Wiring diagram of YSI 709B temperature probe

Digitec 5810 thermometer

The Digitec 5810 digital thermometer uses YSI 700-Series temperature probes. These thermometers were wonderful: they used bright red LEDs instead of an LCD display, and they could measure changes of a hundredth of a degree. Unfortunately, YSI sold off its temperature measurement division many years ago, and replacement probes are no longer available. Even worse, some companies are marketing 400-series probes (which have only two conductors instead of three) as replacements for 700-series probes. However, they are not really compatible. Thus, it's necessary for users to build replacements when the probes wear out. As the probe ages and corrodes, it becomes more inaccurate.

All 700-series probes contained two thermistors. The circuit is shown below. When specifying replacements, 1% precision components are recommended. They must be sealed against moisture with a minimum of material to ensure rapid response. The standard probe for the Digitec 5810 thermometer was the YSI-709B. This probe contains a 6000Ω and a 30000Ω NTC thermistor (measured at 25°) and uses a ¼-inch 3-conductor phone plug. The internal resistors inside the thermometer act as a voltage divider that forms a linearizing circuit with the thermistors, which greatly increases the accuracy. The 5810 digital thermometer has two different range settings, which allow the same probe to be accurate over different ranges by switching in different internal resistors.

From the above circuit, it is clear that the resistance of 1−2 is the sum of 1−3 and 2−3. Below is what I measured for the resistance vs. temperature of an authentic YSI 709B probe. For whatever reason, it is easier to get axial leaded thermistors in higher precision than SMT ones. It is important not to overheat them when soldering. Use zinc thermally conducting paste when making a probe and avoid air spaces, which will slow the response.

One replacement probe is shown above. The thermistors were epoxied at the end of a short piece of gray ¼" PVC tubing, which was then sealed with heat-shrink on the cable end. Another probe was made inside a small piece of aluminum.

The 5810 uses three 1.2-volt 1/2 D type Ni-Cd batteries (e.g., Mouser 547-PS-1/2D-T). They are automatically charged up whenever the thermometer is plugged in. Almost no one ever installed batteries because of their expense, and because you had to remember to unplug the thermometer after 12 hours to avoid heating and overcharging, which would greatly shorten their life.