IF OBJECT_ID('conversions') IS NOT NULL
  DROP TABLE conversions;

IF OBJECT_ID('measurements') IS NOT NULL
  DROP TABLE measurements;

CREATE TABLE conversions
(
  convert_from INT,
  convert_to INT,
  factor DECIMAL (5, 1)
);

CREATE TABLE measurements
(
  m_type INT,
  measurement DECIMAL(18, 5)
);

INSERT INTO conversions
SELECT 1, 1, 1
UNION ALL
SELECT 1, 2, 0.5
UNION ALL 
SELECT 1, 3, 4
UNION ALL
SELECT 2, 1, 2
UNION ALL
SELECT 2, 2, 1
UNION ALL
SELECT 2, 3, 9
UNION ALL
SELECT 3, 1, 4;

INSERT INTO measurements
SELECT 1, 50.5
UNION ALL
SELECT 1, 100
UNION ALL
SELECT 2, 93
UNION ALL
SELECT 2, 76
UNION ALL
SELECT 3, 56;

SELECT * 
  FROM measurements AS m
       INNER JOIN conversions AS c
          ON m.m_type = c.convert_from

DECLARE @source AS NVARCHAR(50);
DECLARE @target AS NVARCHAR(50);

SET @source = '1,2';
SET @target = '2,3';

IF OBJECT_ID('tempdb..#sources') IS NOT NULL
  DROP TABLE #sources;

IF OBJECT_ID('tempdb..#targets') IS NOT NULL
  DROP TABLE #targets;

IF OBJECT_ID('tempdb..#convert') IS NOT NULL
  DROP TABLE #convert;

SELECT pos, element AS c_from
  INTO #sources
  FROM dbo.fn_split(@source, ',');

SELECT pos, element AS c_to
  INTO #targets
  FROM dbo.fn_split(@target, ',');

SELECT s.pos, s.c_from, t.c_to
  INTO #convert
  FROM #sources AS s
       INNER JOIN #targets AS t
          ON s.pos = t.pos

SELECT m.measurement AS original_measurement,
       (m.measurement * c.factor) AS converted_measurement
  FROM measurements AS m
       INNER JOIN conversions AS c
          ON m.m_type = c.convert_from
       INNER JOIN #convert AS conv
          ON c.convert_from = conv.c_from
         AND c.convert_to = conv.c_to