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Reviews > Cook Gear > Utensils > Primus Lunch Jug > Test Report by Roger Caffin


Initial Report     11-Nov-2008

Reviewer Details
Reviewer: Roger Caffin
Age: 63
Gender: M
Weight: 63 kg (139 lb)
Height: 167 cm (67")
Email address:     r dot [surname] at acm dot org
Home: Sydney, Australia

Backpacking Background

I started bushwalking at 14 and took up rock climbing at University with the girl who became my wife and my permanent walking partner. Ski touring and canyoning followed. Winter and summer, we prefer long hard trips by ourselves: about a week in Australia, up to three months in Europe/UK. We prefer fast and light in unfrequented trackless country. We would be out walking, skiing or snowshoeing for at least three months a year. We have now moved to lightweight gear, much to our backs' relief. I designed and made much of our lightweight gear myself.

I am also the maintainer of the Australian aus.bushwalking FAQ web site

Morning tea on Mt Paralyser

Product Information

Manufacturer: Primus
Manufacturer URL:
Year of manufacture: 11-2007 (label on bottom)
Country of manufacture: China
Packaged size (measured):     90 x 90 x 170 mm (3.5 x 3.5 x 6.75")
Jug size (listed): 85 dia x 165 mm (3.3 x 6.5")
Jug size (measured): 85 dia x 165 mm (3.3 x 6.5")
Weight (listed): 355 g (12.5 oz)
Weight (measured): 355 g (12.5 oz)
Capacity (listed): 0.5 L (16.9 oz)
Capacity (measured): 0.5 L (16.9 oz) (see below)
Lunch Jug with soup

Product Claims

There are two versions of the Primus Lunch Jug, as shown here using data from the Primus web site:

Small 265g / 9.3oz 85 D x 130mm / 3.3" x 5.1" 0.35 L - 11.8 oz
Large 314g / 11oz 85 D x 175mm / 3.3" x 6.7" 0.5 L - 16.9 oz

The packaging claims that this is a "Compact lightweight double-walled vacuum bottle with interior and exterior in 18/8 stainless steel. Powder-coated exterior for improved grip and durability. Leak-proof lid with opening which makes it easy to fill, wash and eat out of. Its compact shape combined with its excellent insulating properties makes it perfect for your food, drink or soup. Liquid filled at 95 C (203 F) is still warm (60 C/140 F) after approx 5 hours." The web site has similar claims.

Product Description

The Primus Product Claims and the two product pictures (courtesy Primus) do cover most aspects of the Lunch Jug. The first product photo shows the Lunch Jug as being a grey body, but black would be a better description of the colour. The top and bottom sections are protective plastic shrouds, while the central black part (with 'PRIMUS' on it) is the powder-coated steel flask. The lid is also black plastic. The three black plastic parts are nothing if not robust.

The inwards bulge slightly below the top of the stainless steel section shown in the second product photo is actually part of the sealing mechanism. The lid screws down - the threads are just visible in the second product photo. A central 'plug' inside the lid goes inside the neck of the Jug, and a rubber sealing ring on that plug presses against this inwards bulge to provide the seal. This means that the seal is inside the Jug, not at the top of the black plastic outer body. This is very common with vacuum flasks.

Thermal properties

The same photo shows the Lunch Jug holding what is probably meant to be tomato soup. The top of the soup is shown as slightly below the inwards bulge, and there is a reason for that. Room has to be left for the sealing plug part of the lid to be inserted into the container. When I poured the specified 0.5 L of water into the Lunch Jug the top of the water came to almost exactly the same position. Yes, I have been able to squeeze a little more liquid in and get the lid to seal, but the extra amount is very small. For this reason I am inclined to accept the Primus claim of 0.5 L as being correct.

Of course, an insulated vacuum flask is only really useful if it does insulate. The web site does not include any information of the degree of insulation, but the box the Lunch Jug came in does have a neat little graph on it, as reproduced here (again, courtesy Primus). The text with the graph says:

"Approximate values, measured with an ambient temperature of 20 C (68 F). The vacuum bottle is pre-heated for 5 min with water at 95 C." (95 C is 203 F)

The box does not say whether the vacuum bottle gets pre-cooled for the cold graph. This can be done of course.

First Impressions

Field testing will show how well the vacuum flask works, but I will make one comment here. I found that screwing the lid onto the robust thread (visible in the second product photo) is not as smooth as might be expected. It does seem that the plastic parts stick together a bit, inhibiting the rotation. I mention this here because my own experience with making things out of different plastics has shown that some of them do not slide as well as others. In particular acetal (Delrin) plastic shows a bad tendency to stick to itself like this. I do not know what plastics are used on the Lunch Jug, but they are hard like acetal.

Apart from the small problem with the lid, my initial impression is that this is a very robust unit. I probably won't get to test it with hot liquids: we are moving into a hot summer here in Australia and hot soup is not really on the agenda. Instead I will probably test it with cold things on day walks. According to the graph shown here the contents should still be quite cold after 6 hours. The idea of cold fruit salad for lunch on a very hot day has a certain appeal!


Long Term Report - 6-February-2009

Test Conditions & Locations

I took the Primus Lunch Jug on several day walks during the Test period. Given the small capacity and high base weight I did not take it on any overnight walks. As mentioned in the Initial Report, it was summer here in Australia when I was testing this Lunch Jug, so taking a vacuum flask of hot tea or soup did not seem like a useful idea - anyhow, I have a stove for that. Taking something cold in mid-summer seemed a much better idea, but 0.5 L of plain cold water is a bit of a waste. My plan instead was to take cold fresh fruit salad, prepared the night before. I tried several methods of making sure the contents of the Lunch Jug stayed really cold.

Using Ice Blocks

Mt Solitary

The first trip was a 'day' trip over Mt Solitary, an isolated mesa-style mountain in the middle of a valley near Katoomba, a tourist town in the middle of the Blue Mountains near Sydney. The day was in the low 20s C (around 70 F) but the humidity after recent rain was very high - well over 80%. Height ranged from 150 m (490') to over 900 m (2,950'). I put 'day' in quote marks as we (my wife and I) started at 8:00 am and finished at 20:00 pm, some 12 hours later, going from right to left in the photo above. (The trip was much longer than what's visible, but that gives you the idea.) It was a rather long day.

Fruit salad

I took several steps to help the flask keep everything cold during a hot summer day:

Ice blocks

On the Mt Solitary trip we had lunch at 13:00 pm, on top of the cliffs in the middle of the photo. After eating our normal lunch we hauled out the Primus Lunch Jug and opened it up. The fruit salad was served into our cups, leaving the Ziploc bag behind. The fruit was satisfactorily cold, and was consumed with great pleasure. Afterwards I hauled the Ziploc bag out to see whether there was any ice left. Indeed, about 1/3 of one ice block and 1/2 of the other ice block were still there! They were extracted, mixed with water, and the chilled water was drunk with great pleasure as well.

In many ways, this was the acid test for the insulation of the vacuum flask. That the ice cubes had lasted from very early in the morning when we packed our rucksacks for the day, through to lunchtime, meant that there really is adequate insulation to keep most anything reasonably cold on a day walk. Well - do remember that I helped things along by chilling the Lunch Jug down first, and I did add ice blocks at -10 C (14 F). But this is reasonable.

Smugglers Ridge Creek

I also tried using just one ice block from the freezer instead of two, to see if that would have enough 'cold' to last until lunch time. Less ice block meant more room for fruit salad of course. The photo to the right shows Calna Creek, which is near my home. The rock is sheet sandstone: we have a lot of that.

We had lunch sitting on boulders in the middle of the Calna Creek, with the fruit salad for 'afters'. The rainforest cover provided shade - it was a hot day. Calna Creek The right hand side of the photo shows the remains of the single ice block: not a lot, but still some ice remaining. While there is any ice left the interior of the Lunch Jug should be very close to 0 C (32 F), which is very satisfactory on a hot day. The insulation of the Lunch Jug is not too bad.

Using Frozen Juice

On some later trips I arranged the 'ice' differently. I'll describe this using a day walk down a little un-named creek also near home - one which we call 'Smugglers Ridge Creek'. The trip was far shorter in distance, but as much of the creek travel was done at under 1 km/hr (~0.5 mph), it still took a fair while. It was all off-track, and the scrub was 'a bit thick' in places. The weather was in the high 20s C (low 80s F), and the humidity was around 70%. We had to work hard on some sections to get through the creek-bank jungle.

More cold fruit salad

The night before the walk I poured some plain unsweetened apple juice into the Primus Lunch Jug and put that in the freezer, again at -10 C (14 F). It froze at the bottom - mostly. I think most of the water froze out first, leaving a very concentrated solution of apple juice. This did not freeze very well, even at -10 C (14 F). The whole arrangement may have been a bit sludgy. Never mind, there was a fair bit of ice in there. In the morning (about 6:30 am) I filled the Lunch Jug up with our home-made fresh fruit salad and screwed the lid on carefully. We packed up and went.

Lunch was around 13:00 pm. We ate our normal lunch first, then opened up the Lunch Jug and ladled the fruit salad out. As you can see, some of the frozen water from the apple juice had survived, in a sludgy sort of fashion, and this ended up on top of the fruit salad. The concentrated apple juice had mixed in with the fruit. Well, on a hot Australian summer day, who's complaining about a bit of ice with their fruit salad? Certainly not us!

The point of this example is that I don't have to use a plastic bag to hold the ice. This can also be done with other fruit juices, like orange juice. But we don't try it with fruit itself: most fruits do not like being frozen like that and can go squishy.

Some Problems

However, the Lunch Jug is not perfect: we have had some problems with the lid. The lid itself and the neck of the Jug are shown above in an earlier photo. The lid has a plug in it which fits inside the neck of the Jug. There is a rubber sealing ring around the edge of the plug. When the lid is screwed onto the Jug this sealing ring is pushed down into the neck and the rubber ring is compressed. The sealing ring keeps the liquids inside, while the plug insulates the end of the container.

The lid, tilted

When I filled the Jug with fruit salad I had to make sure that there was room left at the top for the plug to go in: the Jug cannot be filled to the brim. That's OK of course. But even after making an adequate allowance for space when filling the Jug, I found that sometimes the lid did not want to seat properly. The Lunch Jug is shown here (to the right) done up but nearly empty. The tilt in the fitting of the lid is visible, and I am not sure why it goes like this.

When this happened I undid the lid and then replaced it; sometimes that worked straight off. Other times it took several goes to get the lid to seat squarely. I did wonder whether I might have crossed the threads, but the misalignment was nowhere near that severe. As far as I could see that was not the case. It did seem possible that the sealing ring was jamming in the neck when the lid was cold and the rubber hard. I have met this problem before on other equipment, when the designers failed to allow for the hardening of the rubber in an O-ring at freezing temperatures.

It may be that I am doing something wrong with the lid. The lid uses a solid single-start thread with three full turns of thread provided on the Jug part. But I can only get a fraction over one whole turn on the lid before it jams solid. If I push it hard I can (maybe) get another quarter turn, but then undoing the lid is extremely difficult. There seems to be a mismatch between the design of the body and the lid. The lid seems to be jamming on the rubber seal.

Compounding the problem of getting the lid square and undoing it when tight was the slipperiness of the lid. The serrations around the edge are visible in the photo, but in reality those serrations are very shallow and do very little to improve my grip on the lid. This seemed especially a problem when I had just taken the Jug out of the freezer and filled it. The cold Jug and the high humidity of Sydney in summer meant that I was usually trying to do up a wet Lunch Jug, and sometimes one with a little fruit juice spilt down the side. This made the plastic rather slippery.

Cleaning the Lunch Jug has proven to be quite easy. Normally it just needs a rinse. The wide mouth helps of course. And it has taken being tossed around inside my pack very well, with no marks visible.

Final Summary

I am quite pleased with the insulation properties of the Lunch Jug - given that I froze it beforehand to -10 C (14 F) and included some ice in it to help.

The 0.5 L capacity is just big enough for two people - more fruit salad might have been nice! It would be very adequate for one person.

The Lunch Jug seems very robust: I have had no problems there.

The lid has given some problems, and I think the whole design of the lid, the thread and the rubber seal needs to be revisited and improved a bit.


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Reviews > Cook Gear > Utensils > Primus Lunch Jug > Test Report by Roger Caffin

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