Change results in column to display different

I have a column in the following select startement called displayvalue. Displayvalue is a varchar column. Primary everything entered into this column is numeric, but there are times when NR is entered. In the select query if it sees NR, I would like to have NR changed in the resultset to NULL or blank. NR doesn't come up all of the time, but I have not idea how to do this in the select statment.

Here goes..

SELECT SAMPLE.SAMPLEID, SAMPLE.U_WORKORDERNUMBER, SAMPLE.U_SAMPLEDATETIME, SDI.PARAMID, SDI.DISPLAYVALUE
FROM SAMPLE SAMPLE, SDIDATAITEM SDI
WHERE ( (SAMPLE.SAMPLEID = SDI.KEYID1) AND
(SAMPLE.U_WORKORDERNUMBER = '0060') AND (SAMPLE.U_SAMPLEDATETIME > '31-DEC-2003') AND (SDI.PARAMID = 'BOD') )
ORDER BY SAMPLE.SAMPLEID ASC, SDI.PARAMID ASCSELECT SAMPLE.SAMPLEID, SAMPLE.U_WORKORDERNUMBER, SAMPLE.U_SAMPLEDATETIME, SDI.PARAMID,
(CASE WHEN ISNUMERIC(SDI.DISPLAYVALUE) = 0 THEN Null
ELSE SDI.DISPLAYVALUE END) SDI_DISPLAYVALUE
FROM SAMPLE SAMPLE, SDIDATAITEM SDI
WHERE ( (SAMPLE.SAMPLEID = SDI.KEYID1) AND
(SAMPLE.U_WORKORDERNUMBER = '0060') AND (SAMPLE.U_SAMPLEDATETIME > '31-DEC-2003') AND (SDI.PARAMID = 'BOD') )
ORDER BY SAMPLE.SAMPLEID ASC, SDI.PARAMID ASC