# Insertion Length of RTD/TC

A

#### amrit

I am facing this problem in calculation of insertion length of RTD and Thermocouple. The data provided to me is:

Duct Diameter is 4500mm and
length from head to starting of duct should be 550mm.

Thanks...

R

#### Roy Matson

There was a time when we would try to get the probe near mid way which would be 4500/2 + 550 = 2800 however this is much too long, if there is any significant velocity in the duct the probe will act like a vortex shedding flowmeter and may even snap off. Presumably the temperature profile is fairly even so just pick a number like 150mm that way you can cut down on cost and also the number of different models.
A thermocouple measures the temperature at or near the tip, a modern RTD has a measuring length of about 15mm so it's almost the tip as well.

Normally you use the length of the nozzle which is typically about 150mm and add how far you want the probe to extend into the flow lets say 100mm, The probe would have a "U" length of 150 + 100 or 250mm then pick a standard available length say 10".

Depending on how hot the duct is you may need a lagging extension (so it doesn't get buried in the insulation). If it's not too hot I would use the standard length nipple or nipple-union-nipple between the thermowell and the head or transmitter otherwise they look silly.

Hope this helps

Roy

> I am facing this problem in calculation of insertion length
> of RTD and Thermocouple. The data provided to me is:

> Duct Diameter is 4500mm and length from head to starting of duct should be 550mm.

R

Dear sir,

please note that insertion length calculation =
4500/2 = 2250 + 300mm (After duct out side, for t/w)

this calculation for T/C AND RTD

REGARDS

R

#### Roy Matson

I just explained that you do not need to have the probe extend to the middle of the duct, this makes it too expensive, to make sure the thermowell will not Vortex it should be given a "Wake Frequency" - Thermowell strength calculation).

Better to decide on a standard penetration e.g. 4" into duct, that way you may find you have a bunch of identical thermowells.

Roy

> please note that insertion length calculation =
> 4500/2 = 2250 + 300mm (After duct out side, for t/w)

> this calculation for T/C AND RTD