SNAP-AIRTD Generic Input resistance (ohm) measurement needs additional calculations

Applies To:

SNAP-PAC-EB1
SNAP-PAC-EB2
SNAP-PAC-EB1-W
SNAP-PAC-EB2-W
SNAP-PAC-SB1
SNAP-PAC-SB2
SNAP-PAC-R1
SNAP-PAC-R2
SNAP-PAC-R1-W
SNAP-PAC-R2-W
SNAP-B3000-ENET
SNAP-UP1-ADS
SNAP-UP1-M64

Versions Affected:

Versions affected: Firmware R9.3c or earlier
Problem is improved in version: R9.3e for PAC units

Symptoms:

If a SNAP-AIRTD module is configured as a Generic Input (0x01) Point Type in order to read the RTD Resistance (ohms), the Scaled Units (ohms) and the Raw Counts values are inaccurate; they erroneously include half of the lead resistance. In order to obtain accurate values, it is necessary to read the Raw Counts value and perform some mathematical processing.

Workaround:

For firmware R9.3c or earlier.
Read the point's Raw Counts from the Opto Memory Map and perform the following calculations:

Compensated RTD Resistance Scaled Units (Ohms) = [ (Raw Counts / 65536) - (Raw Counts bit-wise AND 65535) ] * SNAP-SCALING

Uncompensated RTD Resistance Scaled Units (Ohms) = [ (Raw Counts / 65536) + (Raw Counts bit-wise AND 65535) ] * SNAP-SCALING

where SNAP-SCALING = module input range / 25000. See the specific module data sheet for the input range.

You can find the memory map address of the Raw Counts for each point by using PAC Manager

For firmware R9.3e
The Raw Counts field and Scaled Units (ohms) are uncompensated values (they include the full lead resistance). PAC Control 9.3 and prior does not have the ability to put the point into compensated mode (with the lead resistance removed). PAC Control 9.4 will provide this capability in a future release.

Read the point's Raw Counts from the Opto Memory Map and perform the following calculations:

Compensated RTD Resistance Scaled Units (Ohms) = [ (Raw Counts / 65536) - (2 * (Raw Counts bit-wise AND 65535)) ] * SNAP-SCALING

where SNAP-SCALING = module input range / 25000. See the specific module data sheet for the input range.

Questions?

Contact: Opto 22 Product Support.
Phone: 800-835-6786 or 951-695-3080
Email: support@opto22.com

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