Measuring the temperatures of hot fluids can be easier and safer using the BruProbe, a make-it-yourself digital thermometer with the temperature sensor mounted in the end of a 30-in. brass wand. This article provides step-by-step instructions for making one at home.

People have been brewing beer for centuries. Long before glass carboys and air locks, the ancients enjoyed fermented beverages. These libations were prepared using crude methods by today's standards, yet they must have been palatable because kings and commoners alike continued to make them.

Today, home brewers continue these traditions with (in most cases) less crude methods. Although commercial brewers can afford the highest of high-tech equipment, home brewers usually can't or choose not to. Most home brewers, as with most artisans, have their own adaptations of commercial equipment or ingeniously self-designed gadgets.

One ever-present factor in brewing, especially with all-grain recipes, is hot water and the need to monitor its temperature. Although commercial brewers can afford to install dedicated thermometers on each vessel, most home brewers have one or two thermometers that they use to measure mash, sparge, and chilled wort temperatures.

Every home brewer who has measured the temperature of near-boiling water in a deep pot on a cold January morning knows the routine; reach down in the steam until your knuckles burn then retract a bit and read the thermometer through the steam. I don't know about other people, but my X-ray vision doesn't cut the steam, and I don't care for scalded knuckles. This aversion to steamed knuckles prompted me to design the BruProbe.

OVERVIEW

Large-scale integration techniques have enabled chip designers to provide complicated systems on a signal chip. The BruProbe uses a digital voltmeter chip (Teledyne, part number TC7106A) in conjunction with a commercial temperature sensor (National Semiconductor, part number LM34CH). The voltmeter chip includes the voltage reference, analog-to-digital converter, and the electronics required to drive an LCD display. The temperature sensor integrated circuit (IC) is contained in a three-lead transistor case, and the chip is laser trimmed so no adjustment circuitry is required.

The thermometer consists of two assemblies: the probe and the display case. The probe has one electronic component. The display case contains the main electronics.

PROBE ASSEMBLY

The probe consists of a length of 7/32-in. o.d. brass tubing, a 5/16-in. brass acorn nut, and the LM34 temperature sensor. Drill out the acorn nut as deeply as possible with a #13 (0.1850-in.) drill bit. This step provides a snug fit for the top of the sensor case, which has a nominal dimension of 0.1865 in. (the maximum dimension for the LM34 sensor is 0.195 in., so if you happen to get a big one you will have to experiment). Next, drill the nut with a 7/32-in. (0.2188-in.) drill bit, creating a counter bore. Be careful to drill on center because the diameter of the hole will be close to the diameter of the hex of the nut. You could use a larger nut, but a larger nut will increase the mass of the probe and increase the time it takes to get a reading. (If you can avail yourself of a small lathe you could also make this piece out of solid brass rod.) The brass tubing outside diameter was chosen to be the same size as the outside diameter of the sensor; the cap can then fit over both the sensor and the end of the brass rod (see Figure 1). Cut the tab on the sensor and trial fit the pieces.

Figure 2 shows a schematic of the probe wiring. The 2-k-ohm, 1/8-W resistor should be soldered to the lead of the sensor as close as possible to the case. The sensor lead connected to the case can be discarded because the brass tubing serves as the ground return.

The next steps require soldering, and because this device will be used in the preparation of food - beer, that is - use lead-free solder. Solder a length of wire to one end of the brass rod. Insert the sensor into the acorn nut. Tolerances in the sensor case and the drill bit can cause a loose fit. If the fit is not snug enough in the smaller diameter to press-fit the sensor, solder the sensor's case to the nut. This should be done as quickly as possible to avoid thermal damage to the sensor. Connect the remaining sensor lead and the resistor to two lengths of #28 insulated wire, and cover the connections with heat shrink tubing up to the case of the sensor. Be sure the insulation on these wires will withstand boiling temperatures. Next, thread the wires into the rod end opposite of the wire soldered previously, slipping the acorn nut over the end of the rod and solder the nut to the rod. Again, use as little heat as possible.

The probe can be tested using a voltmeter. Connect a 9-volt battery and the meter as shown in Figure 3. Set the meter to read in a 2-volt range. Multiply the meter reading by 100 (for example, 0.624 volts = 62.4 °F), which will give you the current temperature of the probe. In use, you will find that you can actually measure the difference in temperature between the top and bottom of a vessel as small as a coffee mug.

The LM34 sensor is available in several ranges, each designated by the suffix. The sensor described above is the LM34CH, which has a temperature sensing range of 0-250 °F. The wand assembly from JB Distributing (Table I) comes with the LM34DZ, whose range is 32-212 °F. For those who prefer centigrade, use the LM35 sensor (range: -10-+90 °C). That completes the probe.

DISPLAY CASE ASSEMBLY

The circuit could be constructed using wire-wrap techniques if care is taken in the analog sections. The best way to build the circuit is with an etched circuit board. A kit that includes an etched circuit board and detailed instructions for construction is available from JB Distributing.

S1 in Figure 4 is the range switch. In the up position, the meter range is 1.999 volts (which equates to 0-199.9 °F), and in the down position 19.99 volts (0-1999 °F). This switch also controls the display of the decimal point, so that the display reads in degrees Fahrenheit rather than volts. These ranges allow the display of the entire 0-250 °F range of the sensor IC.

The schematic shows two potentiometers. VR1 provides calibration of the analog section for passive component tolerances. VR2 provides calibration of the high range display. Calibration is simple: get a small-mouth vacuum-insulated (Thermos-type) bottle, three-quarters fill with ice, and fill the rest of the way with water. Stir a bit and let stand for a couple of minutes. The temperature of the water will be very close to 32 °F.

Insert the probe into the container, set S1 in the up position, and carefully adjust VR1 until 32.0 is displayed on the LCD display. Although 32.3 or 31.7 could be more accurate than any bimetal or liquid thermometer you've ever used for brewing, adjust it as close as you can. Next, set S1 in the down position and adjust VR2 so that the display reads 32.0.

Since building the original unit, I have added a rotary switch to the basic design, and I can monitor the temperature at up to six places in my gravity-feed brewing system (including the hot-liquor tank and several points in the recirculating mash unit).

If you have any questions, feel free to contact me through BrewingTechniques or on the internet at mac@mv.mv.com.

Robert McIlvaine, Jr., an electrical engineer with 15 years' experience dealing with automated process control and computer-integrated systems.