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see the little envelope symbol ? Do you see the little CHAIN symbol in between them ?
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The result is THIS:
I have been looking at thermocouple vacuum gauges like this one or this one
I am hoping to find out whether there is a way to readily interface such a gauge (or any other, if someone knows of one) to an arduino, like with this for instance.
Hi there,
So, I'm building a freeze-dryer and need to find a way to measure the level of vacuum I am at. I have been looking at thermocouple vacuum gauges like this one here or this one here . I am hoping to find out whether there is a way to readily interface such a gauge (or any other, if someone knows of one) to an arduino, like with this https://www.adafruit.com/products/1778 for instance. I'd also be open to alternative suggestions. I've been working on this problem for weeks, but I don't have the electronic know-how to deal with it, so any help is GREATLY appreciated!
Hi Ryan,
This is my first post, but it is something I know a little about... I would look into a pressure transducer rated for PSI-A (pounds per square inch - ABSOLUTE). This is a special type of transducer which is referenced to an absolute vacuum. If you leave a transducer of this type open to the air at sea level it will read apout 14.7 psi. You can get them with varoius outputs. I prefer 4-20 mA for my industrial work, but for arduino a 0-1 volt or 0-10 volt would be easier to measure with the analog input. I would look for a transducer with a range of 0 to 1 atmosphere absolute or 0 to 15 psi-a. This would give you a range of open to maximum vacuum available.
Without a data sheet for either of the thermocouple vacuum gauges, it would be challenging to come up with a useful Arduino interface circuit. The circuit on the belljar.net site is a start, but many bits of information are missing.
If you do come up with a functioning circuit, you will still need some way to calibrate it. Do you have a calibration method in mind?
To be honest with you, I think I can build any circuit with a schematic but have no experience with vacuum
guages so if you can find a schematic for the circuit , I can probably walk you through it.
rdfeil:
I would look into a pressure transducer rated for PSI-A (pounds per square inch - ABSOLUTE).
Robin
Hi Robin, thanks for the reply! This looks great, but I can't seem to find anything like it that doesn't cost hundreds, and right now my cap is about 50 for this sensor. It's a good lead though, I'll add it to the growing list of possible solutions to my dilemma.
jremington:
Without a data sheet for either of the thermocouple vacuum gauges, it would be challenging to come up with a useful Arduino interface circuit. The circuit on the belljar.net site is a start, but many bits of information are missing.
If you do come up with a functioning circuit, you will still need some way to calibrate it. Do you have a calibration method in mind?
Remington,
the best calibration method I can think of which is available to me is to calibrate it to ambient (max) and then to the triple point of water. That particular circuit does seem like it could be helpful (I really am not sure, my electronics chops are rudimentary), but it does not provide a means of interfacing with an arduino. Also, what bits are missing?
The belljar.net article gives some specs for three types of thermocouple vacuum gauges (DV-6M, 531 and 6343) and each of those would work with the circuit given on that page, with some component variations. All of those gauges are available on ebay, with the cheapest being this one (I think).
If you buy that, come back and we can help you get it working. Approximate calibration is possible using the data and information on the belljar.net page.
If you buy a different gauge head, you will need to know the heater current and resistance as well as the thermocouple voltage. Your idea to calibrate against ambient pressure is not a good one, as those gauges are intended to be operated at only rather low pressures. To quote the belljar page:
Many T/C tubes don’t do well when operated at atmospheric pressure. To preserve your tube, don’t apply filament power until you are sure that you are drawing a vacuum in the system. Also, avoid getting contaminants in the tube and position it at a location in the system plumbing where oil cannot back up into it.
jremington:
The belljar.net article gives some specs for three types of thermocouple vacuum gauges (DV-6M, 531 and 6343) and each of those would work with the circuit given on that page, with some component variations. All of those gauges are available on ebay, with the cheapest being this one (I think).
If you buy that, come back and we can help you get it working. Approximate calibration is possible using the data and information on the belljar.net page.
If you buy a different gauge head, you will need to know the heater current and resistance as well as the thermocouple voltage.
I appreciate the help. I actually have some level of comprehension regarding the bell jar example, but where I get lost is how that could be hooked up to arduino. Also, this method seems to require 2 potentiometers to zero in and then manually start up, and my machine is meant to be automated from power-up, so those would be problems to work around, preferably before blowing my sensor budget rather than after :D. btw, anyone know about how I could go about seeing if that adafruit board (or any other) would work as a more plug and play solution?
The cost of these transducers is high. Keep an eye on Ebay and you might find one surplus for pennys on the dollar. Sorry for sending you down a path that is outside the budget. I keep thinking like I am designing for my industrial customers... oops.
where I get lost is how that could be hooked up to arduino
The circuit on the belljar.net page is intended to output 1.0 volts at full scale (highest vacuum). That can easily be measured directly by the Arduino, using the 1.1 V ADC internal reference, or the maximum output can be increased to 5.0 volts by changing the feedback resistor on the op amp.
That does look very intriguing. There is one slight hang-up though, if my assessment is not too far off: the accuracy of that gauge is advertised at 1% of full range (-1,2 Bar gauge pressure) .01*3 Bar=30 mbar minimal accuracy. If that is right, this gauge would not be much good at the needed pressures (10mbar to .1 mbar absolute). Unless I'm mistaken.
jremington:
The circuit on the belljar.net page is intended to output 1.0 volts at full scale (highest vacuum). That can easily be measured directly by the Arduino, using the 1.1 V ADC internal reference, or the maximum output can be increased to 5.0 volts by changing the feedback resistor on the op amp.
Remington,
That's what I thought it meant. I am, however, a bit confused about the mA circle on the left. Surely that doesn't mean there are two meters hooked up to this thing, does it? Also, if I was to go that route, would I have to have this circuit, as well as two carefully programmed digital potentiometers to have it work automatically? I'm not sure I have the programming chops for it, let alone budget XD, I'm really a more mechanical type of guy.
The meter on the left of the figure is required to set the current for the gauge heater. In turn the current is set by a variable resistor, the value of which depends on the voltage of the heater power supply. If you have a regulated heater power supply, a multimeter could be used and after adjustment, probably could be removed from the circuit.
For example, the 6343 gauge head requires 15-18 mA according to the belljar.net page. Unless you can find a proper data sheet, you don't know the resistance of the heater filament at the operating temperature (at typical operating pressures) so the meter in conjunction with the variable resistor is needed to set that value.
In a freeze dry system you could get water vapor into the gauge head and/or the pump, which would shorten their lives considerably. How will you deal with that?
I think there is something wrong but I really don't know. Your calcs make sence, but seem like a lot of error for 1%. Here is another unit from a very reputable manufacturer click. I am not sure what the output is, some Halperns were resistive like a pot. You could call the manufacturer and get design info and accuracy from the horses mouth. That way there would be no guessing. That is what I would do. Another idea is to search the big names and keep an eye on the bay. Some good names are: Ashcroft, Noshok, Rosemount ($$$$), Honeywell, Bourns etc....
The OP is correct in wanting to achieve pressure of less than about 4 millibar. You generally want to be below the triple point of water. See Freeze drying - Wikipedia
Granted, 4 millbar is somewhat above the useful measurement range of a typical vacuum thermocouple gauge.
That same article addresses my earlier question about water vapor getting to the pump and gauge.
Furthermore, a cold condenser chamber and/or condenser plates provide a surface(s) for the water vapour to re-solidify on. This condenser plays no role in keeping the material frozen; rather, it prevents water vapor from reaching the vacuum pump, which could degrade the pump's performance. Condenser temperatures are typically below −50 °C (−60 °F).
jremington:
The OP is correct in wanting to achieve pressure of less than about 4 millibar. You generally want to be below the triple point of water. See Freeze drying - Wikipedia
Granted, 4 millbar is somewhat above the useful measurement range of a typical vacuum thermocouple gauge.
That same article addresses my earlier question about water vapor getting to the pump and gauge.
All in all, a challenging project for a hobbyist!
That's right! gotta be below that triple point, and with some decent margins at that. I'm looking to normally operate around -30F and .1mbar (roughly. I still need a good amount of wiggle room) Thermocouple gauges usually have a range of about 1.333 mbar down to .001333 mbar. As for the vapor, you are quite correct, and that has been accounted for in my first prototype. I have a pump, a vacuum chamber, and a cold trap beneath it powered by dry ice and methanol. I'm looking to upgrade to a hacked refrigerator, which should bottom out my methanol at about -30F. And yes, it has been a very large challenge, but it's been a fun one I've been working on a long time. This last sticking point is almost the only thing holding me back from doing this within budget. Very frustrating
