Thermocouples

Information on temperature measurement and thermocouples by Peak Sensors
and fromTC

UK Thermocouple suppliers

Here are some suppliers, generally cheaper than the kiln manufacturers/dealers
Mitsco thermocouples - good value R and K thermocouples
Avery Weigh-Tronix thermocouple calibration - inexpensive calibration, but to a low temperature for kilns
Sterling Sensors - wide range, but R/S are very expensive
TMS Europe - thermocouples, loggers, ceramic tubes etc.
Omega - thermocouples and many other sensors and loggers
RS - data loggers etc.

Accuracy

There are two standards for thermocouple accuracy: IEC 60584 is the global standard, whilst the USA has it's own standard, ASTM E230. The thermocouples are the same, it's just the two standards bodies differing on their accuracy levels.
The table below gives error values for temperatures at intervals of 100°C. All errors are averages, and are ± the temperature - seeing errors that are double the average is not at all uncommon. Where the temperature is above the maximum temperature for that thermocouple type, according to it's standard, the error is given in italics'' - errors in this range are likely to exceed those I have given.
Note that both standard have 2 levels of accuracy. In the ASTM standard, you have Standard and the more accurate Special thermocouples. In the IEC standard, Tolerance Class 1 are the more accurate, and 2 the standard ones.

K Type

Temperature C	ASTM Special	ASTM Standard	IEC Class 1	IEC Class 2
0	1.1	2.2	1.5	2.5
100	1.1	2.2	1.5	2.5
200	1.1	2.2	1.5	2.5
300	1.2	2.2	1.5	2.5
400	1.6	3.0	1.6	3.0
500	2.0	3.7	2.0	3.7
600	2.4	4.5	2.4	4.5
700	2.8	5.2	2.8	5.2
800	3.2	6.0	3.2	6.0
900	3.6	6.7	3.6	6.7
1000	4.0	7.5	4.0	7.5
1100	4.4	8.2	4.4	8.2
1200	4.8	9.0	4.8	9.0
1300	5.2	9.7	5.2	9.7

R and S Type

Temperature C	ASTM Special	ASTM Standard	IEC Class 1	IEC Class 2
0	0.6	1.5	1.0	1.5
100	0.6	1.5	1.0	1.5
200	0.6	1.5	1.0	1.5
300	0.6	1.5	1.0	1.5
400	0.6	1.5	1.0	1.5
500	0.6	1.5	1.0	1.5
600	0.6	1.5	1.0	1.5
700	0.7	1.7	1.0	1.7
800	0.8	2.0	1.0	2.0
900	0.9	2.2	1.0	2.2
1000	1.0	2.5	1.0	2.5
1100	1.1	2.7	1.0	2.7
1200	1.2	3.0	1.3	3.0
1300	1.3	3.2	1.6	3.2

Maintenance

Thermocouple - Fix it before it fails