MicroVoltage fuse (Now "Sensing the Flame-Front of a Plasma")

[davidwinth]

I have not had any luck finding a fuse that will burn out fast at voltages below a volt. Is there no such thing? Thanks!

 

[phinds]

I have not had any luck finding a fuse that will burn out fast at voltages below a volt. Is there no such thing? Thanks!

Fuses don't operate on voltages so your question makes no sense.

 

[Tom.G]

The voltage rating of a fuse is the upper limit it is designed to handle. Fuses are used to limit the current in a circuit. Here is one source for fuses, from a low of 0.002Amp (2mA), up to 6000Amps. You can probably find one close to what you are looking for here.
https://www.fusesunlimited.com/data-sheets

 

[davidwinth]

O.k., thanks for the input. I didn't see a fuse that acts fast enough and at the low voltage/current I need, so I will try another tack.

I am using the circuit shown below to detect the arrival times of flames as they travel along a flat surface where a combustible liquid is poured. The gaps are above the surface, such that when the flames pass over the gap, the gap is filled with the ions associated with flames. The ions are conductive, so that the DAQ registers a sudden spike in voltage at the flame arrival. This works for detecting the flames just fine, as I get a sudden spike up to 1-2 Volts detected by the DAQ when the flames arrive. I put some diodes in line (the diamonds) so as to prevent back-signals, but that hasn't helped much if at all.

There is a problem though. Since all the gaps share a common ground, I get the signal from one gap interfering with the signal from another gap. I have 8 gaps and 8 slots on the DAQ, along with a common ground. So my idea here was to put a fuse on each gap, such that once the gap triggered it would open the circuit on that gap and no more signal would come from that one. But the flames move fast, on the order of 100m/s and I don't think any of those fuses will burn fast enough.

Does anyone have any idea how I could modify this system to prevent the signals from mixing? Thanks!

 

[Tom.G]

Insufficient data.

1. What is the power source?
2. What is its frequency response?
3. What is the input impedance of the DAQ?
4. How long are the wires?
5. What wire gauge?
6. Are the wires in a cable or individual conductors?
7. Are the wires individually shielded (metallic shield)?

A photo would help.

 

[Averagesupernova]

I would say you are having cross-talk between the wiring. Due to the nature of the sensors (very high impedance) this is the likeliest cause in my opinion. Can you tell more about your setup?

 

[davidwinth]

1. What is the power source?
2. What is its frequency response?
3. What is the input impedance of the DAQ?
4. How long are the wires?
5. What wire gauge?
6. Are the wires in a cable or individual conductors?
7. Are the wires individually shielded (metallic shield)?

I am using four 12-volt batteries in series.
I have no idea what the frequency response of a battery is.
I am using a NI USB 6008 DAQ, and the manual does not list the impedance. It is a "Voltage input" device.
The wires are 8 feet or less, 16 gauge, single strand.
The wires are single - no cable.
The wires are not shielded. They are only insulated.

Thanks for your response.

 

[davidwinth]

I would say you are having cross-talk between the wiring. Due to the nature of the sensors (very high impedance) this is the likeliest cause in my opinion. Can you tell more about your setup?

Is the cross talk because the signals all share a common ground? I was thinking perhaps of putting them each on their own circuit, but then I would need a DAQ and battery setup for every gap and that seems ridiculous! See my reply to the previous poster. Perhaps that will be enough information.

Thank you for your reply.

 

[Tom.G]

I agree there is crosstalk, my questions were aimed at finding where it was occurring. Here are a couple easy things to try.

1. Change the spacing between input leads to see if the interference level changes. If the interference increases when the leads are bundled near to each other and decreases when they are widely seperated, they are talking to each other.
   If true, Step 2, else Step 3
   
2. You don't state the geometry or size of the "gaps". It will increase the signal voltage if you decrease the size of the gaps and increase the area of the opposing surfaces of each gap. This might be sufficient but may increase the crosstalk.
   • 2a. Add a 47kΩ resistor on each DAQ input channel to Ground.
   • 2b. Use shielded wire for the input leads.
3. If the battery is something small like coin cells, use bigger cells, at least "C" cells. The Gold Standard would be a Lead-Acid rechargable. (probably overkill for this!)

There could still be other problems at this point, if so post some photos of the setup and we'll try again.

In step 2a the 47kΩ resistors will decrease the signal by about a factor of four. If the crosstalk is from stray capacitive coupling between the input leads this should show a greater than four times reduction. That's assuming the capacitive coupling is less than the capacitance of the gaps. Of course you could try it on one or two channels to see if it helps.

p.s. The data sheet for the DAQ lists Input Impedance as 144kΩ
(http://www.ni.com/pdf/manuals/375295c.pdf)

 

[jim hardy]

I am using four 12-volt batteries in series.

You are overranging your analog inputs. I hope they're not ruined.

your crosstalk isn't in the wiring it's in your multiplexer
applying voltage greater than your multiplexer's analog inputs can hold off let's it leak into the other inputs

From user manual linked by @Tom.G (@ doesn't work with Chrome, either)

add voltage dividers to bring voltage within range of your analog inputs

EDIT OOPS! Make that all EIGHT places !

 

[Tom.G]

your crosstalk isn't in the wiring it's in your multiplexer
applying voltage greater than your multiplexer's analog inputs can hold off let's it leak into the other inputs

This works for detecting the flames just fine, as I get a sudden spike up to 1-2 Volts detected by the DAQ when the flames arrive.

 

[Averagesupernova]

I have used a hi-pot tester set for several thousand volts in series with a DVM with 10 Megohms input impedance and whatever I wanted to check leakage on. In one case it was a batch of relays. Solder flux would impregnate into the plastic as the PC board went through the wave solder process. The DVM would show zero volts until you breathed on the relay. Then it would read up to several volts indicating leakage on the relay. I would say the same situation is going on here except with 48 volts. This is how the OP is sensing flame. The data acquisition system and the gap between the conductors that sense the flame form a series circuit and until there is a short circuit or excessive leakage across the probes, the data acquisition system should be fine. Yes, a chance is being taken but I don't think this is what is causing the trouble the OP is seeing. It is capacitive leakage on the wires.

 

[jim hardy]

applying voltage greater than your multiplexer's analog inputs can hold off let's it leak into the other inputs

http://www.analog.com/media/en/training-seminars/tutorials/MT-088.pdf

Manufacturers learned to build multiplexers that'll hold off voltages greater than their supply voltage for industrial applications.

Example -
when your plant computer loses power and its multiplexer's internal supplies go to zero,
suddenly the multiplexer becomes an electrical "sponge" drawing current from whatever circuit it is supposed to measure . That loading can change indication.
Plant Operators become rightly unhappy when their board indicators all sag because somebody switched off the computer. Further there's a General Design Criterion for nuke plants that says 'Reactor Control and Protection shall be designed to not give confusing indications'.
RTP and others make multiplexers that sport "high power off impedance " and a prudent engineer will specify them where his computer is connected in parallel with plant equipment.

https://www.maximintegrated.com/en/app-notes/index.mvp/id/5299

Fault-Protected Switches for Overvoltage Protection to ±36V
The supply-voltage rails for an analog switch restrict the allowed input-signal voltage range. (Refer to the Designs for Signal Handling section above.) If the input signal exceeds the supply-voltage rails, the device can latch up or be permanently damaged. Normally this restriction is not a problem, but in some cases, the input signal might be present while the supply voltage to the analog switch is turned off. (This can happen if the system supply-voltage sequencing causes the input signal to be present before the supply-voltage rail comes up.) Transients outside the normal range of the power supply can also cause latchup or permanent damage. New fault-protected switches and multiplexers guarantee overvoltage protection of up to ±36V and power-down protection of ±40V, along with rail-to-rail signal handling and the low RON of a normal switch. The input pin, moreover, assumes high impedance during fault conditions, regardless of the switch state or load resistance, and only nanoamperes of leakage current can flow from the source.

My plant's computer front end used mercury wetted relays that fail open on power loss.

four twelve volt batteries in series , 48 volts, is too much for that multiplexer. Why does it report 1 to 2 volts instead of 48 ?

EDIT Oops that 'gap' looks like a contact, confused my thinking... What voltage does a meter report ? If you're exceeding 10 volts that multiplexer is overranged.
i'd expect it to malfunction by 15 .old jim

 

[jim hardy]

In light of my mistaking your "gap" for a relay contact,

it would be prudent to measure just how much voltage gets applied to that multiplexer through the gap. Is it enough to upset the multiplexer?
A voltmeter would take care of that question, what is the voltage measured by meter ?
Do your results change when you use only one 12 volt battery ? 12 volts shouldn't perturb multiplexer.

Secondly - flames flicker. You may need a software solution to detect initial flame front and ignore that channel afterwards.

just thinking out loud ,
and admitting my mistakes.

old jim

 

[davidwinth]

Thank you for the responses - everyone. I sincerely appreciate your time and input, as this problem has bugged me for a while.

I never see a voltage of more than 4 volts at the very most when the flames fill the gap. Usually it would be half that. I found that I had to use a higher voltage than just one battery because otherwise the signal was too low to detect in noise.

The gaps are about 2 mm in size and each gap pair is several inches apart. I polish the copper of the gaps to maximize conductivity.

For this: 2a. Add a 47kΩ resistor on each DAQ input channel to Ground.

I am not an electrical guy, obviously. Is this saying put a resister between each channel straight to ground, as in the yellow line in the first channel below? Won't that sort of short out the circuit? Are the diodes I put inline with the signal inputs doing anything? My hope was that they would increase the slope of the voltage spike and thereby shorten the time it has to interfere.

As far as shielded cables, does this mean I should get a bunch of single strand wires with metal shielding and simply replace the wires I already have one by one, or would it be enough to get a bundled cable like found here and use this one thick cable for all the wiring I have?

 

[Tom.G]

Change the spacing between input leads to see if the interference level changes. If the interference increases when the leads are bundled near to each other and decreases when they are widely seperated, they are talking to each other.
If true, Step 2, else Step 3

The reason this was the first step is to determine _if_ there is electrical crosstalk between the channels due to the wiring. The applicability of any "fixes" is decided by the "cause" of the interference.

For this: 2a. Add a 47kΩ resistor on each DAQ input channel to Ground.
I am not an electrical guy, obviously. Is this saying put a resister between each channel straight to ground, as in the yellow line in the first channel below? Won't that sort of short out the circuit?

Yes; by a factor of four. If the present signal is 2V then the added resistor will reduce that to 0.5V. Depending on the details of your gaps and of the wiring, this may reduce the interference by a factor greater than four.

Are the diodes I put inline with the signal inputs doing anything?

They are reducing your peak input voltage by 0.3V to 0.7V.

My hope was that they would increase the slope of the voltage spike and thereby shorten the time it has to interfere.

Sorry, they won't help with that.

As far as shielded cables, does this mean I should get a bunch of single strand wires with metal shielding and simply replace the wires I already have one by one

Yes. Although you probably want single conductor, Stranded, Shielded wire. Stranded wire has several smaller bare wires twisted together and then the insulation is applied over the group. The advantage is they are more flexible and more tolerant of repeated flexing without breaking. Single strand wires (usually known as "Solid"), are a little cheaper and used where they will not be subject to flexing, as in the power wiring in your house.

or would it be enough to get a bundled cable like found here and use this one thick cable for all the wiring I have?

No, that cable configuration is good for blocking outside interference (it can't get thru the shield) but is worse for interference between the individual wires because they are so close together. A rather poor analogy is if you are talking with someone right next to you, you can easily hear each other; if you are 50 feet apart the sound level is a lot lower.

An additional thought: It could be that the interference you are seeing is caused by convection between adjacent sensing gaps. Can you put a wall between adjacent gaps and see if that helps?

We are about to the point where photos of the physical setup, and of the signal waveshapes you are seeing, are needed so we have more information to work with. Additional data points would be the waveshape obtained when the DAQ is programmed to read only one channel, two adjacent channels, and two distant channels. Specifically reprogram the DAQ for these readings as the minimum time between samples is determined by the number of channels scanned.

 

[Averagesupernova]

I would try running two conductor cable to each sensor using the second wire as a drain. The positive side of the battery hooks to the sensors the way it already is, the opposite side of the sensors hook to one wire in the two conductor pair, and the second conductor is left open at the sensor end but hooked to the negative terminal of the battery at the data acquisition end. This should help considerably with crosstalk. Plain old zipcord is cheap, you could try that. Or you could just use two conductor running the positive to each sensor individually.
-
If you have some three conductor cable lying around you could try using that running the positive from the battery to each sensor individually along with the drain as I described.
-
There are several options for you to think about.

 

[davidwinth]

Yes; by a factor of four. If the present signal is 2V then the added resistor will reduce that to 0.5V. Depending on the details of your gaps and of the wiring, this may reduce the interference by a factor greater than four.

Yes. Although you probably want single conductor, Stranded, Shielded wire. Stranded wire has several smaller bare wires twisted together and then the insulation is applied over the group. The advantage is they are more flexible and more tolerant of repeated flexing without breaking. Single strand wires (usually known as "Solid"), are a little cheaper and used where they will not be subject to flexing, as in the power wiring in your house.

We are about to the point where photos of the physical setup, and of the signal waveshapes you are seeing, are needed so we have more information to work with. Additional data points would be the waveshape obtained when the DAQ is programmed to read only one channel, two adjacent channels, and two distant channels. Specifically reprogram the DAQ for these readings as the minimum time between samples is determined by the number of channels scanned.

Thanks again. Below is a sample signal, with the interference I am talking about. This setup was a little different that the one I described earlier. Due to the sample frequency limitations you are aware of, I actually hooked the input to the DAQ from every third line to the same channel. So that channel 1 gets the voltage spike from sensors 1,4,7,10 - and channel 2 gets the voltage spike from sensors 2,5,8 - and channel 3 gets input from sensors 3,6,9. That is how I prefer to do it so I only have to divide the sample capacity of the DAQ by 3 instead of by 8 (or use two DAQs).

Anyway, as you can see from the signal, when the first sensor is triggered, the signal lingers and doesn't drop off very quickly. I would like to fix that. But the main problem is that after the first signal, subsequent signals from one channel show voltage on the other channels and this makes it harder to tell which one is actually being triggered. I have placed dots on the curves indicating the flame triggering that particular channel.

 

[davidwinth]

I would try running two conductor cable to each sensor using the second wire as a drain. The positive side of the battery hooks to the sensors the way it already is, the opposite side of the sensors hook to one wire in the two conductor pair, and the second conductor is left open at the sensor end but hooked to the negative terminal of the battery at the data acquisition end. This should help considerably with crosstalk. Plain old zipcord is cheap, you could try that. Or you could just use two conductor running the positive to each sensor individually.
-
If you have some three conductor cable lying around you could try using that running the positive from the battery to each sensor individually along with the drain as I described.
-
There are several options for you to think about.

I very much appreciate you taking the time to write some suggestions. Would each conductor pair still need to be shielded? Would I still benefit from shorting each channel input to ground with a resistor as suggested earlier, or is this a completely separate suggestion?

Thanks

 

[Averagesupernova]

I very much appreciate you taking the time to write some suggestions. Would each conductor pair still need to be shielded? Would I still benefit from shorting each channel input to ground with a resistor as suggested earlier, or is this a completely separate suggestion?

Thanks

Do it exactly as I described. What is referred to as a drain is actually a type of shield. You can try the resistor but I am afraid it will cause enough loading to attenuate the signal so much that it is unreadable. Do you know about zener diodes? How you could use one to limit the voltage on your data acquisition inputs to a safe-for-the-daq level? This would protect the daq without loading the sensors.

 

[Tom.G]

Nice graph, and sneaky idea about combining signals on fewer inputs. That gives a sample rate of 3333Hz, or 300uS per channel.

Insertion:

• Try disconnecting the signal wires from one DAQ channel and see if there is still signal interference from the disconnected one. Disconnect the wires at the DAQ and otherwise leave them in about the same position. If there is still interference, also disconnect the unused wires from the gaps. This will help locate any crosstalk between the wires.

End Insertion

The waveforms could be due to overloading the DAQ inputs. Could you post a run using a 12V battery instead of 48V and, please include the time axis on the graph this time? Also a zoom of the leading edge of the first pulse would be useful so we can see the risetime from baseline to peak, and hopefully see how many samples are on the rising edge.

The trailing edge slope could be real, could be the input capacitance of the DAQ retaining a charge, or overloading the DAQ input. Try adding a resistor on the first channel from input to ground. Any value you might have between 33k and 150k will suffice to show the long fall time is real or an artifact. If the falling edge has the same slope with the added resistor as without it, that indicates the probability of a real signal, i.e. combustion products are lingering around in the gaps. Or it might be easier to supply some forced air movement (fan) as an easy fix to clear the gaps.

At this point I suspect the big spike in the Red channel is a real signal, i.e. flash ignition (explosion) of fumes at that sensor.

 

[davidwinth]

I have some old data with a lower voltage. I will have to dig around for it. In the mean time, here is another image with the data points included and the time labels. I must have been using another DAQ to collect this which has a higher sample rate. Thanks again.

 

[Tom.G]

After a close look and a wee bit of analysis, that looks like real data to me, without any electrical artifacts. Overall, the long trailing edge on the first pulse is not DAQ input capacitance retaining a charge, the shape of the curve is wrong for that. If this was taken with diodes in the input circuit, that would confuse the interpretation though. If this was taken with diodes, please post a waveform without diodes.

It looks like the overall experiment lasts about 10mS. Is that correct?

I'm assuming the flame-front is traveling in the direction Green, Red, Blue (Series 3, 2, 1), that seems to fit the traces shown. Is that correct? The fact that the Red pulse occurs after the second Green pulse also argues against electrical crosstalk.

I believe the next step is to modify the experimental setup by trying to physically isolate the sensors from reactions in other regions. Two possible approaches come to mind.

• Use a fan, aimed perpendicular to the line of sensors, to clear combustion products and decrease interactions.
• Place a barrier between sensor pairs to avoid combustion product mixing and interaction. The barrier of course would have to be far enough above the fuel to have minimal (acceptable) impact on the flame-front but close enough to reduce combustion product spreading to other sensors.

Please post a photo of the experimental setup as it would avoid us having to play 20-questions to understand the overall situation.
Include approximate dimensional information.

 

[davidwinth]

Yes, the order is correct. I am confused by the green rising before the red too (and this trend continues, if you look at the larger image) . I can't think of a possible physical explanation for that. That's why I assumed it was something electronic. Even though I'm an admitted newbie at electronics, I know there are no flames there, so I'm going to have to say that it must something to do with the electronics. It's too big of a coincidence to say that some combustion products just happened to stimulate the first sensor at almost the same time the flame front reaches the second sensor.

I can't use a fan in the experiment because the whole point is to measure the velocity that the flames travel at.Is my idea for a quick acting fuse starting to sound more reasonable? ...frustrating.

 

[CWatters]

Can you try putting fuel on sensor 2 _only_ to see if you still get a spike on 1?

 

[Tom.G]

Can you try putting fuel on sensor 2 _only_ to see if you still get a spike on 1?

If you still get the interference on channel 1, then try it with channel 1 disconnected at the DAQ. This will isolate things enough to see if the problem is in the DAQ.

If the chnl. 1 interference disappears with input #1 disconnected from the DAQ, reconnect the #1 input, disconnect the wire at the #1 sensor gap and move it out of the way. Then repeat the test with fuel only at #2.

 

[Tom.G]

This is what I call a "DUH! - of course" moment.
While falling asleep last night, I realized an easy test for electrical crosstalk is to disconnect the battery from one or a few sensor gaps and see if the crosstalk still occurs. If the crosstalk disappears from the un-powered sensors then the signals are real and the problem lies somewhere in the mechanical configuration of the flame/sensor hardware. If the crosstalk is still there, then electrical interference is the problem.

 

[davidwinth]

This is what I call a "DUH! - of course" moment.
While falling asleep last night, I realized an easy test for electrical crosstalk is to disconnect the battery from one or a few sensor gaps and see if the crosstalk still occurs. If the crosstalk disappears from the un-powered sensors then the signals are real and the problem lies somewhere in the mechanical configuration of the flame/sensor hardware. If the crosstalk is still there, then electrical interference is the problem.

I unconnected all but the gaps in the first line from the battery, but left them in place other than that. So to be clear, I disconnected the battery from gaps 2,3,5,6 and 8 such that only gaps 1,4 and 7 had power (Series 1 in the plot below). The funny thing is, this signal looks about as good as I have seen, even better than some results when all the gaps had battery power! So according to your last post, I am definitely seeing cross-talk? Why are the non-powered gaps showing signals just as big as the powered??

I think it is clear I need to order some sheilded wire, but will that be enough? I also did a run with this configuration but with a 100k resister between the powered line and the DAQ ground. The voltage was cut down quite a bit as you said, but not much else looked different.

 

[Rive]

Something is fishy. It does not seems like a wire-crosstalk for me. More like some resonant or reflecting behavior associated with the flame front.

I would try this with only one gap present, and then again with only two gaps, but different placement.

 

[Tom.G]

1. Please post a photo of the experiment. I may be able to do a partial verification of your results.
2. What is the material used for the gaps?
3. What is the fuel you are using?
4. What results if the first gap is disconnected from the battery?
5. What results if all of the battery-side electrodes are completely disconnected from the battery and each other?

I understand that your purpose is to investigate the flame-front movement, however please try a run with a fan blowing across the flame bed. This will help with my current speculation that the plasma from the flame is spreading to other gaps before the actual flame does. Yeah, sounds weird! But please try it.

with a 100k resister between the powered line and the DAQ ground. The voltage was cut down quite a bit as you said, but not much else looked different.

"Powered line" is a little ambiguous. Is the resistor placed from a DAQ signal input to Ground? If that is the case and the trailing edge slope of the signal didn't change, then capacitive charging of the DAQ inputs is NOT the problem. (One down, how many more to go??)

p.s. Looks like the flame hung around the first sensor for awhile.

p.p.s. These flame-front evaluations are usually done with a high speed video camera. I'm beginning to see why.

 

[jim hardy]

I found a manual at http://www.ni.com/pdf/manuals/371303n.pdf

By GND i trust you mean one of these GND pins on the analog side, and NOT the earthing pin of a wall receptacle..

I've been unable to find how one tells that front end software whether it is to expect your inputs to be wired single ended or differential.

 

[davidwinth]

I found a manual at http://www.ni.com/pdf/manuals/371303n.pdf

I've been unable to find how one tells that front end software whether it is to expect your inputs to be wired single ended or differential.

View attachment 222883

Yeah, I have never understood that part of the device either, Jim Hardy.

Tom G. Looking at the results from yesterday's tests with clear eyes this morning, it seems the resistor did make the slope on the backside drop off faster. The image is below. As a reminder, this is with the 2nd and 3rd line disconnected from the battery, so that only the first line is powered (gaps 1,4,7). And a 100k Ohm resister is run from line 1 channel input to DAQ ground. I only have one of those resistors, or I would have tried it with all three lines.

I want to go get several resistors to try out this morning because the signal is too low with 100k Ohm. Would a larger or smaller resistor make the signal bigger? If I try with a 50K Ohm resistor, will that make the signal bigger or smaller than the 100k Ohm? Also, would the shielded cables help this part too? I would like to order them but they are expensive and I will hold off until I get someone saying it should help. Unfortunately, my boss will not let me post pictures of the experimental setup. I apologize for that. The gaps are made from Aluminum wires, 20 gauge.

 

[Tom.G]

@davidwinth If the material of the flame bed is conductive, connect it to the DAQ Ground. Also what is the material?

I've been unable to find how one tells that front end software whether it is to expect your inputs to be wired single ended or differential.

That is in the "Interactive Control Panel" software that comes with the hardware. See: http://www.ni.com/example/54386/en/

http://www.ni.com/cms/images/devzone/tut/Interactive_Control_Panel_2.JPG

 

[davidwinth]

Tom G., so it is on the software side that the signal is interpreted as individual or differential mode?

 

[Tom.G]

Tom G., so it is on the software side that the signal is interpreted as individual or differential mode?

Not necessarily (and probably not). The software likely reconfigures the hardware for single-ended or differential operation.

I want to go get several resistors to try out this morning because the signal is too low with 100k Ohm. Would a larger or smaller resistor make the signal bigger? If I try with a 50K Ohm resistor, will that make the signal bigger or smaller than the 100k Ohm?

In this usage, the smaller the resistor the smaller the signal, but the faster the trailing edge. Another possibility is to add an amplifying stage between the sensor gap and the DAQ input. That way you can adjust the resistor value at the amplifier input for best (compromised) results and make up the signal loss with the amplifier. For testing you could even use an Audio amplifier with a resistor across the input, then feed the DAQ with the amplifier output. The amplifier must be able to handle an input signal up to a few volts, so an amp with a "Line" input would be best. A starting value for the input resistor could be 15k Ohm, or get a 50k or 100k pot (potentiometer, variable resistor). If you can't beg/borrow/steal an amp with a Line input, an input resistor around 4.7k would be a value to start with.

Also, would the shielded cables help this part too? I would like to order them but they are expensive and I will hold off until I get someone saying it should help.

Probably not worth it at this point. If they would not be exposed to much heat, say less than 140F, they are pretty cheap, around $15 per 100Ft. You can find it as Microphone cable, Guitar cable, or just Shielded Cable with a Google search. If you are in the USA you could even use coax cable as used for cable TV. Home Depot has it for around the above price, it is pretty hard to work with though. Again, some of the Home Depot stores also carry shielded cable.

Bummer on the photos but I understand. How about the material of the flame bed? And the fuel could be helpful too.

Technical stuff for those lurking here:
The overall project is trying to characterize flame-front movement using spark gap style sensors feeding a DAQ. No spark occurs, the gaps are powered by 48VDC and the flame plasma provides a current path to the DAQ. The plasma impedance is in the megohm range and the DAQ input resistance is 144k with the capacitance unspecified. This yields a current source of 7uA to 70uA with flame presence and just the DAQ input circuit otherwise. The stated problems are long fall time of the signal and crosstalk between signals. Many details of the setup are unavailable to us so we are playing 20 questions and 'try this and report.'
End Technical stuff