Hardware Overview > Sensors and Probes > HCD-Sx > Humidity and Temperature Calibration/Adjustment

Adjustment consists in making the humidity and / or temperature values measured by an instrument agree as closely as possible with a number of known reference conditions. As-Left data is the result of an adjustment.

Temperature: the temperature signal of the device is adjusted based on the calibration points present in memory. The type of adjustment depends on the number of calibration points:

o1 calibration point: offset adjustment (equivalent to a 1-point adjustment).

o2 calibration points: offset and slope adjustment.

The HCD uses a 4th degree polynomial to change the raw data generated by the temperature sensor to a linear response.

oA 1-point adjustment changes the offset used by the temperature A/D converter. This is equivalent to changing the value R0 (resistance value of the RTD at 0˚C) in the 4th degree polynomial used by the HCD.

oA two-point adjustment changes both the offset used by the temperature A/D converter and the coefficient A (slope) of the 4th degree polynomial used by the HCD.

In addition to the user generated values, the HCD retains in memory the factory defaults for the value R0 and the coefficients of the 4th degree polynomial. The factory defaults cannot be changed by the user and are always available to return the device to its original condition.

Adjustment accuracy depends both on the number and on the distribution of the calibration points over the temperature range to be measured.

Humidity: the HCD adjusts the raw humidity data provided by the sensor so as to agree with the calibration points present in memory. The type of adjustment depends on the number of calibration points:

o1 calibration point: general offset adjustment (equivalent to a 1-point adjustment).

o2 calibration points: offset and slope adjustment.

o3 or more calibration points: offset, slope and linearization adjustment.

The HCD retains in memory two sets of tables. The combined data from both tables is used to change the humidity sensor raw data to a linear signal.

The accuracy of the user adjustments depends both on the number and on the distribution of the calibration points. The most accurate adjustment results are obtained by using several calibration points, equally distributed over the humidity range to be measured.

IMPORTANT:

oBe sure that the reference environment is stable and that the instrument to be adjusted has equilibrated with the environment.

oAlways verify that the instrument temperature measurement is accurate. If necessary, adjust temperature prior to any humidity adjustment.

oTo obtain the best accuracy when calibrating humidity, we recommend using a sequence of increasing humidity values. In this manner the effect on the calibration data of the humidity sensor hysteresis is practically eliminated (sensor hysteresis is a temporary positive shift in the sensor response at low humidity following exposure of the sensor to high humidity).