My rebreather "isolates" the Primary from the Secondary using 2k ohm 1/8 watt resistors inline with the shared O2 cells.
Is this a sensible arrangement?
if so, would it make same to use an inline resistor on the positive wire of the O2 cell to isolate the OSTC O2 board from the Primary and Secondary?
Resistors for O2 board
-
heinrichsweikamp
- Posts: 4464
- Joined: 13 May 2007, 18:07
Re: Resistors for O2 board
Hello,
O2 cells are a current source and need a load resistor (typically 10kOhm) shunt to work. Personally, I would use independent O2 cells for the board and the original eCCR controller/monitor system.
regards,
Matthias
O2 cells are a current source and need a load resistor (typically 10kOhm) shunt to work. Personally, I would use independent O2 cells for the board and the original eCCR controller/monitor system.
regards,
Matthias
-
Anonymous User
Re: Resistors for O2 board
heinrichsweikamp Wrote:
-------------------------------------------------------
> Hello,
>
> O2 cells are a current source and need a load
> resistor (typically 10kOhm) shunt to work.
> Personally, I would use independent O2 cells for
> the board and the original eCCR controller/monitor
> system.
>
> regards,
> Matthias
Where possible it would be best to use independent O2 cells, but this not always possible.
Where it is not possible, the question is how could isolation be provided.
The resistor I am referring to is in addition to the load resistor and it is placed inline with the positive wire of the O2 cell (not across the positive and the negative like the load resistor).
-------------------------------------------------------
> Hello,
>
> O2 cells are a current source and need a load
> resistor (typically 10kOhm) shunt to work.
> Personally, I would use independent O2 cells for
> the board and the original eCCR controller/monitor
> system.
>
> regards,
> Matthias
Where possible it would be best to use independent O2 cells, but this not always possible.
Where it is not possible, the question is how could isolation be provided.
The resistor I am referring to is in addition to the load resistor and it is placed inline with the positive wire of the O2 cell (not across the positive and the negative like the load resistor).
-
heinrichsweikamp
- Posts: 4464
- Joined: 13 May 2007, 18:07
Re: Resistors for O2 board
Hello,
The series resistor together will result in a voltage divider making the readings false...
OSTC user andersnasman has included the board with existing sensors: http://www.heinrichsweikamp.com/read.ph ... 99#msg-899
regards,
Matthias
The series resistor together will result in a voltage divider making the readings false...
OSTC user andersnasman has included the board with existing sensors: http://www.heinrichsweikamp.com/read.ph ... 99#msg-899
regards,
Matthias
-
Anonymous User
Re: Resistors for O2 board
Hi!
Can you explain with a numerical example how the series resistors would make false the reading?
I tested the mvolt in air pre and post resistor and the difference made by the resistor was only 0.01 mvolt.
Maybe in O2 this would be larger, but would it be 4.78 * 0.01 = 0.05 mvolt?
Would not these differences be negligible?
Can you explain with a numerical example how the series resistors would make false the reading?
I tested the mvolt in air pre and post resistor and the difference made by the resistor was only 0.01 mvolt.
Maybe in O2 this would be larger, but would it be 4.78 * 0.01 = 0.05 mvolt?
Would not these differences be negligible?
-
heinrichsweikamp
- Posts: 4464
- Joined: 13 May 2007, 18:07
Re: Resistors for O2 board
It make a difference because there are 10kOhm load resistors on the board. If you remove them, like andersnasman did, series resistors are not a problem. With worst case scenarios in mind (short circuit at the board), a series resistor will not avoid false readings at the main-monitor because the current source in the cell will not see the correct load resistance.
regards,
Matthias
regards,
Matthias
-
Anonymous User
Re: Resistors for O2 board
All O2 cell boards including the OSTC have a load pull down resistor across the + and - of the O2 cell (as required by the O2 cell manufacturer).
The OSTC has 10 kohm. Another make of dive computer has 200 kohm. Shorting the motherboard of the dive computers (or the O2 boards), would negate this load, but this would be negated only for the specific channel reading the specific cell.
So, let us assume for a moment that Cell 2 channel is shorted. Hypothesis:
1. This should not affect the reading of Cell 1 and Cell 3 (in either Primary, Secondary, or OSTC) as Cell 1 and Cell 3 have their own independent load resistor and cicuitry. Is this correct?
2. Cell 2 reqadings would be affected as the OSTC motherboard in our assumption has shorted and is therefore non-functional. If only the load resistor of the OSTC had shorted, the same would apply. However, the question is, would the Cell 2 readings in the Primary and Secondary be affected where Primary and Secondary each have their own load resistor in each of the channells?
With repsect to 2, in Air (I will try in O2, but I do not have it handy), I shorted the channel 2 + and - of the wire leading to the tertiary (non-Primary and non-Secondary) electronics, and this has no meaningful effect on their mvolt or pPO2 readings.
So, I do not thibnk that it is true that "the current source in the cell will not see the correct load resistance" (other than of course for the channel on the specific electronic which has suffered the short), because all the other channells each have their own load resistor.
Losing one load resistor in one channel of one electronic board doe snot seem to meaningfully affect the others provided they have their own load resistor.
The OSTC has 10 kohm. Another make of dive computer has 200 kohm. Shorting the motherboard of the dive computers (or the O2 boards), would negate this load, but this would be negated only for the specific channel reading the specific cell.
So, let us assume for a moment that Cell 2 channel is shorted. Hypothesis:
1. This should not affect the reading of Cell 1 and Cell 3 (in either Primary, Secondary, or OSTC) as Cell 1 and Cell 3 have their own independent load resistor and cicuitry. Is this correct?
2. Cell 2 reqadings would be affected as the OSTC motherboard in our assumption has shorted and is therefore non-functional. If only the load resistor of the OSTC had shorted, the same would apply. However, the question is, would the Cell 2 readings in the Primary and Secondary be affected where Primary and Secondary each have their own load resistor in each of the channells?
With repsect to 2, in Air (I will try in O2, but I do not have it handy), I shorted the channel 2 + and - of the wire leading to the tertiary (non-Primary and non-Secondary) electronics, and this has no meaningful effect on their mvolt or pPO2 readings.
So, I do not thibnk that it is true that "the current source in the cell will not see the correct load resistance" (other than of course for the channel on the specific electronic which has suffered the short), because all the other channells each have their own load resistor.
Losing one load resistor in one channel of one electronic board doe snot seem to meaningfully affect the others provided they have their own load resistor.
-
heinrichsweikamp
- Posts: 4464
- Joined: 13 May 2007, 18:07
Re: Resistors for O2 board
Hello,
Your assumptions are correct. In the end one has to decide which level of redundancy anyone wants. With worst case, I meant seawater flood of the O2 Monitor electronic (resulting in three shorted cells) + stuck solenoid at the same time. If the 2nd monitor has sensors on it's own you'll have at least a chance to switch to bailout in time.
regards,
Matthias
Your assumptions are correct. In the end one has to decide which level of redundancy anyone wants. With worst case, I meant seawater flood of the O2 Monitor electronic (resulting in three shorted cells) + stuck solenoid at the same time. If the 2nd monitor has sensors on it's own you'll have at least a chance to switch to bailout in time.
regards,
Matthias
-
Anonymous User
Re: Resistors for O2 board
heinrichsweikamp Wrote:
-------------------------------------------------------
> Hello,
>
> Your assumptions are correct. In the end one has
> to decide which level of redundancy anyone wants.
> With worst case, I meant seawater flood of the O2
> Monitor electronic (resulting in three shorted
> cells) + stuck solenoid at the same time. If the
> 2nd monitor has sensors on it's own you'll have at
> least a chance to switch to bailout in time.
>
> regards,
> Matthias
I am in full agreement that a separate set of 3 O2 cells for each electronic device would be the ideal solution.
The resistor system to share existing O2 cell is only second best.
However, as some people are sharing O2 cells without any resistors whatsoever inline, this would seem to be the most risky solution and one to be avoided.
...just measured the resistors the rebreather manufacturer said were 2K - but they are 10k... so looks like 10k resistors are used inline and not 2K.
-------------------------------------------------------
> Hello,
>
> Your assumptions are correct. In the end one has
> to decide which level of redundancy anyone wants.
> With worst case, I meant seawater flood of the O2
> Monitor electronic (resulting in three shorted
> cells) + stuck solenoid at the same time. If the
> 2nd monitor has sensors on it's own you'll have at
> least a chance to switch to bailout in time.
>
> regards,
> Matthias
I am in full agreement that a separate set of 3 O2 cells for each electronic device would be the ideal solution.
The resistor system to share existing O2 cell is only second best.
However, as some people are sharing O2 cells without any resistors whatsoever inline, this would seem to be the most risky solution and one to be avoided.
...just measured the resistors the rebreather manufacturer said were 2K - but they are 10k... so looks like 10k resistors are used inline and not 2K.