Photoresisistor flame scanner

[op9]

Has anyone had practical experience with running long lengths of cable between a photoresistor flame scanner and a burner controller? More specifically a Siemens LAL2.65 and a QRB1 scanner. The documents say it can be run up to 200mtrs with RG62 coax. I have had some other advice not to use it over 20-30mtrs or there will be problems???
RG62 is a low capacitance coax (47.57pF/mtr) and an impedence of 93ohm. Also any of that coax is not robust and has a solid wire as its core.

As impedence and capacitance effect is to do with AC or a changing waveform which is more applicable with say self checking UV signals, I cant see why it would be critical with a simple photoresistor which is just a variable resistance passing a DC current of say 10-35micro amps.

I would like to use Dekoron screened cable which is more robust. Just for comparison a twisted pair version has a capacitance to ground of 141pF/mtr. Again saying is this going to be an issue with DC current? Resistance is not a problem as it is 13.7ohm/km.

Thanks in advance,
Rod

Rod Nissen.
Combustion & Engineering Diagnostics

 

[IRstuff]

After all of that, what is the resistance of the sensor?
The issue with the resistance and capacitance is how long it will take for the detection to propagate down the cable and how much time you have.

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[op9]

When covered and not detecting light the resistance of the sensor would be >1 Mohm. It starts passing a holding current of >4microamps at about 230kohm and at max light intensity a current of about 30microamps at 8kohm.
Rod

Rod Nissen.
Combustion & Engineering Diagnostics

 

[IRstuff]

What's the threshold set for?

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[op9]

Please explain!!!

The controller flame relay holds in at >4 microamps and as I said before saturates at around 30 microamps. On a diminishing light source the flame relay drops out at < 4 microamps. There is no adjustment for this. Crude but it works. KISS principle.

Rod Nissen.
Combustion & Engineering Diagnostics

 

[danw2]

I have no direct experience with long cabling, but I do have a bar stool opinion.

Given the approval process necessary for flame safeguards (from multiple agencies worldwide), I would expect that Siemens' spec for cable distance is useable.

Did you notice that the twisted pair cabling limit was 20-30m?

I suspect that whomever was telling you 'trouble beyond 20-30m' was inferring twisted pair, not coax. Coax is the exception in that kind field wiring and it wouldn't surprise me if most people weren't even aware of the application of coax for this kind of wiring.