Urgent! The issue with the thermocouple length!

This post was last edited by wang9117 on 2011-11-9 00:03. There is no such thing as an \"effective length\" for thermocouples; what exists are the total length L and the insertion depth l. What you refer to as effective length is likely the length of the measuring section, which should be: insertion depth – connector thickness – device wall thickness. In fact, when it comes to using thermocouples for temperature measurement, everyone should know that only the tip is effective; therefore, it’s necessary to ensure that the tip reaches the temperature measurement point you need.

Reply to 2# wang9117: Boss, is the number I calculated correct?

Reply to 3# wangyu84255911: Your calculation does not take into account the wall thickness of the equipment

You’re including the length of the thermocouple inserted into the device, along with its wall thickness; check to see if that meets your requirements

There are many types of thermocouples, especially the accompanying sleeves. To be able to handle issues effectively, it’s best to consult relevant books and sample materials in detail. Based on what you said, it’s likely a flange-type thermocouple; the effective length refers to the distance from the flange sealing surface to the end of the thermocouple. It is not certain that the entire length range is in contact with the process medium, as there is still a certain distance between the flange surface and the process medium (such as the inner wall of a container or equipment). In my opinion, as long as you believe that the tip of the thermocouple is in full contact with the medium, that’s sufficient. For equipment without insulation, try to install it as deep as possible or in a location where there are no dead corners in the medium (such as at elbows); for equipment with insulation, an insertion depth of about 5 centimeters is sufficient. The premise is: whether it’s a measuring container or a pipe, whether you can predict where there will be dead zones and where there won’t. The standards recommend using the maximum possible depth, as if that would ensure perfection; the downside is that the thermocouples tend to break frequently. I’ve seen in foreign countries that the insertion depth has nothing to do with the dimensions of the pipeline equipment, while in China there’s an insistence on establishing a relationship between them – people need to wake up! The text mentions \"whether the effective length (i.e., the length inserted into the pipeline) is 850 – 140 = 710 mm\" – what exactly do you want to know about this? Is it possible to insert 710mm or 850mm all the way into the pipe? There’s no need. If we are to talk about the effective length, I think it’s the 850 value. This “theory” is not suitable for thermal resistors.

It depends on the conditions of the equipment, the location of the measurement points, and the wall thickness. For pipes, this value is generally between 1/3 and 1/2. The specific installation method also matters, whether it involves flange sleeves, threaded sleeves, welded sleeves, etc. Details can be found in the HGT 21581-2010 manual on automatic control installation.

Let me tell you something else: the temperature measurement points of thermal resistors and thermocouples are generally not at the very top; they are usually located about 3-5 cm away from the top. This varies from one manufacturer to another. Many people think that the measurement point is right at the very top of the enclosure, but this is a misconception

Reply to 6# zxg.wylton: Why is it not suitable for thermal resistors?

The measurement tip of a thermal resistor is relatively long, for example 6 centimeters, while a thermocouple has its sensing part only at the very tip.