Take crushed ice (cube ice works too, but I'd suggest crush in a blender) and add it to a glass. Fill the majority of the glass with crushed ice. Add a small amount of tap water. You want the crushed ice consuming volume from the bottom of the vessel to the top. If the ice floats off the bottom, pour off some water or add more crushed ice. Let the temp stabilize for 2-3 minutes. Before taking the reading, ensure the ice is not floating. Again, pour off any water to get the ice to sit once more on the bottom. Insert the thermometer and gently stir the ice 'slurry'. Try not to touch the walls of the container. The goal here is to gently mix the solution so there aren't pockets of heat. Check the temp on the thermometer. After stirring for a few seconds the temp should read 0C or 32F. If it does not, refer to the manufacturers guidelines on how to adjust your thermometer. If you don't have a manual, the web is a terrific place to find one.

One of the important items often overlooked in brewing is the simple thermometer. Most dial indicator and electronic thermometers have a means to adjust them if they are calibrated incorrectly at the factory or are knocked out of alignment, but many times people leave it to chance that the manufacturer did this for them already.When I purchased a smoker grill, a friend advised me to calibrate the dial indicator thermometer on the grill before I used it, using an ice 'slurry'. My findings were that the thermometer was 10 degrees too hot, and needed adjustment. For this particular thermometer, the adjustment is a nut on the backside that had to be turned with a small wrench.With water and mash temps being critical for making good homebrew, spend some time testing your thermometer before using it, and calibrate it if it is off. Even if a thermometer is new, don't automatically assume the factory set the temps correctly. Even if they have great quality control, something could have happened in shipment to affect that. Same goes for existing thermometers, test them periodically to see if they are still accurate.There are many different types of thermometers, and most all of them can be used for brewing, but there are strengths and weaknesses to each, which I will outline belowthermometers are mechanical in nature, and this type of thermometer has a needle that swings across the face of the instrument to show the temperature, connected to a bimetallic coil strip/spring inside the housing. The adjustment for these is typically a small screw or nut on the back side of the thermometer that allows you to adjust the needle. By adjusting it you are winding the coil more loosely or tightly when you turn the adjustment.The benefits of this thermometer type are that they are simple to adjust, they are cheap, and widely available. The simple design does not require batteries or much concern in keeping it dry. The negatives of this type are that like anything mechanical it needs more care to keep it in calibration, and the 'read' on the thermometer is not as quick as today's digital thermos.' thermometers use a liquid inside a glass tube which rises as heat is applied to the 'bulb' end. I use the term mercury thermometer loosely, since I haven't seen a true mercury thermometer since my childhood. Mercury is toxic, and most, if not all liquid thermos today are fluid filled (not mercury). Again, these thermometers are cheap and readily available.For years I used a 'candy thermometer' from a discount store for keeping my temps in check. These generally float, which is a positive. They aren't the easiest to read at some angles, are relatively slow to read, and there is no ability to adjust them. If they are off, your only choice is to be aware of the 'offset' between the actual and the measured value, and compensate for that. An example would be if you tested alongside another calibrated thermometer and the value was high by 10 degrees, you would have to subtract that from each of your readings. These work by the expansion of the liquid as it is heated, which forces the liquid upward in the thermometer housing.Some fluid-filled thermometers are acrylic encased, and others are glass. This to me is a huge negative when working around food or drink. Broken glass is not an adjunct any of us care for in our beer, and aside from that if we bump this type of thermometer off a table it is going to shatter and make a mess that can injure someone.thermometers come in all shapes and sizes. They use thermocouples to read temperatures with relative speed. A thermocouple is generally two dissimilar conductors that form one or more junctions, and it is at the junctions where the temperatures are read. For most digital temp probes, this is at the very tip. The difference in electrical current is used to calculate the temp. Since everything is digital, most have a circuit that will allow you to calibrate the thermometer by manipulating a button, switch or screw. Some digital thermometers are water proof, some are water resistant, and some are not designed to be used around liquids. Do your homework to understand the differences, and look for one with an IPX4 rating for splash-proofness.You need at least a splash-proof one, since you may accidentally dunk it, or handle it with wet hands. Some more expensive models are sealed and allow immersion, but I see no need for this unless you want extra dunking insurance, since you only need to immerse the tip to get a reading.Some digital thermometers have replaceable probes, and others are built-in and are not. You also can get probes that can stay in constant immersion if you like, and you can get various lengths of probes for many models.The benefits of digital thermometers are a quick read with a nice digital display (some are even backlit) and almost all of them have a function to switch between F and C. The replaceable probe feature is nice, since the brains of the thermometer are in the base, and a faulty/damaged probe can be detached and replaced.The downside of digital is that they can be expensive if you want one with an extremely quick read. Additionally, they take batteries that have to be replaced. They are more prone to water damage than a dial or fluid-filled thermometer, since there are electronic components that make up the system. However, a good quality digital with splash-proof features is reliable and not terribly susceptible to water intrusion, but you could permanently damage one if you dropped it into near-boiling wort for a long period of time.thermometers are useful for applying to fermenters, and use liquid crystals to produce a color change at a particular temp. These are sometimes used for forehead thermometers, and aquariums/herpetariums. At least one company makes a 'thrumometer' that uses the same technology to measure temps flowing through an aluminum tube, so output of a liquid can be measured. These would not be useful for measuring mash or strike water temps, since they cannot take the heat. Additionally, if you do use them on fermenters be aware they are not very accurate and they do not hold up well to water. The good news is they are cheap, and cannot be broken (although water immersion will damage them to the point they stop working).thermometers are handheld and are the only 'non-contact' thermometer in the list. I find tons of uses for them elsewhere, but not in my brewing. This type takes its measurement by the amount of infrared radiation emitted by the object it is pointed at. I find most bargain IR thermometers to be highly inaccurate. If you run across one, point it at your hand or arm, and note the temp variance. My IR thermometer typically reads 85F on human skin, when that should be closer to 98.5F. That is quite a bit of variance. Additionally, with the steam involved in brewing, I am not sure how an IR thermometer would work. If anyone is using one for brewing with success, please post that in the comments.So which is best? Based on the pros and cons of each, I'd say for measuring water and mash temps your best bet is a good digital thermometer (something like a Thermopen or Thermopop), or if that is not in your budget find a good dial thermometer that can be calibrated and is easy to read. If you go digital, find one that is rated IPX4 or IPX7 for water resistance. Also consider the battery type and how readily you can get those batteries at your local store. An auto-off feature can help prolong the battery life.Regardless of which type you get, ensure that it covers the temperature range for brewing.Maverick smoking thermometer (note the high reading. The probe is not designed to read below 100 degrees). I included this to show an example with a detachable probe.So I have my thermometer now, how do I calibrate it?Once you have your thermometer dialed in, happy mashing!