Assistance needed with RTD application (SCXI-1121/1320). Not enough excitation.

[jwc1979]

My current application is configured to monitor (measure) RTDs.  I have an 1320 terminal block connected to an SCXI-1121 module.  This interface is used to measure thin film RTDs using the two-wire method.  My application has worked for several years using a certain type of RTD that has a resistance of 500 ohms at 0 C and ~ 820 ohms at ambient temperature (22 C).  However, I am now trying to interface with a different model RTD.  This model is of the same type (thin film with two wire application) but its response slope is different.  It's 0 C resistance is ~1100 ohms and its resistance at 22 C is ~ 1800 ohms.  When I connect this type of RTD and change my equation to account for the difference in response, my readings get maxed out at 60 C.  I used a potentiometer to test the old set-up and it appears the readings max out at approximately 1.2 kohms.  The new RTDs resistance appears to high for the set-up to read it.  I am using current excitation of 0.15 mA.  I thought that maybe the excitation was too low and reconfigured for an excitation of 0.45 mA.  However, this did not improve the situation.  I was still unable to measure any change of resistance with the higher impedence RTD.  What could I possilbly try next with the configuration?  I am absolutely sure that my linear equations are correct for both RTDs and that I have properly applied them through our software application.

Thanks,

James

[Logan_K]

The SCXI-1121 current excitation is designed to handle up to 10kohms of resistance, so I would check your jumper-configurable gain settings on the module to make sure that you are providing a large enough input range for your sensor. At 1.2kohms, using the .150mA current setting, your voltage output would be 0.18V, (using the .450mA setting the voltage would be .54V). The current excitation is also jumper configurable, by the way. So, you will need to reduce your gain if you want to avoid railing your inputs. The SCXI-1121 manual talks about how to change the jumper settings for gain: http://www.ni.com/pdf/manuals/320426c.pdf, and after you change those settings you will need to update your SCXI configuration in MAX to reflect the new gain settings per channel.

Regards,

Logan K