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Equipment & Gear: Kitchen Thermometers
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Cooking For Engineers



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PostPosted: Thu Jan 05, 2006 11:49 pm    Post subject: Equipment & Gear: Kitchen Thermometers Reply with quote


Article Digest:
Since a major aspect of cooking is the heating of ingredients, temperature is an important topic. In cooking, we are primarily concerned in the temperature of our cooking environment (such as deep frying oil) and the temperature of our food (like a roast). With some foods, if the temperature of an ingredient has passed a certain point, it's texture and flavor changes such that we call it overdone. With other dishes, like pot roast, a minimum temperature must be reached to produce the desired effect. With lean steaks and roasts, there is a very small window which enables us to serve the dish as the perfect temperature. A thermometer is an invaluable tool in the kitchen.

What is a thermometer?
A thermometer is a device that measures changes in temperature. (Temperature is the average molecular kinetic energy in a substance.) If a thermometer is calibrated to a known temperature, then it can be used to accurately determine absolute temperature.

Some thermometer types
All thermometers work by measuring a change in a material that reacts to temperature changes.

A mechanical or dial thermometer uses two strips of different metal bonded together. The different metals have dissimilar expansion rates as temperature increases. Because of this, the bimetallic strip curls when the temperature changes. When constructed in the shape of a coil, the curling metal can be used to alter the angle of a needle. A dial thermometer is constructed by placing the needle in front of a static background and marking the background with known temperatures.

Bulb thermometers are based on the expansion of liquids. A liquid is encased in a thin transparent tube with a small reservoir at one end. As the liquid in the reservoir expands or contracts due to temperature change, the level of the liquid in the tube also changes. The thermometer is read by matching the level of the liquid with markings that label known temperatures. Mercury is a common liquid used in bulb thermometers, but should not be used in the kitchen because mercury is a poison and unsafe if the thermometer is broken.
Most bulb thermometers for kitchen use are filled with an alcohol (stained red for easy reading).

Electronic or digital thermometers operate based on an electronic component that reacts to different temperatures. Most digital kitchen thermometers use a thermistor (thermoresistor) - a device whose resistance changes due to temperature. When a current is passed through the thermistor, a voltage drop occurs. This voltage can be measured and a small microcomputer calculates the temperature based on the voltage. The temperature is then displayed for the user to read. Another technique, is to take advantage of the principle that joining two different metals causes a voltage potential between them that is dependent on the temperature of the junction (Seebeck effect). By reading the voltage between two metal wires joined at one end, the temperature can be calculated. Unfortunately, connecting a voltmeter to the thermocouple introduces more thermocouple junctions which alter the readings. By using a technique called cold-junction compensation (outside of the scope of Cooking For Engineers), the voltage can be properly read and temperature accurately calculated. Because of their high accuracy, thermocouple thermometers are popular in engineering and scientific practices, but their high average sales price (well over $100) generally precludes them from home kitchen use.

Many other types of thermometers using other methods of measuring temperature (like infrared thermometers that measure surface temperatures) are available, but not commonly used in a household kitchen.

Instant-read thermometers is a marketing name given to any type of thermometer (digital or dial) designed to be plunged into the food being cooked when it is nearly finished cooking to determine internal temperature. The thermometer does not work instantaneously and usually should not be allowed to persist in cooking temperatures (the thermometer needs to removed).

Probe thermometers are digital thermometers with probes connected to long wires. The probes hold a thermistor and is placed within the food while a connected base station reports the current temperature. Almost all probe thermometers have an alarm facility to alert the cook when a preprogrammed temperature has been reached.

Fry or candy thermometers are designed to measure the temperature of a cooking liquid (usually oil or sugar). The thermometers are partially submerged into the liquid and the temperature monitored during the cooking process.

Speed Test
When using a thermometer, speed is an important factor. Ideally, we'd like to know the temperature of the oil or the roast at this instant. If a thermometer takes a long time to report the temperature, the food could become overcooked or lose a lot of heat while you're waiting for a reading.

I pulled out five different thermometers (3 instant-read, one probe, and one fry/candy thermometer):<li>ThermoWorks Thermapen - A digital instant-read thermometer that uses a thermocouple; $85</li><li>ThermoWorks RT-301 Low Cost Pocket thermometer - A digital instant-read thermometer that uses a thermistor; $14</li><li>Polder Cooking Thermometer - A digital probe thermometer with timer, clock, and alarm - $20</li><li>Taylor Classic Style Meat Dial Thermometer - A basic dial thermometer - $10</li><li>Taylor Candy and Deep-Fry Thermometer - A fry/candy bulb thermometer - $13</li>
I tested the speed and accuracy of the thermometers by submerging them in ice water at 32°F (0°C), ambient temperature water at 70°F (21.1°C), and boiling water at 212°F (100°C). I tested the time it took for the thermometer to produce an accurate reading from (1) ambient to freezing temperature, (2) freezing to boiling temperature, and (3) ambient to boiling temperature. I also recorded the time it took for the thermometer to reach an approximate reading by recording times at 40°F (4.4°C) and 200°F (93.3°C). Two of the thermometers were incapable of indicating freezing temperatures, so I could not record the ambient to freezing time. However, I did place those thermometers in the ice water for several minutes before plunging them into the boiling water for the freezing to boiling times. All of the thermometers reached the reference temperatures as expected.

The tests were run several times and the average times are shown in the following table. The times in [brackets] represent the time it took to reach the ballpark value of either 40°F (4.4°C) or 200°F (93.3°C).

<th>Model</th><th>Ambient to Freezing</th><th>Freezing to Boiling</th><th>Ambient to Boiling</th>
ThermoWorks Thermapen3 sec [1 sec]3 sec [1 sec]3 sec [1 sec]
ThermoWorks RT-30131 sec [15 sec]24 sec [13 sec]18 sec [9 sec]
Polder Probe Thermometer16 sec [9 sec]32 sec [11 sec]36 sec [10 sec]
Taylor Dial ThermometerN/A80 sec [24 sec]70 sec [22 sec]
Taylor Candy ThermometerN/A45 sec [17 sec]33 sec [14 sec]

The Polder probe thermometer took around four seconds to produce any change in reading. For example, after taking the probe from ice water and plunging it into the boiling water, the base module continued to register 0°C for four seconds before changing to around 60°C. I think the slow update time caused the thermometer to lose a few seconds when it might have been internally registering boiling or freezing when the display was not.

Also, I donned heat resistant gloves while testing the Taylor dial thermometer in the boiling water because it required me to hold the thermometer at the edge to the hot pan. The radiant heat and steam would have been dangerous without protective gloves.

As you can see from the results, there is no comparison between the Thermapen thermocouple thermometer and any of the others. The speed is amazing - about one second for a ballpark value and two more for dead accurate. Three seconds is such a short amount of time that the time it takes to position the probe in the middle of the roast is almost the same time it will take for the Thermapen to take a reading. At $85, it is quite expensive, but compared to other thermocouple thermometers, this device is a bargain. It also had the highest maximum temperature (572°F or 300°C) which makes it capable of measuring the temperature of deep frying oils as well.

The Taylor dial thermometer was so slow that I was afraid that my gloves would start getting too hot for me to hold the thermometer. Its thick probe also makes it useless except for large roasts, except you'll need to open the oven door for a full minute before you could guess if your roast was between medium-rare or medium.

Conclusions
If you only have space for one thermometer, save up and treat yourself to the ThermoWorks Thermapen. It's fast. Very fast. It also comes calibrated with documentation to prove it. You can even calibrate it yourself (which is a feature missing in almost all digital thermometers)! The probe tip is very thin (like a thick needle), so measuring the temperature of a 1/4 in. steak is no problem at all. The temperature range is greater than most thermometers and it's also really easy to read. Recently, I've seen other companies selling unbranded thermometers of similar shape and design to the Thermapen, but at a discounted price. I've heard these thermometers may use thermistors and are much slower, so it's best not to take a chance and buy direct from ThermoWorks.

A fry/candy thermometer is also nice to have around if you do any confectionery or deep-frying in a pot.

The Polder probe thermometer is one of the lower cost probe thermometers I've seen and works like a charm. It is a bit slow so I always set it a few degrees below my target temperature. If you make roasts or anything that cooks for a long time (even bringing a large pot of water to a boil), this is your "fire-and-forget" thermometer.

The others? Save your money.

ThermoWorks Thermapen
[IMG]

ThermoWorks RT-301 Low Cost Pocket thermometer
[IMG]

Polder Cooking Thermometer
[IMG]

Taylor Classic Style Meat Dial Thermometer
[IMG]

Taylor Candy and Deep-Fry Thermometer
[IMG]
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elal1862
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PostPosted: Thu Jan 05, 2006 11:50 pm    Post subject: Reply with quote

I think you're completely missing the point on the "meat dial thermometer".
This little critter's sole purpose is to stick it into a large roast and keeping it there during the entire preperation (hint: oven & dishwasher safe).

Its continuous temperature measurement, makes the (sluggish) response time completely irrelevant. In fact, it beats all the others at this point: Split-second peek vs. >2 seconds while-poking-holes-into-your-juicy-roast, letting all those nice juices out.

And with a $10 pricetag, I can live with its single-purpose nature :-/
Just my $0.0268247
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PostPosted: Thu Jan 05, 2006 11:51 pm    Post subject: Reply with quote

My concern is the accuracy of the themometer. I like the ability to calibrate the device. I assume you use ice water and boiling water as your standard
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chennes
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PostPosted: Thu Jan 05, 2006 11:52 pm    Post subject: Reply with quote

Actually, it seems like most people who write up these "thermometer comparisons" stack the deck in favor of the Thermapen. While there is no doubt that is is a great thermometer when speed is the primary consideration comparing it to continuous-read thermometers (all three non-ThermoWorks models in this test) is not particularly useful. For example, I would never use it for a roast because it forces you to open the oven door. In this circumstance the Polder (with its alarm) or the Taylor classic dial are ideal choices. A better test of this type of thermometer is a durability and accuracy test, not a speed test. I personally like to use the analog continuous-read thermometers for oil and sugar because they give a better idea of the rate of change and they are easily mountable on the side of the pan. Speed is only important in a small subset of the things most people cook. I have no doubt that the Thermapen is a fantastic thermometer (I have yet to read a negative review) but most people would be better served by spending the $85 on a candy thermometer, a roast thermometer and a cheap "instant-read" thermometer. And spend the rest on ingredients Smile.
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PostPosted: Thu Jan 05, 2006 11:52 pm    Post subject: Reply with quote

My oven door doesn't have a window, so a dial meat thermometer doesn't work so good. I have a model similar to the Polder and I love it for just about everything.
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Michael Chu



Joined: 10 May 2005
Posts: 1631
Location: Austin, TX (USA)

PostPosted: Thu Jan 05, 2006 11:52 pm    Post subject: Reply with quote

re: meat thermometer usage

You might be right about the usage model of the Taylor Meat Thermometer. The thing is, years ago, my roommate put my meat thermometer in whatever it was he was baking/roasting and it broken because of the oven heat. When I got home, he gave me this thermometer as a replacement. No where could we find the words "oven-safe" and since the last Taylor meat thermometer died (unfortunately, I did not get to see what happened to it), we assumed the thermometer was NOT designed to be used continuously in the oven (which in my mind made it much less useful).

re: Polder probe thermometer
I love my Polder probe thermometer. Truthfully, I didn't use my replacement Taylor meat thermometer for very long because I decided to pick up a probe thermometer. It works great, but you do have to keep in mind that when the meat is at 135įF, the thermistor is not. Nevertheless, I've had great success using the Polder and tried to use it for everything and was quite satisfied. Once I started using a Therapen though, the Polder just sits on my refrigerator as a timer and the probe is wrapped up in a bag in a drawer to be used only when roasts or long barbeques are being prepared (not a common activity for me). Grilling, pan frying, baking (breads and some confections), and deep frying, I use the thermapen now. When I told Tina that I recommended people save their money up and buy the Thermapen, I thought she'd disagree (she's into practicality and efficient use of money), but she said I did the right thing. Watching me use the thermapen, she realized that there is no better instrument in our kitchen for measuring temperature and that it works on a whole different level than any of our other thermometers.

re: Thermapen
I think I might update the article with a section on anecdotal information on what I've found good and bad while using the thermometers (instead of a speed test). After using the Thermapen over and over, it's almost painful to go and use the other instant-read thermometers. If the thermometer is "accurate" (i.e. able to display the correct temperature as time approaches infinity), which all of my thermometers were, then the only thing to compare is speed. The faster the thermometer reacts and displays the reading, the faster YOU will be accurate. The other thermometers will lag - even a thermometer that is inserted into the food throughout the cooking process. For many this lag time won't end up affecting the quality of the meal, but if you're trying to cook a perfect steak everytime, it sure helps to have the best tools around. In addition, you'll find yourself measuring temperatures more often but taking less total time doing it. Often, I just want to know how a meal is progressing (what temperature the scaling milk is, how hot has the chicken breast gotten, what temperature is the oil, etc.) and I'll just pop the thermometer in, read, out, move on. I wouldn't even consider spot checks with any of my other thermometers. The thermapen has opened up a whole new usage model - simply by providing 1 second ball park readings. (This is the whole dial-up vs. broadband argument I had with my parents. They just read basic webpages and send e-mail. Do they need high-speed internet? Not really, but once I convinced them to get broadband - no easy task - they read and send e-mail much more often and actually like popping on and looking something up on the web. Their total data transfer isn't that much more than before, but now they really USE the internet. Having a thermapen gets you to start using a thermometer.) Of course, if you don't have the money - never will have the money - buy the Polder probe thermometer.

re: digital instant-read thermometers
Never leave them in the oven. LCD displays cannot survive oven temperatures. In fact, I don't like the digital fry thermometers either because they can't really survive the constant heat from the oil or candy either.

re: calibration
The Taylor dial thermometer is calibrated to boiling point only. The Thermapen is calibrated to both freezing and boiling. You should be able to alter the temperatures to match your particular altitude. The other thermometers cannot be self calibrated.
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PostPosted: Thu Jan 05, 2006 11:53 pm    Post subject: Reply with quote

Just a question:

Does the calibration function that uses boiling water as a control take atmospheric pressure into acount? This may not be an issue where I live, but I can see it being problematic in other regions or if you were to calibrate during a period of abnormal weather conditions.
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PostPosted: Thu Jan 05, 2006 11:53 pm    Post subject: Reply with quote

1) You've missed what is certainly the best value in digital probe thermometers - The [url=http://www.shopping.cutlery.com/Forms/shopping/*ws4d-db-query-Show.ws4d?*ws4d-db-query-Show***013334***-eProducts***-***shopping(directory)***?shopping/results(S).html]DeltaTRAK Professional Test Thermometer[/url]. It has a very fast response time, accurate readings, and it's less than half the cost of the Thermapen. This is the workhorse in my kitchen.


2) Most modern ovens will come back up to speed very quickly when you open the door. I think Harold McGee did some tests with this and found very little impact on a roast of opening the door a few times during cooking.
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ejm_repost
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PostPosted: Thu Jan 05, 2006 11:54 pm    Post subject: Reply with quote

Adam, we have an ancient electric oven (I'm guessing our stove/oven is from the 1970s) and the maxim of losing 25 to 50 degrees Fahrenheit every time the oven door is opened seems to be about right. I confess I haven't done any timing to see how quickly the oven temperature climbs back up those 25 to 50 degrees though.

To account for this heat loss, I always preheat the oven to 50F more than is called for in the recipe and then as soon as putting whatever it is in the oven, I turn the dial down to the recommended temperature. This ensures that the oven is basically at the correct temperature in the initial stages. (I'm baking bread mostly)
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PostPosted: Thu Jan 05, 2006 11:54 pm    Post subject: Reply with quote

I have a wireless digital thermometer and it's great. Stick it in the roast and walk away, alarm beeps when it's reached the right temp!
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PostPosted: Thu Jan 05, 2006 11:55 pm    Post subject: Reply with quote

Hmm, seems as though many people have many opinions. Some based on fact and some on personal choice. I've been through many thermometers over the years. Mainly because I either wear them out, loose them or snap them in half when I sit down (in my rear pocket eh).

Having to stick your roast and open your oven door is a fact of life. The trick is to open it, get your business done and close the door. Professional bakers have to open their ovens frequently to turn the cookie sheet 180 degrees. Get over it, it isn't a deal breaker.

Not too long ago I bought a Taylor wireless rig for 30 bux. I thought it was a good place to start. Worse 30 bux I ever spent, here's what I had to say about it:

http://www.cyberbilly.com/meathenge/archives/000671.html

That being said, Michael suggested spending the money for a good rig. Since I've already blown 30, at least. What's another 50? I NEVER regretted buying the thermapen and suggest others do the same. Besides, you can get them in colors, I got red.

Biggles
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PostPosted: Thu Jan 05, 2006 11:56 pm    Post subject: Reply with quote

Calibrating your thermometers to boiling and freezing points of WATER?! What kind of amateur engineers are you? Any true engineer would calibrate his thermometer to absolute zero... at least, that's what a real man would do...
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PostPosted: Thu Jan 05, 2006 11:58 pm    Post subject: Reply with quote

I agree with the previous post. You need to stick the thermometer in the meat, stick the whole think in the oven and let it beep at you when it hits the preprogrammed temperature. You canít go opening the oven all of the time poking it with a hand heald. It will take you 25% to 50% longet to cook and you could easily miss your mark!

I have used the Polder as well as a number of similar units (Williams and Senoma, Acu Rite and a few others). Not too sure who the real manufacturer is, but I doubt it is any of the previously mentioned. Basically, these all use the same controller and there are slight cosmetic differences. These units arenít bad. I have calibrated them against a thermometer used by a Hobart service engineer that is +/- 1F. The units I used that day were pretty close (something like +/- 3F).

To me, this wasnít the issue. The actual thermometer and lead wire absolutely suck. They canít take high humidity and I have even overcooked part of the lead wire in the grill. I probably have gone through 7 or 8 of them in 3 years. I use to use them in brewing (where I monitor the temperature of the mash over 1 hour). I submerge the thermometer part but keep the lead dry. This isnít good enough and on 2 occasions I killed the lead.

A friend of mine has a remote unit from Brookstones (not too sure who the ďmanufacturerĒ is). The lead is really sweet and about 5 mm thick (as opposed to the chinsey units with a 2 or 3 mm thick lead). I have not calibrated this unit, but it is really robust. It is also remote where you can have the thermometer in the grill and the controller inside.

Jim
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PostPosted: Thu Jan 05, 2006 11:58 pm    Post subject: Reply with quote

Jim again. The guy posting previous to me is correct - but I donít know if he/she knows it. Calibrating to boiling water is not exactly 212F (or 100C). It depends on the solid content of the water and the atmospheric pressure. If you are in Denver, you are not going to be close to 212F. Probably more like 205F.

Also, calibrating to freezing is stupid unless you are working in that temperature area. Who gives a hooie if you hit 0C if you want to make sure you are getting medium rare steak? It is in a totally different operating range and it is not a realistic calibration.

2 cents
Jim
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Michael Chu



Joined: 10 May 2005
Posts: 1631
Location: Austin, TX (USA)

PostPosted: Thu Jan 05, 2006 11:58 pm    Post subject: Reply with quote

re: calibration

If you know the temperature that water should boil at in your altitude, then when you calibrate, simply adjust the thermometer to read the appropriate temperature.

Also, calibrating to one known simply guarantees that at that particular temperature, the thermometer will report an accurate value. Usually, thermometers are calibrated to boiling, which is still pretty far from 135įF. Being able to calibrate to freezing as well, can fix any first-order inaccuracies in the thermometer.
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