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Generally speaking, the grounded junction offers the best compromise of performance and reliability. It is the best choice for general purpose measurements.
Select ungrounded if the lead wire will be shielded and attached to the sheath. Also select the ungrounded junction to avoid ground loops between instruments,
power supplies and the sensor. |
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| Illustrations (X-Ray) |
Junction Type |
Application Notes |
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Grounded Junction |
The sheath and conductors are welded together, forming a completely sealed integral junction.Recommended in the presence of liquids, moisture, gas or high pressure.The wire is protected from corrosive or erosive conditions. Response time with this style approaches that of the exposed junction. |
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Ungrounded Junction |
On this type the thermocouple junction is fully insulated from the welded sheath end. The ungrounded junction is excellent for applications where stray EMFs would affect the reading and for frequent or rapid temperature cycling. Response time is longer than for the grounded junction. |
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Dual Ungrounded Junction |
Two separate thermocouples are encased in a single sheath. The isolation would prevent ground loop errors if wired to separate instruments. Only available as ungrounded junctions. |
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Exposed Junction |
Thermocouple wires are butt welded, and insulation is sealed against liquid or gas penetration. This junction style provides the fastest possible response time but leaves the thermocouple wires unprotected against corrosive or mechanical damage. |
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| 9) What is the response time of mineral insulated thermocouples? |
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The smaller the diameter, the faster the thermocouple will respond. Grounding the junction will also improve the response time by approximately 50 percent.
This is based on the sensor achieving 63.2 percent of the final reading, or to the first time constant. It will take about five time constants to obtain
steady state readings. Since you are actually interested in the temperature of the surrounding medium, accuracy depends on the ability of the sensor to conduct
heat from its outer sheath to the element wire. Several factors come into play. The most commonly noted is "time constant" (thermal response time). Time constant,
or thermal response time, is an expression of how quickly a sensor responds to temperature changes. As expressed here, time response is defined as how long it takes
a sensor to reach 63.2 percent of a step temperature change. Response is a function of the mass of the sensor and its efficiency in transferring heat from its outer
surfaces to the wire sensing element. A rapid time response is essential for accuracy in a system with sharp temperature changes. Time response varies with the
probe's physical size and design. |
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| Average Response Time Still Water (seconds) |
| Readings are to 63.2% of measured temperature. |
| Sheath Diameter |
Grounded Junction |
Ungrounded Jonction |
| 1/2 |
0,500 |
15 |
20 |
| 3/8 |
0,375 |
8 |
11 |
| 5/16 |
0,313 |
5 |
7 |
| 1/4 |
0,250 |
2,2 |
4,1 |
| 3/16 |
0,188 |
1 |
2,3 |
| 1/8 |
0,125 |
0,5 |
1,1 |
| 1/16 |
0,062 |
0,22 |
0,4 |
| 1/25 |
0,040 |
0,04 |
0,13 |
Temperature Response Time Of Still Water At 80 Celcius
Measured with a Thermocouple having a .250" Sheat Diameter. |
| Time Constant |
Response Time |
Step Change |
Measured Temperature |
| Grounded |
Ungrounded |
| 1 |
2,2 |
4,1 |
63,2% |
50,6 |
| 2 |
4,4 |
8,2 |
86,5% |
69,2 |
| 3 |
6,6 |
12,3 |
95,0% |
76,0 |
| 4 |
8,8 |
16,4 |
98,2% |
78,6 |
| 5 |
11,0 |
20,5 |
99,3% |
79,4 |
| 6 |
13,2 |
24,6 |
99,8% |
79,8 |
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1) What do thermocouple letter designations mean?
2) What are thermocouple color codes?
3) What letter of calibration should I use?
4) What is the initial accuracy of temperature sensors?
5) Is there a maximum length for thermocouples and thermocouple wiring?
6) What sheath material is the best for my application?
7) What are the maximum temperatures of thermocouples?
8) What type of junction should I use?
9) What is the response time of mineral insulated thermocouples?
10) Where should my sensor be placed?
11) What should the thermocouple resistance measure?
12) The sensor appears to be reading incorrectly. What might be wrong? |
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