sense to use thermistors, rather than have four amplifier chips to to give sufficient power to use the meters I'd chosen.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
The full datasheet ( linked in my previous post ) has a chart of accuracy vs temperature and resistance vs temperature on page 86.
To use them you just use a normal DMM/panel meter/etc which is set to read resistance. A cheap dmm which only reads to the nearest ohm will work fine, even at -200c the resistance changes about 5ohms per degree.
@ash - RTDs are a little different than thermistors, a RTD is just a coil of wire (platinum in these devices), the resistance of which changes more or less linearly with temperature. A thermistor is a semiconductor device which has a roughly exponential temperature dependence (either positive or negative depending on the doping)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
... wrote ...
The full datasheet ( linked in my previous post ) has a chart of accuracy vs temperature and resistance vs temperature on page 86.
To use them you just use a normal DMM/panel meter/etc which is set to read resistance. A cheap dmm which only reads to the nearest ohm will work fine, even at -200c the resistance changes about 5ohms per degree.
@ash - RTDs are a little different than thermistors, a RTD is just a coil of wire (platinum in these devices), the resistance of which changes more or less linearly with temperature. A thermistor is a semiconductor device which has a roughly exponential temperature dependence (either positive or negative depending on the doping)
Sorry, I missed the link to the full data sheet. It still says 2% at -200, which equates to 4 degrees C, as I interpret it.
I believe these are thin film, rather than 'a coil of wire', but I get the gist.
NIST calibration would give greater accuracy, but at what cost, etc?
(I always find these datasheets hard to follow, they never seem to say things in plain English.)
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
IamSmooth wrote ...
They seem nice. I don't want to take on a secondary project. Would I be able to simply put an Ohm meter across the leads, measure the resistance and just compare it to a chart to get the temperature? This would give me a solution that only costs a few dollars.
Sounds good, but you need an ohmeter that is accurate and, more importantly, stable minute-to-minute and month-to-month. It might be handy to have a good, stable, low-tempco metal resistor with an ohms value close to your RTD operating point. Let me know if you want some R's measured with four or five digit traceable accuracy. Or buy some spec'd much, much tighter than 1%.
For temperature calibration, think about reference temperatures that you can access (from time to time) inexpensively. The closer to actual usage, the better. Freezing point of water Freezing point of mercury Evaporating point of dry ice Boiling point of nitrogen, corrected for ambient pressure
I skipped BP of propane, because the stuff sold as propane is usually LPG and includes heavier hydrocarbons. BP of Dust-Off or air conditioner refrigerants might be useful, depending on the specified purity.
Perhaps "..." or a datasheet can address the sensitivity of bare RTD's or probes to thermal shock, and their chemical compatibility with various cold fluids.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
@ash they are specified to 2% linearity, however as long as you take into account the higher order terms in the expression given they are specified to "±0.3°C or 0.6% of temperature, °C (R0 ±0.1% trim), whichever is greater", which is what is shown in the chart in the upper right corner of the datasheet.
Things are starting to get a bit off topic, if people want to talk more about getting sub degree accuracy temperature measurements it might be time for a new thread. There is a lot more to it than just buying a sensor that is specified as such! Agilent has a pretty good app note for temperature measurement which covers most of the effects you need to consider to get to within a few tens of millikelvin. To get degree accuracy out of a sensor like this does not require going to such great lengths, especially because it is a 1000ohm device instead of a 100ohm device, so the lead resistance and self heating effects are reduced by an order of magnitude.
@klugesmith using boiling nitrogen or sublimating dry ice, the sensor should be able to handle being dunked in LN2, etc, although I have never tried with a thin film RTD. We usually have them mounted on a block of copper along with the device that needs to be cooled. These sensors are encased in a block of ceramic with teflon leads, so it should be compatible with just about anything.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Update:
I am getting close to firing up my unit. I just have to lathe a teflon mounting plug for my dewar container. I have the pump, I've coiled my stainless tubing, I have a dewar, needle valve and a digital thermometer capable of reading temperatures under -320F. I also have coaxial tubing that I will try to use to regenerate the cold gas. I am hoping the layer of air between the two tubes will afford some degree of insulation.
I have also constructed a filter to extract water, CO2 and hydrocarbons.
I constructed a blast shield out of 1/2" thick lexan. I want to test a piece on the gun range and see what happens when I fire a 9mm at it.
Hopefully, in a few days I will see if I get liquid nitrogen and oxygen. Once I have something I will document the process along with sources for purchasing the components for anyone else that wants to do this.
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Condesation/Fractional distillation of air
