SU800850A1 - Air relative humidity sensor - Google Patents

Description

(54) RELATIVE AIR MOISTURE SENSOR

one

The invention relates to hygrometry and can be used in automatic measurements of relative air humidity at positive and negative temperatures.

A device is known which contains a stabilized alternating voltage source, an amplifier at a transistor, a humidity sensing element connected between an alternating voltage source and an amplifier input through a diode, a logarithmic attenuator and a measuring device connected in parallel to the output of amplifier 1.

The exponential change in the resistance of the sensing element of humidity is converted into a linear change in the current of the microammeter due to a logarithmic attenuator. However, the accuracy of the logarithm is not great due to the absence of a direct connection between the current through the sensing element and the logarithmic attenuator. The presence of a transistor between them increases the temperature error of the device.

The closest in technical essence to the proposed is

a relative air humidity sensor comprising a source of alternating voltage, a ceramic moisture sensitive element connected to its output connected to the input of a logarithm equipped with an operational amplifier, the negative connection circuit of which contains logarithmic diodes I.

0

The disadvantage of the device is the presence of a temperature coefficient of a cerium titanate sensor equal to 0.3% / ° C.

Taking into account that the effect of temperature on the gain factor is small, the total temperature coefficient of the known hygrometer is 0 ,. Therefore, the use of a hygrometer without a thermal compensation chain when changing

0 - air temperature leads to temperature error, which reduces the accuracy of measuring relative humidity.

The purpose of the invention is to improve the accuracy of measuring the relative viscosity of air.

The goal is achieved due to the fact that a divider is inserted between the input of the operational amplifier and the logarithm of the hands of the qimi diodes

voltage, the shoulders of which contain diodes, g included in opposite directions parallel to each other.

The drawing shows a diagram of the device.

The device contains a stabilized source of alternating voltage 1, a logarithmator 2, built on an operational amplifier 3, covered by a negative feedback circuit 4 containing logarithms; diodes 5 and b, connected in parallel, the voltage divider 7 whose shoulders contain diodes 8, 9, 10 and 11, connected anti-parallel and ceramic moisture sensitive element 12 connected between the output of the source 1 of alternating voltage and the input of the logarithm 2

The device works as follows.

The conductivity G of the ceramic moisture sensitive element 12 is related to the relative air humidity by an exponential relationship. The current arising in the ceramic moisture sensitive element 12 under the action of the voltage of source 1, the proportions of the conductivity of the ceramic moisture sensitive element 12. Negative feedback circuit 4 keeps the input voltage of the logarithm 2 close to zero, and the current through diodes 8 and 9 are equal to the current through the ceramic moisture sensitive element 12.

The current through diodes 5 and 6 is equal to the sum of the currents through diodes 8.9 and 10 and 11. If diodes 8, 9, 10 and 11 have the same characteristics, the current through diodes 5 and b is equal to twice the current through diodes 8 and 9. Output voltage logarithm 2 is equal to the sum of the voltage drops across diodes 8, 9, and 5.6 and is related to the current through the diodes by a logarithmic dependence. Thus, the output voltage of the logarimeter 2 is associated with a linear relationship with relative humidity. The output voltage of the logarithm 2 decreases with. increase in temperature. The conductivity of the ceramic moisture-sensitive element 12, and hence the current, grows with increasing temperature, which leads to an increase in the output voltage of the logarithmator 2, i.e. Thermal compensation occurs. Full thermal compensation is achieved when the condition is met.

St -Shch-%

where, is the sensitivity of the output voltage of the logarithm 2 to temperature and relative humidity;

5, the sensitivity of the ceramic performance,

moisture sensitive element to temperature and relative humidity.

0 This condition is provided by selecting the type of diodes 8, 9, 10 and 11 of the voltage divider 7. For example, replacing diodes 10 and 11 with lower-impedance ones as compared with diodes 8 and 9 and poz will increase the current through the diodes.

5, b, thereby increasing the sensitivity of $ 2 in order to withstand the condition of thermal compensation.

The device improves the accuracy of measuring the relative humidity of air, which increases its efficiency when used in hydrometeorology due to more accurate weather forecasting, reduces the amount of waste in the industry when monitoring the process conditions for humidity in furnaces and chambers and increases the storage time of equipment in rooms with controlled humidity.

Claims (2)

1. U.S. Patent No. 3,196,682, Clause 73-336.5 1966. her 2. Humidity. Principles and methods of measuring humidity in gases, M., Hydromerodizdat, 1967, V.1, p. 283 (prototype). 12 T L- ±.