Posts Tagged Bead
Thermocouple response time
Posted by Paul Austen in Response Time, Thermocouple Attachment, Thermocouples on March 20th, 2009
The response time of a thermocouple is affected by several factors
The main factors affecting thermocouple response time are thermocouple bead size and the conducting medium including attachment method.
Thermocouples response time is measured as a “time constant.” The time constant is defined as the time required for a thermocouple’s voltage to reach 63.2% of its final value in response to a sudden change in temperature. It takes five time constants for the voltage to approach 100% of the new temperature value.
Thermocouples attached to a heavy mass will respond much slower than one that is left free standing because its value is governed by the temperature of the large mass. A free standing (exposed or bare wire) thermocouple’s response time is a function of the wire size (or mass of the thermocouple bead) and the conducting medium. A thermocouple of a given size will react much faster if the conducting medium is water compared to still air.
Here are some typical time constants of various free standing thermocouple bead sizes (bead size is typically 2 times the diameter of the wire) in these conducting mediums:
Wire (AWG) Bead Size (inches) Still Air (sec) Water (sec)
42 0.003 0.07 0.003
40 0.005 0.25 0.02
36 0.010 1 0.05
30 0.020 4 0.17
NOTE: Remember it takes five time constants for a thermocouple to reach 100% of the final temperature value so the above time constants must be multiplied by 5 to get the total time.
So the most common sizes (30 AWG or smaller) of thermocouples used to attach to surfaces or components will have fast enough response time to accuracelly measure the temperatures of reflow solder process which tend to change no faster then 5 degrees/second. If one wishes to measure the air temperature, 36 AWG is common since the air is always moving, and the chart reflects “still air” response times.
The main reason for selecting thermocouples of a specific size is to match the size of the surface or point where the thermocouple is to sense temperature. 36 AWG is a good compromise between cost, size, and strength. Much smaller and it is too easy to break. Much bigger and it may be bigger then the component or attachment point.
One other factor in selecting thermocouples is the heat source. In wave soldering, the heat is typically from the bottom of the assembly and the thermocouples are attached on top. Here the thermocouples will be cooler then bottom of the assembly causing them the sink the heat as it flows from the bottom to the top. Smaller thremocouples will reduce the heat sinking effect.
In reflow soldering, both the top and the bottom of the assembly are heated at about the same rate causing the thermocouple wire and its bead to heat as fast as the assembly. Some times the thermocouple can heat faster then the assembly because it is closer to the heat source and can act as a heat source to the component. This is often true where there is exposed thermocouple wire where the insulation has pulled back from the bead more then 0.5 inches. Keeping the insulation closer to the bead prevents this in most cases.
Thermocouple Size or Gauge
Posted by Rex Breunsbach in Size or Gauge, Thermocouples on March 18th, 2009
There is more than one way to specify thermocouple size.
Thermocouples are made when two conductors (wires) of different metals (alloys) are connected together to form a “junction.” This junction, or connection between the two conductors, is typically made by melting the two conductors together using a torch or a flash welding process. The size of the thermocouple is typically specified by the size of the two conductors, however, rather then the size of the junction formed where the conductors are melted together. The junction size is typically 2.5 time the wire diameter or less. Since the junction can vary somewhat, it is not the best way to specify the thermocouple size. So we us the wire size. Below are several of the most common ways to specify the size of a thermocouple:
- Gauge (American Wire Gauge, or AWG)
Wire gauge is common in the US and has meaning in the electronic and electrical fields. It’s handy because it keeps you from having to say (or write) long decimal numbers like 0.005 inches in diameter when you can just say 36 gauge. However, it’s upside-down in that as gauge number goes up, wire diameter goes down. There is a ratio between the gauge size and the diameter in inches:
Wire Diameter (inches) = 0.005 * (92^((36-AWG)/39))
As messy as this is, we still use AWG to call out thermocouple wire size. Here is a table of some common wire gauge sizes and their diameters in inches:
AWG Diameter (inches)
22 0.0253
24 0.0201
26 0.0159
28 0.0126
30 0.0100
32 0.008
34 0.0063
36 0.005
38 0.004
40 0.0031
- Wire diameter
We also size thermocouple wire by the diameter of the conductors. Each of the two conductors will be the same diameter, of course. See the above table for typical conductor conductor diameters use in the US.
- Square Millimeters (mm²)
Most other countries in the world use what’s called cross sectional area to specify the wire size. This is nothing more then the area of the circle formed by the conductor if you were to look flat at the end of the conductor. You know the area of a circle is:
Area =∏*radius²
And since the rest of the world is metric, this area is in millimeters (mm²). Common wire sizes are in nice round mm² numbers which means common sizes do not match up well with the AWG sizes. The table below shows the mm² sizes for the AWG gauge sizes:
AWG mm²
22 0.326
24 0.205
26 0.129
28 0.081
30 0.051
32 0.032
34 0.020
36 0.013
38 0.008
40 0.005
The most common thermocouple wire gauge sizes used for reflow or wave soldering in the US are: 30 and 36 AWG, and some 40 AWG
A common size in other countries is 0.03 mm², which as you can see from the table above is neither 30 nor 36, but real close to 32 AWG. The method used to specify a thermocouple size really depends on where (what country) you are buying it from. Although we can all convert, and most make equivalent sizes, what you will hear on the street will be AWG size in the US and area in millimeters most anywhere else in the world.
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