Hello,

I have an old ted 60 oxygen analyser that I would like to use with an oxygen sensor that is not original to the analyser. Problem is the original sensor generates a voltage of 25mv in normal air as the sensor I would like to use generates 11mv. This means I am not able to calibrate the analyser to the starting point of 21%. I contacted teledyne and they were so kind to send me the schematic of the analyser. Could someone tell me there is a way to modify the analyser so I can use the sensor cells I have available?

regards

tim

 

There is a SPAN adjustment. Does this not give you the desired range?

If not, the gain of U3 is determined by R18/R17. You can adjust this ratio in order to change the gain.
The simplest solution is to put a 1M-ohm trim pot or 1M-ohm resistor in parallel with R17.

 

Min is 2.9 and max is 6.2 that I can get with the original calibration .

 

Try to replace R12 , 750 kOhm to 1.8MOhm

 

Thanks for the suggestions,I will give it a go

 

Succes,changed r17 and the calibration is spot on mid range off the calibration pot.

 

I was suggested by someone that it would be better to change r22 to the value of r17/r18 parallel. Would this be better? I used 220k for r17 and r18 is 1M. Value for r22 would be 180k,is that correct?

 

I was suggested by someone that it would be better to change r22 to the value of r17/r18 parallel. Would this be better? I used 220k for r17 and r18 is 1M. Value for r22 would be 180k,is that correct?

Thats right. Use a precision type resistor for R17 /18

 

Thats right. Use a precision type resistor for R17 /18

R17 was a normal resistor so I also used a normal one. R18 and 22 don’t look like normal resistors so maybe they are precision ones. So best is to use precision ones for all?

 

R17 was a normal resistor so I also used a normal one. R18 and 22 don’t look like normal resistors so maybe they are precision ones. So best is to use precision ones for all?

There's no point in using all precision resistors. Only those resistors need to be precise whose deviation significantly changes the output signal voltage

 

Ok,thanks

 

Someone also asked me some questions about no thermistor being connected as the sensor I am using doesn’t have one and the original did.
he said something about this thermistor having a big impact on the amplification of the first opamp and the input bias being crooked when there is no signal from the thermistor. Being a novice to electronics this is all Chinese to me,will this be a problem? I tested it yesterday and it seems to work fine.

 

It depends on the usage of the instrument. What is its application?
Personally, I would leave things as they are.

 

Originally it was used for medical applications so I think I needed to be very precise. Therefore the thermistor for temperature compensation I think. I will use it for diving nitrox. I checked and the analysers you can buy in the shops don’t have the temperature compensation either.