What is the accuracy of an SCC-RTD01

[paulmw]

I need to know what, or how to calculate, the accuracy of an SCC-RTD01 from the specifications and could use some assistance.

 

My total system will use a thin-film platinum RTD (100 Ω at 0°C, temp coefficient of 0.00385 Ω/Ω/°C), connected via 4-wire to the SCC-RTD01 installed in a SCC-68 I/O connector block and digitized by a PXI-6251. 

 

NI's online accuracy calculator conveniently provides the calculated accuracy for the PXI-6251.

 Any help is appreciated- 

[Henrik_Volkers]

The spec of the SCC-RTD01 tell me a 1.2% gain error 😮  as the biggest point. However with a RTD simulator this could be calibrated and compensated.

Message Edited by Henrik Volkers on 02-26-2010 09:10 AM

[paulmw]

Thanks for the info on the RTD simulator.  I was not aware of their existance - maybe our calibation guys have one...

Anyway, so I can estimate that the 1.2% gain error will be the majority of the error but I'd like to know (for my own informatoin) how I would include the +/- 250uV (RTI) for the offset error.  The RTI part is confusing me.

Also,  your RTD simulator leads me to a new question (I don't have the equiptment yet so I can't check myself).  Can I store this calibration/compensation value provided by the RTD simulator somewhere in MAX or will I have to write code to do the compensation.

thanks-

[paulmw]

I hope I am calculating something wrong but my estimation for the accuracy for the overall voltage measurement for the system is about +/- 2mV and a resistance accuracy of about +/- 2 ohm.  This translates to an accuracy of only 5 deg. C.  Can this be true?

[Henrik_Volkers]

If I remember right you can enter custom coefficients for the Callendar van Dusen equation. So it should be possible, however it will need some preprocessing to get these values.

[paulmw]

So, I am toying with a new idea that will help even if the my accuracy is this low and I don't have use of an RTD simulator.  What if I use a thermister instead.  They have a much greater sensitivity at my temperature range (my measurements will be at room temperature).  What I am thinking is to use the current source of the SCC-RTD01 only and route the voltage directly to the DAQ by passing the gain stage of the SCC-RTD01 (the SCC-RTD01 has a limit of 400mV input range).  If I read the specification correctly the excitation current can withstand up to 24V (without regulation loss).  I will choose thermistor that has 3k ohms at room temperature so my DAQ range will be +/- 5V.  Any thoughts?

[Henrik_Volkers]

Powerloos in the thermistor? Selfheating?

[paulmw]

1 mw/°C min. in still air (for example thermistor).  So for 1mA and 3k ohm that is 3mW or 3°C self heating, damn-

[Henrik_Volkers]

Even if you don't have a RTD simulator/calibrator  , two 100R  and a good voltage meter (4+ digit) can give you the same answers. Generous people also spend  big Cs in parallel 😉

Connect both R in series to the current source and allow 20 min to settle. Measure alternativ with the SCC/DAQ  and the DVM  the 100mV and 200mV