9219 absolute error

[ltok]

Hi,

I am interested to know what is the absolute error for 9219. From the specification of 9219, there is gain error and offset error. I do not know how this result in the absolute error. Could anyone give me an example for 4 wire Pt100 RTD connection found on page 4 of the link below??

NI 9219 Specifications

http://www.ni.com/pdf/products/us/9219ds.pdf

I would also like to know the maximum sampling rate of 9219 in terms of Samples/seconds.

Thank you & Regards

Lee

[Brian F.]

Hello. 

Thank you for posting to the NI Discussion Forums. 

The maximum sampling rate for the 9219 is 100 S/s/ch (the inputs are simultaneous sampling. 

In terms of computing the absolute error, the gain error and offset error will be additive.  So, for a worst case scenario with a 4 wire Pt100 RTD connection, under normal temperatures, assume that you have .1% Gain error and 2400 ppm Offset error.  Compute the actual error and then add these error values up. 

If you have any further questions about how to do this, please ask and we will be happy to help!

Brian F
Applications Engineer
National Instruments

[ltok]

Hi Brian,

Thank you for your reply.

Maybe I am not precise enough. As I am using 4-wire Pt100 RTD connection and the environment temperature range is 20-100 deg C, I know that the gain error will be 0.1 deg C (0.1% x 100 deg C for the worse case). But how do I get the offset error?? If I am not wrong, it is 2400/1000000 of a range, but what is the range?? Is this range the range of the environment temperature or the range of RTD?? Could you tell me how to calculate that??

Thank you & Regards
Lee

[Stephanie B]

Hi Lee,

In the manual here on page 24, the offset error says the range it uses is the +/- 80ppm with a +/- 60V range (so 120V total). So its ppm of the 120V total range. Hope this helps! Let me know if you have any other questions!

Stephanie

[ltok]

Hi Stephanie

Thank you for your reply.

How do I know the claim 1 is for all modes or only for certain mode?? There is no indication of that claim in the table, not like claim 2 which is for 4-wire and 2-wire Resistance, 10Kohm and 1Kohm.

If that is the range (120V), then what is the total accuracy for a 4-wire RTD, Pt 100 reading at 100 deg C??

Gain Error = 0.5% x 100 = 0.5 deg C
Offset Error = 6400/1000000 x 120 = 0.768 (What is the unit??)
Total Error = 0.5 deg C + ______ (Is it plus 0.768??)

Could anyone please show me the steps on how the calculation is done?? I need to know the total error of a 4-wire RTD, Pt 100 reading at 100 deg C urgently.

Regards
Lee

[Stephanie B]

Hi Lee,

The claim you are mentioning is for the 3- and 4-wire RTDs and Pt1000 and Pt100, and so the mode and range will be those thereof.

The units you are asking about are V (ppm*V=V). You have to convert the voltage to the corresponding degrees C change. The Wikipedia page on this is pretty good. Hope this helps!

http://en.wikipedia.org/wiki/Resistance_thermometer

Stephanie

[ChuckS]

Okay I was good up until someone said +/- 60 v range on a PT100 which really only works around 1v ish with a 1ma source.

What range are you talking about?

[LoganK]

I am not sure where the 60V range came from for RTD mode in this thread. To get the offset error for RTD mode, you take the offset error of the mode you are looking at (6400ppm for 4-wire pt100), then you apply it to the actual range identified on p. 21 of the operating instruction. The ranges supported on the 9219 for RTDs are 5.05kohms and 505 ohms. So, the maximum offset error is 3.23 ohms using the 505 ohm mode. The typical error is obviously much better than this.

-Logan

[ChuckS]

3.2 ohms = 8+ degrees C that is huge !... what good is a 24bit converter

For typical it is 2400ppm  2.4m * 505 = 1.21ohm about 3.5 degrees ... still no better

With 60hz filter 140ppm 140e-6*505 = .0707 equals about 0.2 degrees ...

Is this right, These answer are totally different than discribed above in the early thread.

It would be nice if someone could show us how to calculate this in one message.

As Itok ask ... this is confusing. I dont know what to believe ...

Thanks

Chuck

[M_B1]

Hi Chuck,

Sorry about the confusion.  It looks like your calculations were pretty close.  Here is how you solve for absolute accuracy (error):

Absolute Accuracy = Gain Error + Offset Error

So for absolute accuracy (error) for RTD measurements 4 wire pt100, here is an example case based on the assumption of that you have an RTD with R0 of 100 (i.e. 100 Ohms at 0° C) and it moves 0.3 Ohms each °C.

Case 1: measuring at 25° C +/- 5°

Absolute Accuracy = Gain Error + Offset Error

Gain Error = 0.1% * 100 Ohms = 0.1 Ohms.

The rages are specified on page 21 and are 5.05k or 505 Ohms.

Offset Error = 2400 ppm * 505 Ohms = 1.212 Ohms.

Total = 0.1 Ohms + 1.212 Ohms = 1.312 Ohms

So if moves 0.3 Ohms each °C as in my case then:

Total = +/-1.312 Ohms / 0.3 Ohms/°C = +/- 4.37 °C

I hope this helps clear up how to find the expected accuracy in Ohms and you should be able to determine if is accurate enough for the application based on the specifications for your RTD.