Things to know when choosing a temperature and humidity sensor

When choosing a humidity sensor, you must first figure out what kind of humidity sensor you need; what level of product your financial resources allow you to buy, and weigh the relationship between "needs and possibilities" so as not to act blindly. From our interactions with users, we feel that the following issues are worth noting.

1. Choosing the measurement range Just like measuring weight and temperature, choosing a humidity sensor must first determine the measurement range. Except for meteorological and scientific research departments, those who engage in temperature and humidity measurement and control generally do not need full-range (0-100%RH) measurement. In today's information age, sensor technology is closely integrated with computer technology and automatic control technology. The purpose of measurement is to control, and the measurement range and control range are collectively referred to as the use range. Of course, for users who do not need to engage in measurement and control systems, it is enough to directly choose a general-purpose hygrometer. The following lists some application fields with different requirements for the use temperature and humidity of humidity sensors for users' reference (see Table 1). Users propose measurement ranges to sensor manufacturers as needed, and manufacturers give priority to ensuring that the performance of users' sensors is stable and consistent within the use range, and obtaining a reasonable performance-price ratio is a mutually beneficial thing for both parties.

2. Choose the measurement accuracy. Like the measurement range, the measurement accuracy is also the most important indicator of the sensor. Every percentage point increase is a step up, or even a grade up, for the sensor. Because to achieve different accuracies, the manufacturing costs vary greatly, and the selling prices also vary greatly. For example, an imported cheap humidity sensor costs only a few dollars, while a full-range humidity sensor for calibration costs hundreds of dollars, a difference of nearly a hundred times. Therefore, users must tailor their products to their needs and should not blindly pursue them. Manufacturers often give the accuracy of their humidity sensors in sections. For example, the medium and low temperature sections (0-80%RH) are ±2%RH, while the high humidity section (80-100%RH) is ±4%RH. And this accuracy is the value at a specified temperature (such as 25°C). If the humidity sensor is used at different temperatures, its indication must also consider the influence of temperature drift. As we all know, relative humidity is a function of temperature, and temperature seriously affects the relative humidity in a specified space. Every 0.1°C change in temperature will produce a humidity change (error) of 0.5%RH. If it is difficult to achieve constant temperature in the application, it is inappropriate to propose too high humidity measurement accuracy. Because humidity also fluctuates with temperature changes, it will lose practical significance to talk about humidity measurement accuracy. Therefore, to control humidity, we must first control temperature. This is why a large number of applications are often temperature and humidity integrated sensors rather than simple humidity sensors.

In most cases, if there is no precise temperature control method, or the measured space is not sealed, an accuracy of ±5%RH is sufficient. For local spaces that require precise control of constant temperature and humidity, or occasions where humidity changes need to be tracked and recorded at any time, humidity sensors with an accuracy of ±3%RH or above are selected. Correspondingly, the temperature measurement accuracy of the temperature sensor must meet ±0.3℃ or above, at least ±0.5℃. The requirement of accuracy higher than ±2%RH is probably difficult even for the standard humidity generator that calibrates the sensor, let alone the sensor itself. An article from the Humidity Room of the National Center for Standard Materials Research believes that: "It is still very difficult for relative humidity measuring instruments to achieve an accuracy of 2%RH even at 20-25℃."