Calibration relative humidity

Moisture measurements are important in the process industries because moisture can foul products, poison reactions, damage equipment, or cause explosions. Moisture measurements include both absolute-moisture methods and relative-humidity methods. The absolute methods are those that provide a primaiy output that can be directly calibrated in terms of dew-point temperature, molar concentration, or weight concentration. Loss of weight on heating is the most familiar of these methods. The relative-humidity methods are those that provide a primaiy output that can be more direc tly calibrated in terms of percentage of saturation of moisture. 

Figure L Calibration plot for nitrosation potential showing amount of t -nitroso-thiomorpholine formed vs, the square of the nitrogen dioxide concentration. Standard conditions of 50%relative humidity, 25°C, and 1 L/min flow rate for 30 min...

Burettes calibrated at 20°C and 25°C deliver different weights of water for each 10 ml, when weighed with standard brass weights in air at 50% relative humidity (RH) at standard atmospheric pressure, as given below ... 

An environment should have appropriate controls for temperature, pressure, and relative humidity. For aseptic production, cleanroom conditions monitored for particles and bioburden contamination are necessary. Equipment must be validated and maintained with current calibration. Processes must be developed and validated to ensure the production of pure and consistent product. 

A diagram of a typical gas-phase (ozone-ethylene) chemiluminescent ozone analyzer is shown in Figure 6-10. The detector responds linearly to ozone concentrations between 0.003 and 30 ppm no interferences were initially observed. More recently, however, it has been established that, as the relative humidity goes from 0 to 60% and the temperature from 20° to 25° C, water vapor produces a small positive signal that results in an increase of about 8% in the ozone concentration measurement. This potential source of error can be minimized by using humidified, rather than dry, ozone in air streams when calibrating. 

Calibrated Dickson relative humidity and temperature recorder... 

NIR calibration curves were prepared by measuring the key compact attributes on rectangular surrogate tablets, prepared on the laboratory Carver press. The preparation of the calibration curve requires samples with attributes spanning the range of values likely to be encountered during roller compaction. Hence, drug content (for content uniformity), relative humidity... 

The standard conditions are (a) 23°C and 50% relative humidity, and (b) 27°C and 65% humidity, with the latter condition intended for use in tropical countries. Where control of temperature only is required, this is either 23°C or 27°C, and a further atmosphere where neither temperature nor humidity need be controlled is defined as prevailing ambient temperature and humidity. A note drawing attention to the atmosphere 20°C and 65% relative humidity which was used for textiles is no longer included. The normal tolerances are 2°C on temperature and 10% on relative humidity however, provision is made for closer tolerances, if required, of 1 °C and 5% relative humidity. This is a welcome change from previous conditions when the standard humidity tolerances of 5% and 2% were unreasonable in that 2% is virtually impossible to achieve and 5% debatable. It should be noted that 20°C is the usual temperature for calibration laboratories although in most cases the three degree difference will not have a significant effect. 

Reliable laboratory instrumentation is required to guarantee the safety of food products and enforce government regulations. Two main types of commercial instruments are available. One uses chilled mirror dew-point technology, while the other measures relative humidity with sensors that detect changes in electrical resistance or capacitance. Each has advantages and disadvantages. The methods vary in accuracy, repeatability, speed of measurement, stability in calibration, linearity, and convenience of use. 

Smith, PR. 1971. The determination of equilibrium relative humidity or water activity in foods A literature review. The British Food Manufacturing Industries Research Association, U.K. Stekelenburg, F.K. and Labots, H. 1991. Measurement of water activity with an electric hygrometer. Ini. J. Food Sci. Technol. 26 111-116. Stoloff, L. 1978. Calibration of water activity measuring instruments and devices Collaborative study. J. AOAC 61 1166-1178. 

An example is the sensor for methane (Stetter and Li, 2008), which also shows sensitivity to several other oxidizable species. The cell current is calibrated against the concentration of methane in air of 75% relative humidity. The key element in this sensor is the low volatility electrolyte, such as y-butyrolactone, propylene carbonate,... 

Verify the calibration of all instrumentation involved in monitoring the EtO cycle. Examples include thermocouple and pressure gauge calibration, gas leak testing equipment, relative humidity sensors, and gas chromatographic instrumentation. 

To obtain water vapor concentration, output signal intensity by TDLAS is calibrated under a well-controlled environment in variation of relative humidity and surrounding temperature. Figure 10 shows a relationship between water vapor concentration and output signal intensity detected by TDLAS. It is shown that the... 

Some products have specific control requirements for the environment, as indicated during development. Relative humidity, temperature, and even light-level restrictions are parameters that may need to be controlled. If there are control requirements, they should be indicated in the batch record. Documenting the actual results of environmental conditions should be a requirement of the batch record. This can be accomplished by either routine monitoring with a calibrated instrument and then recording these results on a log sheet or by attaching data generated by a recorder (e.g., strip chart, circular chart). 

A significant concern in the use of total radiation pyrometry is that it must be calibrated at the distance it will be from the source because of the influence of the atmosphere. Normal atmosphere contains a small fraction of carbon dioxide and water vapor (the latter dependent on the relative humidity, which varies with the day). When combustion is used for furnace heating (e.g. CH4+2O2 = 2H2O+CO2), water vapor and carbon dioxide are the predominant, atmospheric constituents. As... 

The performance of the sensors tested here can be snmmarized as follows Sensor A (TCD) was not sensitive enongh to detect H2 anywhere, even up to 7000 pL/L in the calibration cell. It was however sensitive to condensing water vapor, reading the eqnivalent of 3000 pL/L H2 at 25 °C and 100 % relative humidity. 

An example of a non-covalent MIP sensor array is shown in Fig. 21.14. Xylene imprinted poly(styrenes) (PSt) and poly(methacrylates) (PMA) with 70 and 85% cross-linker have been used for the detection of o- and p-xylene. The detection has been performed in the presence of 20-60% relative humidity to simulate environmental conditions. In contrast to the calixarene/urethane layers mentioned before, p-xylene imprinted PSts still show a better sensitivity to o-xylene. The inversion of the xylene sensitivities can be gathered with PMAs and higher cross-linker ratios. As a consequence of the humidity, multivariate calibration of the array with partial least squares (PLS) and artificial neural networks (ANN) is performed, The evaluated xylene detection limits are in the lower ppm range (Table 21.2), whereas neural networks with back-propagation training and sigmoid transfer functions provide the most accurate data for o- and p-xylene concentrations as compared to PLS analyses. 

Air Humidity Humidity probes need to be calibrated before use, and the absolute humidity (or both the relative humidity and temperature) needs to be recorded. If the probe temperature is below the dew point of the air in the process, then condensation on the probe will occur until the probe heats. 

---