Since a physical system can be inferred multiple times by measuring the temperature, the temperature has so far been the physical parameter that performs the most electrical measurements. When choosing a temperature sensor, you need to strike a balance between accuracy, durability, cost, and compatibility with the media being tested. For example, for high-volume or one-time inspection applications, small-signal transistors like the MMBT3904 are an attractive option due to their low cost. Although such sensors are relatively simple, precise temperature measurements are required to achieve accurate temperature measurements to eliminate effects such as series resistance.
The LTC2991 system monitor incorporates this sophisticated circuitry that turns a small signal transistor into an accurate temperature sensor. The device not only provides ±1°C accuracy when measuring remote diode temperatures, but also measures its own supply voltage, single-ended voltage (0 to VCC) and differential voltage (±325mV). While the LTC2991 appears to be designed for system monitor applications, its top-of-the-line performance makes it suitable for instrumentation applications such as the high-accuracy wet and dry meters described herein.
Dry and wet meter: far from being as "dreadful" as it sounds
A wet and dry meter is a hygrometer used to measure relative humidity. The hygrometer uses two thermometers, one dry (dry bulb) and the other covered with distilled water (wet bulb). Air is passed over the two thermometers using a fan or by a rocking instrument (as in a "hand shake dry table"). Then, a temperature and humidity map is used to calculate the humidity based on the dry bulb and wet bulb temperatures. On the other hand, there are many calculation formulas for this purpose. In the process of testing the circuit we used the following formula.
Figure 1 A dry and wet meter based on the LTC2991
Figure 1 shows a wet and dry meter based on the LTC2991. When connected to the appropriate input of the LTC2991, both transistors provide wet bulb and dry bulb temperature readings.
The above formula uses atmospheric pressure as a variable, where it is determined by a Novasensor NPP301-100 air pressure sensor (measured with channels 5 through 6 configured as differential inputs). At 100 kPa (the pressure on sea level is approximately 101.325 kPa), the full scale output is 20 mV per 1 V excitation voltage.
In addition, the LTC2991 can measure its own supply voltage, which is the same power rail used to excite the pressure sensor in our circuit. Therefore, it is easy to calculate a "proportional" result using a pressure sensor, thereby eliminating the error caused by the excitation voltage.
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