Accuracy of the PEM2

When creating our own temperature and relative humidity data logger, the PEM2, our priorities were ease of use, accuracy and long-term stability. The design of the PEM2 was informed by our fifteen years of experience with environmental monitoring in libraries, archives and museums. 

Technology and Accuracy
The technology and the calibration of the PEM2 make it a highly accurate monitoring device. The PEM2 measures temperature with a digital sensor that demonstrates consistent linearity across our specified operating range and we guarantee its accuracy to ± 0.5^oC. The PEM2 measures relative humidity with a thin-film polymer capacitance sensor and we guarantee accuracy within ±2% RH in the 10%-90% RH range and ±3.5% RH at extremities. These technologies surpass the requirements for recommended environmental monitoring conditions in storage and display areas. 

NIST- Traceable Calibration Source
We can guarantee the PEM2's measurements because we individually calibrate each PEM2, checking the performance of the temperature and humidity sensors with a five-point calibration routine. The machine we use for calibration is NIST-traceable, which means it is calibrated to a NIST standard (the equivalent of an “official” source of time). We also chose a calibration machine that uses one of the most accurate methods of calibration, a Thunder Scientific two-temperature two-pressure humidity generator.

Hysteresis
By calibrating each PEM2 to five different humidity values and a varying pattern of low and high RH conditions, we are able to check for hysteresis behavior in each humidity sensor; if a sensor demonstrates hysteresis and it can not be corrected, we do not use that sensor.  

Humidity Compensation at Extreme Temperatures
Electronic humidity sensors respond differently when they are exposed to extremely low or extremely high temperatures. This is why not all electronic data loggers are suitable for use in cold vaults or freezer storage. The effect of low temperature will be to cause inaccurate humidity readings well beyond the stated accuracy range. However, it is possible to program an offset into the firmware of the data logger to compensate for the sensor behavior at extreme temperatures. The PEM2 includes this humidity correction firmware and can therefore give reliable humidity readings even when exposed to extreme temperatures.