tractor-trailer question

Well, it’s actually a question about the trailer. I’ve noticed that many trailers have a small, rectangular “door” on the back door of the trailer. Does anyone know what this is for?

If you will notice, those small rectangular doors are typically on refrigerated units. This allows the drive to check his load without losing a lot of the cold air trapped inside.

Yes, that little door is usually found in the a back door of a “temperature-controlled trailer,” often referred to as a “reefer.” I drove a reefer for a long time and enjoyed driving in general. Some shippers would insist that drivers manually check the temperature in the trailer every four hours or so, even though the temperature-control unit has a digital electronic temperature probe in it and can even e-mail temperature reports to headquarters through the “Qualcomm” satellite communications device in most such trucks. For manual checks, I would get out my trusty thermometer, open the little back door, put in the thermometer, close the door for a few minutes, open the door, then check thermometer’s reading, then close the door.

I say “temperature controlled” because that is the technical term for “reefer” hauling. The unit on the front of the trailer is normally a “heat pump” and can be used to cool or heat the load. The only time I ever had to use heat was when delivering some photographic chemicals in winter. The shipper required that the load me maintained at 68 degrees Fahrenheit.

We also hauled a lot of other cargo that did not require temperture control. If the shipper loaded the trailer, which was common, then we would look in through the little door to make sure the load looked like it was braced well. When the shipper loads, the main doors are usually guarded by a seal, so you can’t look in that way.

Funny story:

My wife’s company distributes cheese to retailers. One morning, a regular driver got sick so they called in a temp driver and got a guy that looked like he just gotten out of prison that morning. The same morning, the refrigerated truck broke down. They had some quick local deliveries to make so they sent the temp driver to a supermarket in just a regular van to deliver a small load of cheese.

When the driver got to the receiving dock, the receiving manager saw that he wasn’t in the usual temperature-controlled truck. He asked the temp driver “Where is your reefer?”. The driver looked a little shocked and said “Aw, man. I ain’t got no more reefer.”

Over here these little doors are only fitted to fridge boxes. They are, as mentioned upthread, for taking temperatures, but also for relieving pressure differentials. It can be hard to open the doors because the seals are so good that there can be a partial vacuum inside.

Incidentally, a lot of people assume that the fridge motor blows air from outside to the inside, cooling it on the way. Not so - it re-circulates the air inside - the box is air tight.

I know this is a zombie thread, so I don’t necessarily expect an answer, but if the driver checks the temperature in the trailer and it’s wrong, what can he do about it? Do big trucking companies have chilled warehouses that they can bring the trailer into? Can technician fix the cooling unit without letting all the cold air out, and fast enough to prevent the load from spoiling?

There’s no sense gathering data if you don’t know what to do with it once you’ve got it.

All of the mechanicals for the refrigeration units are on the outside of the trailer, so repairs can be done without the need to unload the trailer. If the problem is beyond a quick fix, then the trailer would be unloaded into a chilled warehouse or into a working trailer for delivery.

There’s a non-zombie thread here asking the same question:

It’s for the cats.

It might depend on whether they think they can get away with it. The insulation is pretty good so even with a big differential inside/outside it takes a few hours to warm up to unacceptable. As well as that, the air will warm up a lot faster than the goods, so as long as the fridge unit is fixed reasonably quickly, they would give it an hour to cool down and hope that the customer doesn’t care about the chart that got “accidentally” destroyed.

When I delivered chilled goods, mostly at 2 degrees C, the customer would usually stick a probe into the little hatch, frown, open the doors, climb in and probe between the packages for an accurate reading.

You don’t take the trailer into the warehouse - hygiene nightmare - The bay doors have a cushion around them which make a pretty good seal when the trailer is backed up tight.