How do pointer thermometers and hygrometers work?

Pointer thermometer.

The principle is an on-site detection instrument for measuring low and medium temperatures. Pointer thermometers can directly measure the temperature of liquid, vapor and gaseous media in the range -80 +500 in various production processes.

The hygrometer measures the complete relative humidity.

The principle: because the temperature of the wet bubble thermometer is wrapped in cotton yarn, the lower end of the cotton yarn is immersed in water, and the evaporation of water makes the temperature indication of the wet bubble thermometer always lower than the temperature indicator (air temperature) of the dry bubble thermometer, and the temperature difference is related to the speed of water evaporation (that is, the relative humidity at that time) According to the readings of the two thermometers, the relative humidity of the air can be found from the table or curve.

Bimetal thermometers work on the principle that two different metals expand differently when the temperature changes. The main element is a multi-layer sheet of metal made up of two or more metal sheets stacked on top of each other. In order to improve the sensitivity of temperature measurement, the metal sheet is usually made into a spiral coil shape.

When the temperature of the multi-layer metal sheet changes, the individual layers of metal expand or contract by different amounts, causing the spiral to roll up or loosen. Since one end of the spiral coil is fixed and the other end is connected to a freely rotating pointer, the pointer indicates the temperature on a circular index scale when the bimetal feels a change in temperature.

Electronic humidity sensor method (there are many types, but this one is more modern).

The humidity is determined according to the conductivity of the moisture sensitive resistor or moisture sensitive capacitor at different humidity.

Household pointer thermohygrometer calibration:The temperature can be calibrated with the air conditioner, put the thermometer in the room, induct it for about one to two hours, and then see if the temperature of the pointer is the same as the temperature of the air conditioner, if the difference is about one degree is normal, the humidity calibration wraps the entire temperature and humidity meter with a wet towel shirt, wraps it for an hour, and then sees if the humidity pointer is at 95% If it is not adjusted to 95%, it can be used.

Meaning:

When reading the temperature of the thermometer, the temperature of the thermometer becomes larger upwards with each small call. When reading humidity, read the relative humidity from the bottom of the dial. The relative humidity decreases from left to right, at 2% per cell.

The hygrometer does not show the degree but the percentage, and the indoor humidity of normal life should be kept at 45%-70% left and collapsed right.

How does an electronic hygrometer work:

Because the temperature sensing bubble of the wet bubble thermometer is wrapped in cotton yarn, the lower end of the cotton yarn is immersed in water, and the temperature indication of the wet bubble thermometer is always lower than the temperature indicator (air temperature) of the dry bubble thermometer due to the evaporation of the water.

Absolute Humidity vs. Relative Humidity:

Absolute humidity: The pressure p of the water vapor in the air is called the absolute humidity of the air

The humidity of the air can be expressed by the density of the water vapor contained in the air, that is, the mass of the water vapor contained in the unit volume of air Because it is difficult to directly measure the density of water vapor in the air, and the pressure of water vapor increases with the increase of water vapor density, the humidity of the air is usually expressed by the pressure of water vapor in the air, which is the absolute humidity of the air.

How to choose a thermohygrometer:

Due to the use of different temperature and humidity measurement principles, the temperature and humidity instruments are diverse, and the actual application environment and requirements of the user should be considered when selecting, such as range, output and display, installation method, sampling method, gas type, material and structure, control and monitoring requirements, environmental hazards, etc. In addition to this, it is important to pay attention to factors such as cost performance and maintenance workload:

Robust: Hygrometer sensors and housings are designed to withstand condensation, drying, extreme temperatures, dust, chemicals, or other contamination.

Cost-effective: When choosing a temperature and humidity instrument, you can't just consider the lowest level, but you should choose it based on comprehensive performance and performance. This includes **, life, maintenance, and calibration costs.

Validation: The method of calibration and whether it is easy to make is considered, even if you don't need high-precision results. Instruments that are easy to calibrate both on-site and in-situ will save you effort.

Quality reliability, average life: when the quality is not good to judge, you can start from the overall impression, inspect the quality appraisal and factory standards, investigate the history, reputation, market share and application of the manufacturer, brand-name products are better than general products, professional manufacturers' products are better than those of other manufacturers, and it is also a good way to consult other users.

Adaptability: When the use is not a single one, the adaptability of the instrument should be considered. Interchangeability: It is generally desirable that the thermohygrometer can be used interchangeably or other probes to match your main unit. Maintenance: Investigate the time requirements for regular cleaning, renewal and replacement of the hygrometer.

Here we introduce a common hygrometer called a psychrometrophic hygrometer.

There are two thermometers with the same specifications, and the difference between the two thermometers is that the bulb of one thermometer is wrapped with soaked white gauze, which is called a wet bulb; The other bulb is in direct contact with the air and is used to determine the air temperature, which is called a dry bulb. When the relative humidity in the air is lower than 100%, the moisture will evaporate and absorb heat, making the temperature of the wet bulb lower than that of the dry bulb. The drier the air and the more intense the evaporation, the greater the temperature difference between the wet and dry bulbs, and the humidity of the environment can be measured by the temperature difference between the two.

Although the principle is very simple, the temperature difference between the wet and dry bulb represents different humidity at different air temperatures, and the relationship is very complicated. There will be a humidity comparison table on the household psychrometer, from which the relative humidity can be read, and some are equipped with a calculation plate at the bottom, and the relative humidity can also be obtained by turning the calculation disk.

The accuracy of the electronic sensor is also high, the humidity is 2%rh, and the temperature is .

The pointer type is generally mechanical and not very accurate; The digital type is mainstream and very accurate, but it is generally dozens of times better than the pointer type.

The digital thermohygrometer measures the temperature of moist air when it reaches saturation, which is a direct result of thermodynamics, with high accuracy and a wide measurement range. Digital thermohygrometers can be used in laboratories, quality control, air conditioning systems and printing processes.

The pointer type is generally mechanical and not very accurate;

The digital type is mainstream and very accurate, but it is generally dozens of times better than the pointer type.

The accuracy of the electronic sensor is better, I hope I can give you advice!

The accuracy of the electronic sensors is also high.

Humidity 2%RH temperature

The moisture-sensitive element of the mechanical pointer humidity meter is made of hair type and metal hairspring coated with polymer hydrophilic plastic material. The principle is a length variation, that is, the element can change its length with the change of air humidity, and the displacement caused by the length change is used to drive the pointer shaft, so that the pointer moves on the dial, so as to realize the humidity measurement function.

A physical property analyzer that measures the humidity of gases. Humidity indicates the amount of water vapor in a gas, and there are two ways to express it: absolute humidity and relative humidity. Absolute humidity refers to the absolute amount of water vapor in a gas, and the most commonly used unit is gram 3.

At a certain temperature and pressure, the water vapor content per unit volume has a certain limit, which is called the saturated water vapor content.

Relative humidity refers to the ratio of the absolute content of water vapor in the gas to the content of saturated water vapor in the same volume of gas at the same temperature and pressure. Hygrometers are used in a wide range of applications, such as in ultra-pure metal smelting, textile processing, production processes such as paper and dyeing, as well as food storage and meteorological measurements, where they are often used to measure or control the humidity of air or industrial process gases.