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Customer Review

565 of 567 people found the following review helpful
1.0 out of 5 stars Two engineers, one cup: a scientific investigation into Mustard's 50ºC claim., 8 Jan. 2016
This review is from: Mustard USB Cup Mug Warmer Coaster - Dark Brown Hot Cookie (Personal Computers)
After receiving this item as a Christmas present, I immediately saw its potential for keeping my tea warm at my desk whilst I'm working in the electronics laboratory where beverages, hot or cold, are forbidden (for obvious reasons). Imagine my disappointment then when, on my first day back at work after the Christmas break, I made myself a nice cup of Earl Grey, went to the lab for a while, and then came back to find my tea cold. Certainly not the 50ºC at which this product claims to keep your tea. It was then that I decided to finally put my physics degree and electrical engineering PhD to good use by subjecting the Mustard Hot Cookie to a scientific investigation.

Figure 1 shows my test set-up. A k-type temperature probe, connected to a Pico-logger (which in turn was connected to a PC), was inserted into a mug of hot water which was placed on top of the Mustard Hot Cookie coaster. The water was decanted into the mug from the boiler in the work's kitchen and the whole test set-up was placed in an environmentally-controlled laboratory (in other words, our electronics lab has air conditioning).

The first test was the control. The mug of water was placed on the coaster, but the coaster was not plugged into the USB port of the PC. The PC logged temperature data every minute for over two hours. The second test was performed with the coaster plugged into the USB port and switched on. It was seen that there was very little thermal contact between the mug and the top of the coaster. The instructions say to use a flat-bottomed drinking receptacle, but as far as I am aware, all common mugs have a rim around the circumference of the base that keeps most of the mug from actually making contact with the coaster. To remedy this, a third test was performed using a thermally-conductive gasket placed under the mug to increase its thermal contact with the coaster (see figure 2).

Figure 3 shows the results. The control test (light blue line) saw the water temperature fall to 35ºC after the first hour and to 26ºC after the second hour. With the Hot Cookie switched on (test 2, brown line), there was only a small improvement, with the water at 39ºC after one hour and at 32ºC after two hours. Surprisingly, the thermally-conductive gasket made to difference to the results seen in test 2 (grey line hidden behind the brown line). These results show that the claims made by Mustard that the Hot Cookie keeps your drink at 50ºC were plainly false. The only thing with an elevated temperature was clearly the hot air spouted by Mustard.

At this point in my investigation, my boss saw what I was up to: i.e., not getting on with my proper work and breaking the rules about having drinks in the laboratory. That was when he suggested that I should use an external power supply to increase the power dissipation of the coaster. A USB port in a PC provides 0.5A of current at a voltage of 5V, giving a total power dissipation of 2.5W (P=VI). USB phone chargers are available however that can provide 2.1A at 5V. That's a power rating of 10.5W. However, the coaster needed to be modified in order for it to draw 2.1A of current. This involved opening up the coaster, removing its internal workings, and installing a 2.2 Ohm resistor rated to 50W. A thermally-conductive compound was used to provide good contact with the top of the coaster and some thermally-resistive packaging foam was used to keep the resistor in place.

Using Ohm's Law (V=IR), in order to get 10.5W of power dissipation, it was calculated that the voltage supplied by the external power supply to the now-modified coaster needed to be 4.8V, thus giving a current consumption of 2.2A: close enough to the aforementioned USB phone charger for the scope of this study. As before, the mug was filled with hot water and placed on the now-modified coaster and the temperature was logged for two hours. Again, this test was repeated using the thermally-conductive gasket.

Figure 3 shows that with the modified coaster powered to give a power dissipation of 10.5W, the temperature falls to just below 43ºC after two hours (yellow line). This time though, the gasket makes a small difference, and after two hours, the temperature is 44.5ºC. This is still 5.5ºC below the claimed 50ºC that is made by the manufacturer.

As well as measuring the water temperature, the temperature of the top of the coaster was noted after if had been left to heat up sufficiently. The unmodified coaster top surface reached 50ºC whereas the modified coaster reached 90ºC. It can be concluded therefore that even though the current design simply doesn't keep the drink at 50ºC, a re-design has the potential of fulfilling this claim. However, there is the added health and safety risk of having a 90ºC hot surface on your desk.

Not satisfied with this result, my boss insisted that I push the modified coaster to its limit by increasing the power dissipation to its full 50W. At the first attempt, the solder to the resistor melted and the wires to the power supply fell off. We re-attached the wires using crimps. However, in switching on again at 50W, the thermally-resistive packaging foam that was placed inside the coaster started to melt and give off toxic fumes. This additional test was therefore abandoned.

Conclusions: this coaster falls well short of the claim that it keeps your drink at 50ºC. Modifying the coaster brings it closer at around 45ºC, but the coaster itself gets dangerously hot. In short, this product just doesn't cut the Mustard. Ha ha ha! Mustard!
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Showing 1-10 of 20 posts in this discussion
Initial post: 19 Jan 2016, 16:33:59 GMT
This is the best review I've ever read on Amazon, how this has not gone viral I don't know. I think you should sell the modified version!

Posted on 31 Jan 2016, 16:24:51 GMT
Amazing. Well done.

Posted on 8 Feb 2016, 13:14:42 GMT
Good stuff :)

In reply to an earlier post on 13 Feb 2016, 14:34:44 GMT
monica says:
It made it to Reddit today, in fact. Sorry I can't show URL as that subreddit name has what amazon considers an obscene word, but it's 'Quit Your -------- [American term for rubbish talk]'.

In reply to an earlier post on 20 Feb 2016, 18:50:27 GMT
I searched for it. It came up with "Engineer vs exaggerated Amazon product claim". No rude words there so I'm wondering whether there's another thread.

In reply to an earlier post on 20 Feb 2016, 19:07:26 GMT
monica says:
That's it. It's the title of the subreddit, i.e. the forum, that amazon would censor.

Ah, I see. I just googled it myself & see it's also on the engineering subreddit. (You can see the name of the subreddit I saw it on, which I got slightly wrong, at the end of the post title there.)

In reply to an earlier post on 21 Feb 2016, 10:46:41 GMT
I don't have a Reddit account (I don't even know what a subreddit is), so may I take this opportunity to comment on some of the posts here. There are several links in the Reddit thread so some of those users may end up reading this.

Hello Reddit users. Thank you for your comments. On the whole, I think most of you found my review informative and entertainment. It got you talking about physics and engineering, which is good. I do bow down to the guy who made a ping pong ball go supersonic though. Now that is engineering!

Some answers to your posts:

1) The ambient background temperature: I did say the lab was air conditioned so assume it's comfortable for human habitation and well within legal limits for a modern workplace. In short, it's around 22ºC.

2) The aforementioned ambient temperature would still be true whether the lab is located in north Wales or in the Australian outback. That's what air conditioning does.

3) As for air currents: the air vents in the ceiling are covered by some diffusing material to eliminate draughts. There were therefore no strong air currents above or around the test set-up.

4) The product was not defective. I did say it drew 0.5A of current: the maximum possible from a PC USM port. Actually though, it drew 0.6A which had the effect of pulling the voltage down from 5V to 4.5V. The power dissipation the device was therefore 2.7W, not 2.5W. It was still rubbish.

5) I didn't take thermodynamics and the thermal properties of the mug into account. I tried to fit a mathematical model to the device but it didn't quite work out as I had hoped. My knowledge of thermodynamics was called into question. Well I did study it as part of my physics degree but it was one of my least favourite subjects and I don't claim to know much about it. However, I would be willing to bet my entire Marillion CD collection that if I knew more about thermodynamics, my tea would still be just as cold. Mankind didn't know about nuclear fusion and the transmutation of the elements until the early 20th century, but that didn't stop the sun from shining for the preceding four billion years. In fact, it is Mustard, not I, who needs to know about thermodynamics and the thermal properties of mugs if a person's cup of Earl Grey is to be kept warm.

6) I will concede the fact that a 4th year mechanical engineering student is more knowledgeable than any electrical engineer with over 25 years' experience in the field. I bow down to you sir. However, I do feel that one really doesn't need any particular qualifications to be able to stick a temperature probe in a mug of water and click "go" on a PC. For presentation purposes though, it is desirable to know how to plot graphs in Excel. They may teach you this at the job centre after graduation as part of some back-to-work scheme for the long-term unemployed.

I hope I have redeemed myself in answering to these obvious oversights in my original review. I understand now why my review was rejected by The Physical Journal of Applied Thermodynamics and Cheap Novelty Gifts. I'll therefore not be expecting a phone call from Stockholm anytime soon, unless it's from the Abba museum demanding that I return Agnetha's gold spangled hotpants.


Dirty Hormonal B.Sc., Ph.D., E.S.C (elemental swimming certificate)

Posted on 10 Feb 2017, 13:07:27 GMT
you have an amazing boss

In reply to an earlier post on 10 Feb 2017, 13:15:24 GMT
Mr says:
It has now, it's all over Twitter!

Posted on 10 Feb 2017, 19:02:27 GMT
Andrew W says:
Is it possible we are misinterpreting the correct use of the "cookie"? After all, conventionally one would dip a cookie in tea rather than placing it under the cup. Did the experimenter test an immersion setup?
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