Response time (Time constant) = The time constant is equal to the time that passes until 63,2% of the temperature leap is indicated. In many cases, the temperature indication does not change according to the exponential function. Therefore, it is useful to indicate the half-time z 0.5 and the 9/10 time value z 0.9. This is the definition of time from the sudden change of temperature to the reach of 50% either 90% of this temperature change. The exponential course shows z 0.5 =0.693 (time constant) resp. z 0.9 = 2.303 (time constant) and the ratio z 0.9/z 0.5 has to be equal to 3.32.
The following table shows the response times of thermocouples and resistance thermometers each with insulated measuring point (form A) and measuring point welded (form B). The measured values are broken down into response times in water and in air:
Response Time of Thermocouples in Water and Air
Insulated hot junction | Response time in | |||
(form A) | Water with 0,2 m/s | Air with 0,2 m/s | ||
| t 0,5 (s) | t 0,9 (s) | t 0,5 (s) | t 0,9 (s) | |
| 0,5 | 0,06 | 0,13 | 1,80 | 5,50 |
| 1,0 | 0,15 | 0,50 | 3,00 | 10,00 |
| 1,5 | 0,21 | 0,60 | 8,00 | 25,00 |
| 3,0 | 1,20 | 2,90 | 23,00 | 80,00 |
| 4,5 | 2,50 | 5,90 | 37,00 | 120,00 |
| 6,0 | 4,00 | 9,60 | 60,00 | 200,00 |
| 8,0 | 7,00 | 17,00 | 100,00 | 360,00 |
Welded hot junction | Response time in | |||
| (form B) sheath-∅ (mm) | Water with 0,2 m/s | Air with 0,2 m/s | ||
| t 0,5 (s) | t 0,9 (s) | t 0,5 (s) | t 0,9 (s) | |
| 0,5 | 0,03 | 0,10 | 1,80 | 6,00 |
| 1,0 | 0,06 | 0,18 | 3,00 | 10,00 |
| 1,5 | 0,13 | 0,40 | 8,00 | 25,00 |
| 3,0 | 0,22 | 0,75 | 23,00 | 80,00 |
| 4,5 | 0,45 | 1,60 | 33,00 | 110,00 |
| 6,0 | 0,55 | 2,60 | 55,00 | 185,00 |
| 8,0 | 0,75 | 4,60 | 97,00 | 310,00 |
Mineral insulated resistance thermometer
sheath-∅ in mm | Response time in | |||
| Water with 0,2 m/s | Air with 0,2 m/s | |||
| t 0,5 (s) | t 0,9 (s) | t 0,5 (s) | t 0,9 (s) | |
| 1,6 | 3,6 | 5,5 | 10,8 | 26,3 |
| 3,0 | 5,2 | 9,8 | 20,0 | 51,0 |
| 6,0 | 10,4 | 23,2 | 46,8 | 121,0 |
These indications are only reference values as the response time depends on the applied RTD.
Mineral insulated thermocouples and mineral insulated resistance thermometers can be bent with a radius of five times the outer diameter of the sheath material. Herewith it must be considered that any bending of the measuring tip over a lenght of 60 mm has to be avoided.