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Laboratory Electronics

The relative humidity and air temperature measurement methods, additional calibration and devices maintenance

Basic measurement taking rules

For the correct relative humidity measurement it is necessary that the humidity sensor reaches beside humidity also the temperature of the air measured. For example, in typical room conditions (20 °C i 50 % R.H.) the temperature of the probe's inside is higher for 1 °C compared to the environment temperature, what causes for the relative humidity to do down to approx. 47%. The relations existing between different parameters that describe climate can be monitored using a program "HUMIDITY CALCULATOR LAB-EL". The equalization of the humidity sensor temperature with the environment temperature counteract the thermal inertia: sensor structures, probe casing and the air that exists in the measurement probe's head. The equalization of those temperatures is done by: heat radiation and moving the heat by exchanging air. The heat radiation meets high heat resistance of the air that surrounds the probe and it has a minimal influence on the equalization of temperatures. The air convection done in a way of physical phenomenon of the air thermal expansion (the warmer air moves up) causes slow air flow. The only effective way of exchanging the air surrounding a humidity sensor is the forced air flow around the measurement probe.

The measurement results stabilization is performed after the time, which depends on the changes in the measurement conditions and on the intensity of air exchange around the measurement probe. For small changes (up to few °C and a dozen or so of  % R.H.) and with the aeration approx. 0,2...0,5 m/s the stabilization of readings with a negligible error compared to the measurement accuracy is performed after approx. 1 minute. For big changes with lack of aeration the time of result settlement increases greatly and can last up to 15...30 minutes.

A very disadvantageous phenomenon takes place when the device is moved around (i.e. in the winter time) from a cold to a warm room and when in the warm room the dew point temperature is higher that the temperature in the cold room. Then a phenomenon of  water out dropping (retting) takes place on the surface of a cold humidity sensor and causes a sudden increase of humidity displayed by the meter. In such case, the measurement results are settled near the real humidity of the warm room not only after equalization of the sensor and air temperatures in the warm room but only after water evaporation from the sensor surface - what can be performed in about 60 minutes. That is why when transporting the device one should protect it (i.e. by a protective cover) from rapid cooling.

In case of placing the probe during the measurements on objects one should put it in a permeable to air place (i.e. on a stand) so the air can freely move around the probe. The probe should not touch any objects with its ventilation holes, because then the temperature and humidity of those objects influences in a not controlled way the measurement results. The probe during the measurement should be held as far as possible from a heat source (human body, heaters, sun radiation etc.) it should not be held in hands.

One should notify that in each room there are local differences and constant air temperature and humidity fluctuations in different places of the same room depending on i.e. opening the door, bringing the merchandise into the room, personnel entrance, switching off the machines, working heating or air conditioning system, sun radiation and wind direction outside of the building. In order to lower the climate parameters differences in a room on should introduce i.e. forced air flow.

When it the time then when one should assume for the measurement of humidity and temperature to be finished, if the expectation time for the measurement results depends on the external sensors? The measurement result changes should be observed in such case. If one can see that the averaged measurement results for each following 10 seconds change monotonically for not more than  0,1°C and 0,1% R.H., then the readings can be stabilized in the scope of measurement accuracy. If the moment measurement results change in both directions for the decimal parts and the average measurement result is constant, then the observed moment variations come from the local climate fluctuations in a room.

During humidity measurements in quick air flows (above 10 m/s) the measurement humidity can be underrated (even for few % R.H.). It is recommended then to place the probe in an additional casing which lowers the intensity of air flow.

Additional calibration - how often ?

The user of the measurement device (i.e. hytherograph) should pay attention to the necessity of periodical checks concerning the correctness of measurement readings.

The frequency of such checks depends on the following conditions:

  • how important it is for the devices to be exactly accurate (i.e. it is more important that the measurement accuracy is exact in the warehouse that stores medications than in the regular office space),
  • devices work conditions (more often, if the devices are exposed to extreme temperatures or humidity),
  • mechanical and chemical pollution, especially the presence of corrosion sensors (i.e. caustic substances vapors).
The more extreme conditions, the device aging process appears quicker (the chance of measurement error increases ). So:
  • if the work conditions are mild (i.e. living areas, office space) then the hytherographs should be accurate for a period of approx. 24 months (and after such period of time they should be checked and regulated in a laboratory),
  • if the work conditions are close to the measurement range limits (i.e. high humidity) and the device is only temporarily used in such conditions (for the majority of time is used in mild conditions) then the hytherographs should be checked every 18 months (and after such period of time they should be checked and regulated in a laboratory),
  • if the work conditions are close to the measurement range limits (i.e. high humidity) and the device is constantly used in such conditions then the first check should be performed after 6 months and the next after 12 months,
  • if the device work in the extreme conditions in the presence of high pollution then the check and regulation should be performed every 6 months,
  • if the control measurements are done by any other different device show errors then the check and regulation should performed immediately.
Checking the readings, maintenance and additional device calibration are normal exploitation procedures and because of it they are not under warranty and they are payable.