Photo 1. The G-Cap sensor calibration workstation includes a humidity- controlled glove box; a QuadTech 7600 LCR meter (far right) for measuring capacitance; and a PC for data gathering, process control, and analysis

Secondary relative humidity (RH) sensors have become almost universally used in OEM applications where RH sensing is required (see sidebar "Some Sensor Basics"). Although an RH sensor might seem a simple device, its proper operation is highly dependent on careful humidity calibration. Unfortunately, few systems integrators are equipped (or inclined) to calibrate RH sensors on the production line where the sensor is added to the final product. The customer usually either bypasses the calibration step and risks winding up with a finished product that does not meet specifications, or designs a dedicated system to test
individual devices. These tests take time
and add cost to the finished product, and their complexity makes them unsuitable for use on the production line. One way to solve these problems is to buy factory-calibrated RH sensors with such slight part-to-part variations as to require no subsequent calibration by the customer.

G-Cap Capacitive RH Sensors

Photo 2. The capacitance of the sensor is measured and compared to the desired value. The die is then trimmed with a punch to match the value.

General Eastern's G-Cap sensors have individual capacitance variations of only 1 pF and part-to-part capacitance variances of 1%, which translate into a 3% variance in RH measurement. The typical industry values of 10%, by way of contrast, translate into an RH variance of ~30%. The degree of interchangeability is such that if the electronics are designed properly, the sensor can be installed directly into a transmitter circuit without post-installation calibration.

The sensors are essentially capacitors consisting of a film with a metalized layer deposited on both sides. Water molecules absorbed by the film change its dielectric constant, so the amount of water absorbed produces a corresponding change in capacitance that allows the sensor's use as an RH detector. While capacitance is the fundamental measurement parameter, the thickness of the metalized layer and its permeability are also crucial. Together, these measurements produce a clear picture of device tolerances, and hence the ultimate performance of the sensor.

Photo 3. The capacitance of the finished sensor is rechecked inside the test chamber by means of a remote test fixture. The data are logged to a PC for process control and analysis.

For the sensor to work, the film must have known and well-characterized hydroscopic characteristics. After the coating process, the sensor die is subjected to a known RH environment (see Photo 1). Its capacitance is measured by a QuadTech 7600 LCR meter (see sidebar "Verifying Capacitance"), and the data are compared to desired values and then used for trimming (see Photo 2). The individual capacitors are then separated, integrated into mechanical packages, and placed in the final test chamber.

Capacitance is measured at this stage as well to ensure that nothing has changed during packaging. A remote test fixture that interfaces with another QuadTech 7600 allows measurements to be made in a highly controlled environment (see Photo 3). Measurement data are displayed on both the instrument and the computer monitor, and logged to a PC for process control and later analysis.

Some Sensor Basics There are basically two types of RH sensors: fundamental (or primary) and secondary. Primary RH sensors perform direct measurements on some physical phenomenon or property. They are based on well-defined thermodynamic principles such as water vapor condensation as a function of air temperature and atmospheric pressure. They are very accurate and easy to understand, but also expensive. Continuously controlled chilled mirror sensors, aspirated psychrometers, electrolytic sensors, and gravimetric instruments are examples of fundamental sensors. A secondary sensor measures the change that takes place in its own characteristics as a result of exposure to a phemomenon or property. Although easy to use and less expensive, secondary RH sensors require the person designing them into a system to know both how they work and the basic principles of humidity. For accurate performance, they must be calibrated. This type of sensor can respond to either relative or absolute humidity by detecting: A change in the length of a material (the operating principle of the extension hygrometer) A change in weight (the principle of the absorption hygrometer) A change in either impedance or capacitance (true of most electronic hygrometers)

Verifying Capacitance The 7600 inductance-capacitance-resistance (LCR) meter shown in Photo 1 can measure and display any 2 of 14 impedance parameters simultaneously. Up to six different tests, each with its own conditions and limits, can be run in sequence with a single push of the start button. Unique to the meter are swept parameter measurements, graphical or tabular, for verification of component and material response to changes in AC test frequency, voltage, or current, without the need for complex programming or an external controller. Test setups can be stored and recalled either from internal memory or from standard DOS-formatted 3.5 in. floppies. The front panel controls can be locked out, with password protection. Measured data can be stored on a 3.5 in. floppy and transferred to a PC for data reduction and analysis. The NIST-traceable 7000-09 Cal Kit permits on-site calibration. Other features include: Automatic calculation and display of instrument accuracy for selected conditions. Self-test routines that verify crucial instrument operation at powerup or when selected from menu 0.05% basic measurement accuracy 7-digit measurement resolution 10 Hz to 2 MHz frequency range 289,900 programmable frequencies Menu-driven interface to IEEE 488.2, RS-232, handler, printer port, and 3.5 in. floppy drive Data storage of 40,000 measurements/disk 0ºC º­p;50ºC operating temperature; -40ºC to 71ºC storage temperature 45% ±5% RH humidity range Meets MIL-T-28800E, Type 3, Class 5, Style E & E

For more information about the G-Cap sensor, contact Paul Savery, General Eastern, 20 Commerce Way, Woburn, MA 01801; 617-938-7070, fax 617-938-1071.

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