If you are going to be using this system a lot, I'd second T Chap's suggestion of using solid state relays. I use a toaster oven to dry ore samples with a slow PWM (as has been suggested here several times). I've had two relays die on me over the last few years. While getting over a year's life out of the relay is okay, they tend to fail at just the wrong time. One the failure occurred in the "on" state which ruined the test in progress. A stuck "on" relay wouldn't be good in a lye bath either.
I understand solid state relays have a better life expectancy. The SSR I've been using in our kitchen oven hasn't had the problems my "clicking" relays have had (though the kitchen oven isn't used as often as my drying oven). I'm going to miss the clicking of the drying oven relay but I'm hoping the SSR wont need replacing near as frequently.
I switched to a SSR in our kitchen because the clicking bothered my wife. I personally like the sound of clicking relays.
JonnyMac, you are awesome! Thanks very much. This might be exactly what I need to open up some new possibilities. Much obliged! BTW, your 1-wire code is working great, too. I've been using it for weeks and it's been a beaut.
As mentioned in other threads, I lack fine digital phtographic equipment, so quick photos are impossible. Also as previously mentioned in this thead, I will probably use the MAX31855 to monitor temperature, as per JonnyMac's suggestion. To switch the heating element on and off, I will use a RS3-1D10-51R relay. I really love some of the RS3 series relays, because they are solid state, they work with a direct hook up to an I/O pin and GND, and I have never had a problem with the ones I have used so far. They are definitely costly, but for industrial control they are nice. I have attached the datasheet just in case anyone wants to spend money.
The setup is fairly basic, just the hot plate area of a Mr. Coffee and no more than 5 fluid ounces of liquid. If I eventually create a lot of boards, I would just duplicate the smaller system, therefore I would never have large vats of heated liquid. Large vats of dangerous or heated chemicals in open containers is not a good idea in my view point. I once worked in a paint factory and there was a two vat collision. Take my word for it, when you get soaked with a fairly strong paint thinner, it is not a pleasant feeling. Additionally, the developer for PCBs should either be replenished or replaced on a fairly regular basis.
Through my experiments I have learned that heating and cooling the liquid is a fairly lengthy process, especially with small wattage heating elements. It appears as though the liquid should be heated to a temp of 100 degrees and then shut down the power because as mentioned the temperature will continue to climb. It will take a bit more experimenting, but with a thermocouple, a MAX31855, a RS3-1D10-51R relay, and a Propeller chip, I believe controlling the temperature will be a breeze.
One problem solved and now it is time for this one.
I am looking for a little input involving thermal transfer theory.
Let's say that I have a dish filled with 3 fluid ounces of liquid, I have a hot plate set at a predetermined temperature of 105 degrees fahrenheit, and I have a constant ambient temperature of 70 degrees fahrenheit. By placing the dish upon the hot plate, approximately what temperature could the 3 ounces of liquid possibly reach?
Nope it was not a trick question. I was just thinking that the ambient temperature would effect the bowl temperature and thereby reduce the the effective temperature of the fluid.
To use an electrical analogy, the problem is one of 2 resisters in series.
One end is at 105F and the other end is at 70F with the fluid somewhere between the ends.
The units placed on the resisters is "thermal resistivity", DeltaDegF/W.
Getting the appropriate values is the hard part.
Generally, the thermal resistance from the hot plate, through the dish and into the liquid would be quite low.
Generally, the thermal resistance from the liquid out into the air is quite high.
Therefore, the liquid would stabilize to a temperature that would be close to the 105F.
Where to find the actual values of the resistivity. I don't have a clue.
In reality you answered my question, because I was wondering if I got a surface to a desired temperture, would a container and liquid therein, be raised to the same temperature.
... I was wondering if I got a surface to a desired temperture, would a container and liquid therein, be raised to the same temperature....
Bruce,
I concur that the liquid in the dish will probably end up at 105 F. That's because your temperature difference (105 - 70) is not very high. You might start experiencing a temperature gradient across your dish if the ambient air were very cold, but it's not going to happen at room temperature. One word of caution: if you need the liquid to be right at 105, you had better check on what kind of hysteresis your hot plate has. A clunky hot plate from Walmart probably has a crappy control system with a sizable hysteresis, so just be aware of that potential error. Also, check on where your hot plate measures its temperature. It's possible you could get high local heating just above a heating coil, for example. So you probably want to know what the hot plate control system is actually using as its reference point. Again, a lot depends on how sensitive your process needs to be.
In my former life I used to make fixtures to measure the temperature performance of electronic stuff. I usually attempted to measure to better than 1/10th degree F. The best way to do this is to use a stirred liquid bath.
Basically I used a heavy block of aluminum hollowed out to hold the liquid. I used a nice hot plate supply the heat. A bit of peanut oil under the block greatly reduced the thermal resistance between the hot plate and the block. Add some insulation around and over the top to increase the thermal resistance to the environment.
Hot plates just don't have good enough thermostats. I always used external temperature controllers. Usually with a thermocouple sensor. The thermocouple would go into a hole drilled into the block.
Another technique is to use "Real Glass" wide mouth thermos bottles. (Cheap Dewar flasks.), with vacuum insulation. In this case the heater is a small power resister in the liquid. Drive the resister with a temperature controller. Works very well for small devices.
Now for shear accuracy one should use a Platinum Pt100 temperature meter. Highly stable and easily calibrated to 0.1F or better. Stick this in the liquid for the final reading.
I have started another thread which describes what I am attempting to do and it will contain any experiments that I perform. The main reason I started a new thread is because my solution will not be Propeller specific and it is more of a general type subject. I am currently writing a new post for this thread and it should be updated shortly. The link provided below will definitely provide up to date information about what I intend to do and the progress that I am making.
Please note that I am not attempting to bring this thread to a close, because temperature control is an important subject, however, it would not be appropriate to discuss my current endeavors here. Please feel free to discuss temperature control in either thread, this one or the one linked below, but the one linked below, will ultimately contain the solution to my current endeavors.
Infrared non-contact temperature sensors are notoriously inaccurate at measuring temperature.
The problem is the neasured value is dependent on the emissivity of the thing they are measuring.
They can be used to measure liquid surfaces but need to be pre calibrated for each surface.
Stick with direct temperature measurement sensors to be accurate.
I have come to the conclusion that direct temperature measurement would be a pain for my current setup. So my next question is, if calibrated properly, are the infrared non-contact temperature sensors reasonably reliable?
Comments
I understand solid state relays have a better life expectancy. The SSR I've been using in our kitchen oven hasn't had the problems my "clicking" relays have had (though the kitchen oven isn't used as often as my drying oven). I'm going to miss the clicking of the drying oven relay but I'm hoping the SSR wont need replacing near as frequently.
I switched to a SSR in our kitchen because the clicking bothered my wife. I personally like the sound of clicking relays.
JonnyMac, you are awesome! Thanks very much. This might be exactly what I need to open up some new possibilities. Much obliged! BTW, your 1-wire code is working great, too. I've been using it for weeks and it's been a beaut.
Thanks again.
Thanks for all the replies.
As mentioned in other threads, I lack fine digital phtographic equipment, so quick photos are impossible. Also as previously mentioned in this thead, I will probably use the MAX31855 to monitor temperature, as per JonnyMac's suggestion. To switch the heating element on and off, I will use a RS3-1D10-51R relay. I really love some of the RS3 series relays, because they are solid state, they work with a direct hook up to an I/O pin and GND, and I have never had a problem with the ones I have used so far. They are definitely costly, but for industrial control they are nice. I have attached the datasheet just in case anyone wants to spend money.
The setup is fairly basic, just the hot plate area of a Mr. Coffee and no more than 5 fluid ounces of liquid. If I eventually create a lot of boards, I would just duplicate the smaller system, therefore I would never have large vats of heated liquid. Large vats of dangerous or heated chemicals in open containers is not a good idea in my view point. I once worked in a paint factory and there was a two vat collision. Take my word for it, when you get soaked with a fairly strong paint thinner, it is not a pleasant feeling. Additionally, the developer for PCBs should either be replenished or replaced on a fairly regular basis.
Through my experiments I have learned that heating and cooling the liquid is a fairly lengthy process, especially with small wattage heating elements. It appears as though the liquid should be heated to a temp of 100 degrees and then shut down the power because as mentioned the temperature will continue to climb. It will take a bit more experimenting, but with a thermocouple, a MAX31855, a RS3-1D10-51R relay, and a Propeller chip, I believe controlling the temperature will be a breeze.
Bruce
http://www.parallax.com/tabid/814/Default.aspx
http://forums.parallax.com/archive/index.php/t-92666.html
And look at that, Chris is even using a RS3 series relay in that project, which I happened to mention in this thread
Bruce
Bruce
Now that you mention it, I remember buying some NTE opto-isolators that were definitely not manufactured in house.
Bruce
EDIT: Now I wonder if I can get the same relays for a cheaper price with the Crydom stamp
I am looking for a little input involving thermal transfer theory.
Let's say that I have a dish filled with 3 fluid ounces of liquid, I have a hot plate set at a predetermined temperature of 105 degrees fahrenheit, and I have a constant ambient temperature of 70 degrees fahrenheit. By placing the dish upon the hot plate, approximately what temperature could the 3 ounces of liquid possibly reach?
Bruce
This scenario is a bit of a trick question.
Since the hotplate is, I presume, temperature controlled at 105F the resultant temperature of the liquid in the bowl would also reach 105F.
Duane
Nope it was not a trick question. I was just thinking that the ambient temperature would effect the bowl temperature and thereby reduce the the effective temperature of the fluid.
Thermodynamics was never my strong suit
Thanks for your input.
Bruce
To use an electrical analogy, the problem is one of 2 resisters in series.
One end is at 105F and the other end is at 70F with the fluid somewhere between the ends.
The units placed on the resisters is "thermal resistivity", DeltaDegF/W.
Getting the appropriate values is the hard part.
Generally, the thermal resistance from the hot plate, through the dish and into the liquid would be quite low.
Generally, the thermal resistance from the liquid out into the air is quite high.
Therefore, the liquid would stabilize to a temperature that would be close to the 105F.
Where to find the actual values of the resistivity. I don't have a clue.
Duane
Thanks for the response. After I posted in this thread, I came up with another idea that I am gioing to run with it.
I appreciate your effort.
Bruce
In reality you answered my question, because I was wondering if I got a surface to a desired temperture, would a container and liquid therein, be raised to the same temperature.
Bruce
Thanks again
Bruce,
I concur that the liquid in the dish will probably end up at 105 F. That's because your temperature difference (105 - 70) is not very high. You might start experiencing a temperature gradient across your dish if the ambient air were very cold, but it's not going to happen at room temperature. One word of caution: if you need the liquid to be right at 105, you had better check on what kind of hysteresis your hot plate has. A clunky hot plate from Walmart probably has a crappy control system with a sizable hysteresis, so just be aware of that potential error. Also, check on where your hot plate measures its temperature. It's possible you could get high local heating just above a heating coil, for example. So you probably want to know what the hot plate control system is actually using as its reference point. Again, a lot depends on how sensitive your process needs to be.
My 2 cents.
In my former life I used to make fixtures to measure the temperature performance of electronic stuff. I usually attempted to measure to better than 1/10th degree F. The best way to do this is to use a stirred liquid bath.
Basically I used a heavy block of aluminum hollowed out to hold the liquid. I used a nice hot plate supply the heat. A bit of peanut oil under the block greatly reduced the thermal resistance between the hot plate and the block. Add some insulation around and over the top to increase the thermal resistance to the environment.
Hot plates just don't have good enough thermostats. I always used external temperature controllers. Usually with a thermocouple sensor. The thermocouple would go into a hole drilled into the block.
Another technique is to use "Real Glass" wide mouth thermos bottles. (Cheap Dewar flasks.), with vacuum insulation. In this case the heater is a small power resister in the liquid. Drive the resister with a temperature controller. Works very well for small devices.
My current favorite controller is from Auber Instruments:
Universal 1/32 DIN PID Temperature Controller. $36.95
And there are many others.
Now for shear accuracy one should use a Platinum Pt100 temperature meter. Highly stable and easily calibrated to 0.1F or better. Stick this in the liquid for the final reading.
Duane
I have started another thread which describes what I am attempting to do and it will contain any experiments that I perform. The main reason I started a new thread is because my solution will not be Propeller specific and it is more of a general type subject. I am currently writing a new post for this thread and it should be updated shortly. The link provided below will definitely provide up to date information about what I intend to do and the progress that I am making.
Please note that I am not attempting to bring this thread to a close, because temperature control is an important subject, however, it would not be appropriate to discuss my current endeavors here. Please feel free to discuss temperature control in either thread, this one or the one linked below, but the one linked below, will ultimately contain the solution to my current endeavors.
http://forums.parallax.com/showthread.php?139257-New-Project-New-Experiment-PCB-Etchant-And-Developer-Heaters
Duane, without any doubt, I appreciate your input, but I changed directions from being a Propeller solution.
Bruce
That may just be the ticket for automating my temperature control.
Thanks
Bruce
Infrared non-contact temperature sensors are notoriously inaccurate at measuring temperature.
The problem is the neasured value is dependent on the emissivity of the thing they are measuring.
They can be used to measure liquid surfaces but need to be pre calibrated for each surface.
Stick with direct temperature measurement sensors to be accurate.
Duane J
I have come to the conclusion that direct temperature measurement would be a pain for my current setup. So my next question is, if calibrated properly, are the infrared non-contact temperature sensors reasonably reliable?
Bruce