efrigerant temperature difference on outside of copper pipe

[chilliwilly]

Its been a while since I have had to do this, so I've forgot. Does anyone know the temperature difference of the inside to the outside of a copper pipe wall? ie 3deg C on the outside, ? deg C on the inside?

I'm topping up a system tommorow and I'm going to check the super heat of the evaporator amongst other things, and I want to do it by the actual inside temperature of the pipework and check it against the temp pressure chart. I don't know if its 134a or 404A thats in there, so I am going to use this method to be some where closer to being sure.

Thanks in advance for any replies

Regards

Martyn

[Brian_UK]

I think that you are expecting too much to be able to measure the temperature inside the pipe.

Use your comparator, it's good enough for most things.

If the system is to be topped up has the leak been fixed and what was put in there before you visit?

[Tesla]

From experience about 2.5degC. But there's nothing like measuring the actual refrigerant temperature - try yellow jacket ritchie superheat calculation kit, it can also rapidly speed up recovery and vac times with the schrouder removal tool.

[Peter_1]

I have mostly a difference of 4°C. If you evaporate at let's say -5°C and you measure direct after the TEV or on the first bend of your evaporator, then you have to measure theoretically -5°C. The difference you measure with your probe is due to the resistance of the copper and influence of the ambient on your probe.

[Tesla]

Hi Peter
This temp diff would depend on how the temp is measured on the outside wall. I did measure a 4deg diff once but that was with a digi-temp just taped to pipe - to get the closest measurement on the outside I used a thermocouple (fluke) with paste, taped and well insulated. I would imagine it would also depend on the wall thickness. As I wrote above there is nothing like direct refrigerant temp measurement - it's easy and it can also be done at suction service valve for some applications.

[chilliwilly]

I think that you are expecting too much to be able to measure the temperature inside the pipe.

Use your comparator, it's good enough for most things.

If the system is to be topped up has the leak been fixed and what was put in there before you visit?

Yes I've repaired the leak, but i'm not sure if its 134a or 404A that's in there. Thats why I'm using the temp/pressure method to determine what's in there, the cost of analising a sample is too expensive.

My probe is the clip on type and I normally just stick a lump of armaflex round the probe to cut down any external influence. I'll just have to suck it and see.

[Peter_1]

Hi Peter
This temp diff would depend on how the temp is measured on the outside wall. I did measure a 4deg diff once but that was with a digi-temp just taped to pipe - to get the closest measurement on the outside I used a thermocouple (fluke) with paste, taped and well insulated. I would imagine it would also depend on the wall thickness. As I wrote above there is nothing like direct refrigerant temp measurement - it's easy and it can also be done at suction service valve for some applications.

Tesla, your right but if you do it on the copper of your machine with your own thermometer, then you know immediately the 'fault' of your meter on your copper.
I mostly use a contact thermometer and I rarely strap nor isolate it. If I can, I measure on the outside of a bent and out of the stream of fans

[chilliwilly]

It was 404A after all, the temperature difference was 5.2 deg C, compared with the reading on my guages and the external temp from my probe. It was a lot nearer match than the other probable refrigerants that could have been in there.

Found another leak though, I must have weakened a spot repair on the evap coil that was done a few years back. I tested the low side to 150 psi with OFN, and the high side to about 400 psi with no drop after tightening up flares here and there.

Anyway its fridging nicely now, I just carried out a temporary makeshift repair to the evap coil with a jubilee/hose clip and 2 puncture repair patches :eek:, until its convenient to braze a spot over it..... "A puncture repair patch", you sometimes have to do these things when its late and you've no porta pack with you.

Thanks for all replies

Martyn

[glenn1340]

I mostly use a contact thermometer and I rarely strap nor isolate it. If I can, I measure on the outside of a bent and out of the stream of fans

On insulated tube I tend to use an ITE insertion probe, just slide the probe parrallel into to insulation until it touches the pipe. It doesn`t damage the insulation and is fully insulated so you get a good temp reading.
When removed you wouldn`t know the the insulation`s been touched.

[TRASH101]

From experience about 2.5degC. But there's nothing like measuring the actual refrigerant temperature - try yellow jacket ritchie superheat calculation kit, it can also rapidly speed up recovery and vac times with the schrouder removal tool.


I completely and utterly agree with Tesla for these reasons.

1) The probe reads the temperature of the refrigerant and is not influenced by poor contact or adverse ambient conditions.( unless it is already made its mark on the system )

2) You are reading the pressure at exactly the same point as the temperature.

In summary :
The closest you can get, in the field, to a definitive superheat/ subcooling result without spendin a lot of time and/or money.