<kellysbradbury@gmail.com> wrote in message news:19e8992d-8874-40da-a63e-b6925b6a0502@googlegroups.com... I am having a difficult problem on a current project so I was wondering if anyone could help. This is a project where I have to measure current generated by a sample at high temperature inside a furnace. The current is in the picoamp-nanoamp range depending on temperature and I have been using a Keithley picoammeter. The furnace is controlled by a PID temp controller connected to a solid state relay. When the relay is off I can get data with no issues but when the relay is on there is lots of noise and I cannot get good data. Unfortunately, the high temperature environment makes it difficult to shield the sample without going to exotic materials like platinum foil. Is it possible to measure picoamps-nanoamps in the presence of lots of noise by using some kind of instrumentation amplifier or something with very good common mode rejection ratio? -Kelly You've received some excellent suggestions re reducing the source of the noise. Assuming you can do nothing about that aspect, the answer to your final question is yes. Back in the late '70's I had to design the production test equipment for Burr Browns then new OPA111 (1pAmax Ib) The production environment was very noisey and little could be done about reducing it. The solution was to force the Ib current onto a capacitor and then measure dV/dT. The result was repeatability/accuracy down in the fA's. The tradeoff was that dT sometimes had to be in the 100mS range. Art
measuring nanoamps with lots of noise
Started by ●October 24, 2015
Reply by ●October 25, 20152015-10-25
Reply by ●October 25, 20152015-10-25
On Sat, 24 Oct 2015 23:39:31 -0800, "Artemus" <bogus@invalid.org> wrote:> ><kellysbradbury@gmail.com> wrote in message >news:19e8992d-8874-40da-a63e-b6925b6a0502@googlegroups.com... >I am having a difficult problem on a current project so I was wondering if anyone could help. This >is a project where I have to measure current generated by a sample at high temperature inside a >furnace. The current is in the picoamp-nanoamp range depending on temperature and I have been using >a Keithley picoammeter. The furnace is controlled by a PID temp controller connected to a solid >state relay. When the relay is off I can get data with no issues but when the relay is on there is >lots of noise and I cannot get good data. Unfortunately, the high temperature environment makes it >difficult to shield the sample without going to exotic materials like platinum foil. Is it possible >to measure picoamps-nanoamps in the presence of lots of noise by using some kind of instrumentation >amplifier or something with very good common mode rejection ratio? > >-Kelly > >You've received some excellent suggestions re reducing the source of the noise. Assuming you can do >nothing about that aspect, the answer to your final question is yes. >Back in the late '70's I had to design the production test equipment for Burr Browns then new >OPA111 (1pAmax Ib) The production environment was very noisey and little could be done about >reducing it. The solution was to force the Ib current onto a capacitor and then measure dV/dT. The >result was repeatability/accuracy down in the fA's. The tradeoff was that dT sometimes had to be in >the 100mS range. >Art > >If one is not in a hurry, you can connect a capacitor across a weak current source for a while, and then measure the voltage. A relay can do the switching. Common-mode rejection can be almost perfect, and you can measure femtoamps. Sort of a flying-capacitor integrator.
Reply by ●October 25, 20152015-10-25
On Saturday, October 24, 2015 at 7:06:57 AM UTC-6, kellysb...@gmail.com wrote:> I am having a difficult problem on a current project so I was wondering if anyone could help. This is a project where I have to measure current generated by a sample at high temperature inside a furnace. The current is in the picoamp-nanoamp range depending on temperature and I have been using a Keithley picoammeter. The furnace is controlled by a PID temp controller connected to a solid state relay. When the relay is off I can get data with no issues but when the relay is on there is lots of noise and I cannot get good data. Unfortunately, the high temperature environment makes it difficult to shield the sample without going to exotic materials like platinum foil. Is it possible to measure picoamps-nanoamps in the presence of lots of noise by using some kind of instrumentation amplifier or something with very good common mode rejection ratio? > > -KellyThanks everyone for your responses! To answer some of your questions: I am going to about 1050 C in an oxidizing atmosphere so that rules out many traditional materials. Basically I have to use platinum wires to connect to the sample. Nobody that I know makes a high temp coax that goes to 1050 C which is why I have virtually no shielding. I don't have to make fast measurements, in fact 1 sample measurement per second would be fine. I have tried ramping to a certain temperature and then unplugging the furnace. All the noise goes away and I can get continuous data with no problem. But that is not a good long term solution since I will need to record data over multiple ramp/dwell/cool cycles which is why I am looking for a better solution. Someone suggested an isolation transformer but that might be cost prohibitive and same goes for a DC power supply. My furnace runs off 120 V at 14 amps. However, I did find a line filter at the local surplus electronics store but I haven't tried it yet. I like the idea of integrating a capacitor. Texas Instruments makes the IVC102 which does this but I am sure there are other solutions as well. Has anyone used this IC? Are there other IC's or solutions that someone could recommend? I will be honest, my electronics skills are amateur but I have breadboarded a few things in my time. Thanks in advance! I really appreciate all your responses. Kelly
Reply by ●October 25, 20152015-10-25
In article <4bb463f6-2db4-4f25-a40d-35344b269f27@googlegroups.com>, <kellysbradbury@gmail.com> wrote:> On Saturday, October 24, 2015 at 7:06:57 AM UTC-6, kellysb...@gmail.com wrote: > > I am having a difficult problem on a current project so I was wondering if > > anyone could help. This is a project where I have to measure current > > generated by a sample at high temperature inside a furnace. The current is > > in the picoamp-nanoamp range depending on temperature and I have been using > > a Keithley picoammeter. The furnace is controlled by a PID temp controller > > connected to a solid state relay�[SSR]. When the relay is off I can get data > > with no issues but when the relay is on there is lots of noise and I cannot > > get good data. Unfortunately, the high temperature environment makes it > > difficult to shield the sample without going to exotic materials like > > platinum foil. Is it possible to measure picoamps-nanoamps in the presence > > of lots of noise by using some kind of instrumentation amplifier or > > something with very good common mode rejection ratio? > > > > -Kelly > > > Thanks everyone for your responses! To answer some of your questions: > > I am going to about 1050 C in an oxidizing atmosphere so that rules out many > traditional materials. Basically I have to use platinum wires to connect to > the sample. Nobody that I know makes a high temp coax that goes to 1050 C > which is why I have virtually no shielding.One can make one's own coax, from platinum tube, platinum wire, and furnace cement (or powdered aluminum oxide, or a few beads on the wire to keep wire away from tube). Need to use a mineral insulation that is still insulating enough at 1050 C. What is the impedance of the sample, at temperature? What is the physical mechanism of current generation?> I don't have to make fast measurements, in fact 1 sample measurement per > second would be fine.Having a measurement gate exactly one second long allows profound suppression of prime power frequencies (50 Hz or 60 Hz) and all harmonics.> I have tried ramping to a certain temperature and then unplugging the > furnace. All the noise goes away and I can get continuous data with no > problem. But that is not a good long term solution since I will need to > record data over multiple ramp/dwell/cool cycles which is why I am looking > for a better solution. > > Someone suggested an isolation transformer but that might be cost prohibitive > and same goes for a DC power supply. My furnace runs off 120 V at 14 amps. > However, I did find a line filter at the local surplus electronics store but > I haven't tried it yet.It is not hard to arrange 14 amps at 110 volts at DC. All it takes is a 25-amp fullwave rectifier bridge and some filter caps. This will not be expensive, and it is well within the reach of an amateur to build such a thing. What are the heating elements made of? At high temperatures, some solid materials are subject to electrolysis if powered with DC. The furnace maker will know.> I like the idea of integrating a capacitor. Texas Instruments makes the > IVC102 which does this but I am sure there are other solutions as well. Has > anyone used this IC? Are there other IC's or solutions that someone could > recommend? I will be honest, my electronics skills are amateur but I have > breadboarded a few things in my time.It requires considerable skill to build good femtoamp electronics, so it may be better to figure out how to use the Keithley picoammeter. Hmm. What is the model number? If it is digital, it should be able to implement measurement windows that are an exact multiple of the period of the power line frequency. This is called NPLS (Number of Power Line Cycles) in the DMM world.> Thanks in advance! I really appreciate all your responses.This furnace controller - is it zero-voltage switching? That will sharply reduce noise. One can buy such controllers off the shelf. Try Omega.com. Although one would expect the SSR to be zero-voltage switching. The input filter may be the answer. And ensure that the shall of the furnace is well grounded. If one replaces the SSR with a mechanical switch, do you get noise when the switch is closed? Joe Gwinn
Reply by ●October 25, 20152015-10-25
On Saturday, October 24, 2015 at 7:06:57 AM UTC-6, kellysb...@gmail.com wrote: I am going to about 1050 C in an oxidizing atmosphere so that rules out many traditional materials. Basically I have to use platinum wires to connect to the sample. Nobody that I know makes a high temp coax that goes to 1050 C which is why I have virtually no shielding. =========================================================================== This might not be useful here, but you could make your own shielded coax with some platinum tubing, platinum wire, and ceramic beads that barely fit into the tube loosely strung along the wire. It would only have to be long enough to get through the oven wall and then for the temperature from conduction to get down to what something like 316L stainless steel could handle so you could switch to SS wire and tubing to then get far enough away to switch to regular coax, or save the extra connections and just make the platinum section long enough so that the cold end is cold enough to connect to regular coax. Once you get outside the oven you will get plenty of cooling by convection, so my guess is that you would only need a few inches of the platinum coax outside the oven so the cost would not be that outrageous. Since you are working at such low frequency it doesn't matter what the characteristic impedance is, you just have to find tubing and beads that fit together off the shelf. You could make a test version from all 316SS that would probably survive for a little while at maybe 700C, especially with an inert atmosphere like nitrogen or argon, so you could see how much good it will do under pretty realistic heater power conditions before committing to the platinum. ----- Regards, Carl Ijames
Reply by ●October 25, 20152015-10-25
On Sun, 25 Oct 2015 10:46:35 -0700 (PDT), kellysbradbury@gmail.com wrote:>On Saturday, October 24, 2015 at 7:06:57 AM UTC-6, kellysb...@gmail.com wrote: >> I am having a difficult problem on a current project so I was wondering if anyone could help. This is a project where I have to measure current generated by a sample at high temperature inside a furnace. The current is in the picoamp-nanoamp range depending on temperature and I have been using a Keithley picoammeter. The furnace is controlled by a PID temp controller connected to a solid state relay. When the relay is off I can get data with no issues but when the relay is on there is lots of noise and I cannot get good data. Unfortunately, the high temperature environment makes it difficult to shield the sample without going to exotic materials like platinum foil. Is it possible to measure picoamps-nanoamps in the presence of lots of noise by using some kind of instrumentation amplifier or something with very good common mode rejection ratio? >> >> -Kelly > > >Thanks everyone for your responses! To answer some of your questions: > >I am going to about 1050 C in an oxidizing atmosphere so that rules out many traditional materials. Basically I have to use platinum wires to connect to the sample. Nobody that I know makes a high temp coax that goes to 1050 C which is why I have virtually no shielding. > >I don't have to make fast measurements, in fact 1 sample measurement per second would be fine. > >I have tried ramping to a certain temperature and then unplugging the furnace. All the noise goes away and I can get continuous data with no problem. But that is not a good long term solution since I will need to record data over multiple ramp/dwell/cool cycles which is why I am looking for a better solution. > >Someone suggested an isolation transformer but that might be cost prohibitive and same goes for a DC power supply. My furnace runs off 120 V at 14 amps. However, I did find a line filter at the local surplus electronics store but I haven't tried it yet. > >I like the idea of integrating a capacitor. Texas Instruments makes the IVC102 which does this but I am sure there are other solutions as well. Has anyone used this IC? Are there other IC's or solutions that someone could recommend? I will be honest, my electronics skills are amateur but I have breadboarded a few things in my time. > >Thanks in advance! I really appreciate all your responses. > >KellyYou could use a small DPDT relay and a capacitor as a flying-cap integrator. Connect the cap to the input leads for one second; then switch it to a DVM and measure the voltage for 100 ms or whatever; switch back to integrate some more. Eventually the cap will need to be discharged, which can be every measurement or just occasionally. Nuclear particle detectors sometimes work this way. Low currents are integrated into a cap, which avoids the Johnson noise of resistors. Various things, like photofets, are used to occasionally discharge the cap.
Reply by ●October 25, 20152015-10-25
In article <4bb463f6-2db4-4f25-a40d-35344b269f27@googlegroups.com>, kellysbradbury@gmail.com says...> > On Saturday, October 24, 2015 at 7:06:57 AM UTC-6, kellysb...@gmail.com wrote: > > I am having a difficult problem on a current project so I was wondering if anyone could help. This is a project where I have to measure current generated by a sample at high temperature inside a furnace. The current is in the picoamp-nanoamp range depending on temperature and I have been using a Keithley picoammeter. The furnace is controlled by a PID temp controller connected to a solid state relay. When the relay is off I can get data with no issues but when therelay is on there is lots of noise and I cannot get good data. Unfortunately, the high temperature environment makes it difficult to shield the sample without going to exotic materials like platinum foil. Is it possible to measure picoamps-nanoamps in the presence of lots of noise by using some kind of instrumentation amplifier or something with very good common mode rejection ratio?> > > > -Kelly > > > Thanks everyone for your responses! To answer some of your questions: > > I am going to about 1050 C in an oxidizing atmosphere so that rules out many traditional materials. Basically I have to use platinum wires to connect to the sample. Nobody that I know makes a high temp coax that goes to 1050 C which is why I have virtually no shielding. > > I don't have to make fast measurements, in fact 1 sample measurement per second would be fine. > > I have tried ramping to a certain temperature and then unplugging the furnace. All the noise goes away and I can get continuous data with no problem. But that is not a good long term solution since I will need to record data over multiple ramp/dwell/cool cycles which is why I am looking for a better solution. > > Someone suggested an isolation transformer but that might be cost prohibitive and same goes for a DC power supply. My furnace runs off 120 V at 14 amps. However, I did find a line filter at the local surplus electronics store but I haven't tried it yet. > > I like the idea of integrating a capacitor. Texas Instruments makes the IVC102 which does this but I am sure there are other solutions as well. Has anyone used this IC? Are there other IC's or solutions that someone could recommend? I will be honest, my electronics skills are amateur but I have breadboarded a few things in my time. > > Thanks in advance! I really appreciate all your responses. > > KellyType K couple works in oxidizing environments and is good up to 1350C Also you have IR. Jamie
Reply by ●October 25, 20152015-10-25
On 26/10/15 08:30, John Larkin wrote:> On Sun, 25 Oct 2015 10:46:35 -0700 (PDT), kellysbradbury@gmail.com > wrote: > >> On Saturday, October 24, 2015 at 7:06:57 AM UTC-6, kellysb...@gmail.com wrote: >>> I am having a difficult problem on a current project so I was wondering if anyone could help. This is a project where I have to measure current generated by a sample at high temperature inside a furnace. The current is in the picoamp-nanoamp range depending on temperature and I have been using a Keithley picoammeter. The furnace is controlled by a PID temp controller connected to a solid state relay. When the relay is off I can get data with no issues but when the relay is on there is lots of noise and I cannot get good data. Unfortunately, the high temperature environment makes it difficult to shield the sample without going to exotic materials like platinum foil. Is it possible to measure picoamps-nanoamps in the presence of lots of noise by using some kind of instrumentation amplifier or something with very good common mode rejection ratio? >>> >>> -Kelly >> >> >> Thanks everyone for your responses! To answer some of your questions: >> >> I am going to about 1050 C in an oxidizing atmosphere so that rules out many traditional materials. Basically I have to use platinum wires to connect to the sample. Nobody that I know makes a high temp coax that goes to 1050 C which is why I have virtually no shielding. >> >> I don't have to make fast measurements, in fact 1 sample measurement per second would be fine. >> >> I have tried ramping to a certain temperature and then unplugging the furnace. All the noise goes away and I can get continuous data with no problem. But that is not a good long term solution since I will need to record data over multiple ramp/dwell/cool cycles which is why I am looking for a better solution. >> >> Someone suggested an isolation transformer but that might be cost prohibitive and same goes for a DC power supply. My furnace runs off 120 V at 14 amps. However, I did find a line filter at the local surplus electronics store but I haven't tried it yet. >> >> I like the idea of integrating a capacitor. Texas Instruments makes the IVC102 which does this but I am sure there are other solutions as well. Has anyone used this IC? Are there other IC's or solutions that someone could recommend? I will be honest, my electronics skills are amateur but I have breadboarded a few things in my time. >> >> Thanks in advance! I really appreciate all your responses. >> >> Kelly > > You could use a small DPDT relay and a capacitor as a flying-cap > integrator. Connect the cap to the input leads for one second; then > switch it to a DVM and measure the voltage for 100 ms or whatever;What about timing jitter in the relay?
Reply by ●October 25, 20152015-10-25
On 26/10/2015 04:46, kellysbradbury@gmail.com wrote:> On Saturday, October 24, 2015 at 7:06:57 AM UTC-6, kellysb...@gmail.com wrote: >> I am having a difficult problem on a current project so I was wondering if anyone could help. This is a project where I have to measure current generated by a sample at high temperature inside a furnace. The current is in the picoamp-nanoamp range depending on temperature and I have been using a Keithley picoammeter. The furnace is controlled by a PID temp controller connected to a solid state relay. When the relay is off I can get data with no issues but when the relay is on there is lots of noise and I cannot get good data. Unfortunately, the high temperature environment makes it difficult to shield the sample without going to exotic materials like platinum foil. Is it possible to measure picoamps-nanoamps in the presence of lots of noise by using some kind of instrumentation amplifier or something with very good common mode rejection ratio? >> >> -Kelly > > > Thanks everyone for your responses! To answer some of your questions: > > I am going to about 1050 C in an oxidizing atmosphere so that rules out many traditional materials. Basically I have to use platinum wires to connect to the sample. Nobody that I know makes a high temp coax that goes to 1050 C which is why I have virtually no shielding. > > I don't have to make fast measurements, in fact 1 sample measurement per second would be fine. > > I have tried ramping to a certain temperature and then unplugging the furnace. All the noise goes away and I can get continuous data with no problem. But that is not a good long term solution since I will need to record data over multiple ramp/dwell/cool cycles which is why I am looking for a better solution. > > Someone suggested an isolation transformer but that might be cost prohibitive and same goes for a DC power supply. My furnace runs off 120 V at 14 amps. However, I did find a line filter at the local surplus electronics store but I haven't tried it yet. > > I like the idea of integrating a capacitor. Texas Instruments makes the IVC102 which does this but I am sure there are other solutions as well. Has anyone used this IC? Are there other IC's or solutions that someone could recommend? I will be honest, my electronics skills are amateur but I have breadboarded a few things in my time. > > Thanks in advance! I really appreciate all your responses. > > Kelly >Why not run the heater from a commercial PID temperature controller which uses pulse-width modulation to turn the heater on and off (e.g. once a second), and program the maximum duty cycle to (e.g.) 90%, meaning there is guaranteed to be a 100ms off-time every second. You can do the measurement during the off-time. It might be challenging to disconnect the heater thoroughly enough to get a clean reading, without using a mechanical relay (which will eventually wear out and may get stuck-on when that happens). Either you would need to find two solid state relays (one for each end of the heater) that can turn off well enough to give good measurements, or you might find a hybrid solution that works ok: Put one solid-state relay that turns on and off the power to the heater, and then in series with that, right before the heater, put an additional double-pole mechanical changeover relay that only ever switches when the solid-state relay is already off. This double pole mechanical relay would disconnect both ends of the heater from the mains and optionally would also connect the heater to ground (perhaps via a 100k resistor, just in case one side of the relay fails to switch). The relay should last a very long time because it would not be switching any current as the SSR would be off when it switches. You could achieve the correct timing using for example some logic gates and monostables or 555-timers etc. that are processing the output PWM signal from the commercial PID temperature controller. This would also go to a monostable that drives the trigger input of the meter used for measurements during the off-time. Chris
Reply by ●October 25, 20152015-10-25
On 10/24/2015 8:06 AM, kellysbradbury@gmail.com wrote:> I am having a difficult problem on a current project so I was > wondering if anyone could help. This is a project where I have to > measure current generated by a sample at high temperature inside a > furnace. The current is in the picoamp-nanoamp range depending on > temperature and I have been using a Keithley picoammeter. The > furnace is controlled by a PID temp controller connected to a solid > state relay. When the relay is off I can get data with no issues but > when the relay is on there is lots of noise and I cannot get good > data. Unfortunately, the high temperature environment makes it > difficult to shield the sample without going to exotic materials like > platinum foil. Is it possible to measure picoamps-nanoamps in the > presence of lots of noise by using some kind of instrumentation > amplifier or something with very good common mode rejection ratio? > > -Kelly >Have you tried twisting your power wires together? Tried twisting the sensing wires together?
