Polatherm Merlin Low temp 2 stage freezer cabinetR404a / Pentane PT100 probe -80c

[Contactor]

Hi Everybody

I've been asked to look at a Polatherm low temp chest freezer to -80c, it's for component testing.

Its a two stage set up, one on R404a and the other is Pentane.

The controller is an LAEMTW11 which I beleive uses a PT100 probe.

I am not HC qualified but have been asked to check the R404 side.

I am not familiar with PT100 probes, I understand any PT100 probe will do as long as it can take the -80c set point.

Does anyone have any experience with these systems / probes, what the control set up is and where I should start, it sits showing -2 and thats all I know.

I think the 404 just takes it below freezing and then the second stage is brought in.

Thanks

[Emmett]

The first stage R404 actually provides cooling to the low stage which is likely not pentane but has pentane in it. you cannot look at just part of this system you need to understand the entire unit or you will never solve the problem.

[Brian_UK]

Some data on the LAE

http://www.hvacrinfo.com/Controllers/lae%20mtw.pdf

[Contactor]

The PT-100 is reading the wrong temperature and somebody has modified the control circuit, I've already had it running to -60.

Point taken Emmett, but can you actually help? It is just Pentane on the second stage, it's not an additive, as you suggest.

What I'm after is specific information to this system, controls in particular, such as the helpful link provided by Brian. Thank you Brian.

Regards

[Emmett]

Contactor
I did not intend to offend! Only wish to help and be helped. I am glad Brian was able to provide valuable information to you.

[norseman]

Emmet.

I can not see other than your advice was good. It is not possible to use a standard Pentane as a second stage refrigerant. The boiling point of Pentane is +36c and it is mostly used as an oil return additive. It is a good solvent and by running around as liquid it will dilute the oil which get a high viscosity on low stage systems.
Cascade systems has to be investigated in a certain way to track down any problems. You really need a manual who tells about specifications for the system. I have never seen this one and could not find any specs on the internett. Anyway a static pressure test at ambient on the low stage, the heat exchanger temperature and if possible how deep the compressor can run at the suction side would be what I would have looked for.

The senors involved on low temperature units would to 99% be a PT 100 or PT 1000 and the few using PT 500.
Such Platinium sensors are very correct and are very linear so they can easily be checked against a simple temperature chart using a digital multimeter. They are very sensitive to moisture so if lucky the one in your freezer can give false reading. Some makers give a damn in the quality and some use a not waterthight type.
I have changed a endless amount of sensor due to moisture in the sensors after a warm up of units.

[Emmett]

Thank you Norseman
Exactly my point, you cannot look at a cascade as two seperate systems you must be able to look at the system as a whole and fully understand how the system should operate in order to determine what is wrong.

Have a Great day.

[Contactor]

Thanks everyone, it's a steep learning curve this one!!