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measuring nanoamps with lots of noise

Started by Unknown October 24, 2015
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
On Sat, 24 Oct 2015 06:06:53 -0700, kellysbradbury 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
can you overshoot the temp, and turn off the controller during the test phase and take a reading as you cross back through the desired test temp point?
On Saturday, October 24, 2015 at 9:06:57 AM UTC-4, 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
Hmm, So I would ask how the noise is getting into your measurement. A guess is perhaps noise on the heater. I had issues with capacitive pickup like that on a low temp probe. I stuck a cap multiplier on the heater power and life was good. But maybe some simpler filtering would be enough. How much power is in the heater circuit. (Current and voltage) George H.
On Sat, 24 Oct 2015 06:06:53 -0700 (PDT), 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
How high is the temperature? How fast do you have to make measurements? Might you be able to run the heater from clean filtered DC instead of AC?
On a sunny day (Sat, 24 Oct 2015 06:06:53 -0700 (PDT)) it happened
kellysbradbury@gmail.com wrote in
<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 gener= >ated by a sample at high temperature inside a furnace. The current is in t= >he picoamp-nanoamp range depending on temperature and I have been using a K= >eithley picoammeter. The furnace is controlled by a PID temp controller co= >nnected 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 diffic= >ult to shield the sample without going to exotic materials like platinum fo= >il. Is it possible to measure picoamps-nanoamps in the presence of lots of= > noise by using some kind of instrumentation amplifier or something with ve= >ry good common mode rejection ratio? > >-Kelly
Maybe it is simpler to see where the noise comes from, heater, electronics, power lines to heater, and filter that?
On Saturday, October 24, 2015 at 6:06:57 AM UTC-7, kellysb...@gmail.com wrote:
> I have to measure current generated by a sample at high temperature inside a furnace. > The current is in the picoamp-nanoamp range .... The furnace is controlled by > a PID temp controller connected to a solid state relay.
So, interference is from coupling (probably capacitive) with an electric heater's wiring? There are two things you can do; you can disable your measurement while the current is on. It's even possible, sometimes, to measure during a small part of the AC phase where the interference is, briefly, minimized. Or, you can try to float the meter (or even the furnace) with transformer isolation, from both AC power and ground. This makes any attempts at shielding (guarding) very effective. Instead of removing the current leak path in the furnace, this means removing the ground return path, everywhere.
On Saturday, October 24, 2015 at 9:06:57 AM UTC-4, kellysb...@gmail.com wrote:
> The furnace is controlled by a PID temp controller connected to a solid state relay. > > -Kelly
Check your PID temp controler. I think some have a output for a SSR and also an output for a regular relay. If so you could switch to a regular relay and DC for the heating power. Add an one shot and a and circuit so the temp reading can only occur with the heater either on or off , but not when the relay is changing state. Dan
Use oversampling and averaging

Or perhaps cross corrolated sampling to a reference signal,mains zero crossing? 

Cheers 

Klaus 
On Sat, 24 Oct 2015 06:06:53 -0700, kellysbradbury 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
The wiring outside the furnace should be shielded as well as you can. Maybe some of the noise is coupled in the external wiring / measurement circuit. It takes a lot of trial and error. If the frequency of the noise is high compared to your sample rate, you might be able to add a low pass filter.
In article <19e8992d-8874-40da-a63e-b6925b6a0502@googlegroups.com>, 
kellysbradbury@gmail.com says...
> > 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 can't use a J type couple that can be embedded? Those normally are incased in a 3/16 inch rod to be connected to ground. Jamie