Instrumentation & Measurement Magazine 23-4 - 81

Fig. 1. Detailed picture and schematic representation of the portable device, and block diagram of the MM.

The HYT 271 (already present in the previous version described in [12]) was kept as a reference for comparing RH
measurements (accuracy ±2% RH at +23 °C (0% RH to 90%
RH) and ±0.2 °C (0 °C to +60 °C) reproducibility ±0.2% RH and
±0.1 °C).
While the HYT 271 already includes integrated electronics
for the conditioning of data, both of the new sensors required
additional conditioning blocks to digitalize and process measured data. More in detail, the H sensor was conditioned
using a customized circuit, placing the sensor in a Wheatstone bridge, and amplifying the output differential voltage
using a rail-to-rail instrumentation amplifier (INA826). Differently, the RTD was conditioned directly using an integrated
converter from resistance to digital, specifically designed for
RTD (MAX 31865 from Maxim). An external resistor was used
to set the sensitivity of the RTD and ADC with delta-sigma
precision (15 bit) to digitize the ratio between reference and
RTD. In order to allow the communication between the different devices and the correct conversion of the data, both sensor
conditioning circuits were then interfaced with the PIC24
microprocessor.

Sensors Characterization
Sensors Static Characterization
The static characterization of the two sensors was performed
by evaluating the steady state response of the sensors to steps
of T or H, controlled using two climatic chambers: a Perani UC
June 2020	

150/70 for T sensor and an Angelantoni MTC120 for H sensor.
The reference sensors used were a Pt1000 and the HIH-3610-1
capacitive H sensor from Honeywell with a repeatability
±0.5% RH an accuracy of about ±2% RH, a stability of about
±1% RH and a linearity of about ±0.5% RH.
Temperature sensor static characterization was performed by
introducing sensors in the climatic chambers and reading measurement with two multimeters Tektronix DM2510G placed
outside. Using an IEEE 488 bus, the two DM2510G were connected to a PC, and the whole measurement procedure was
controlled with an own-written LabVIEW VI. Steps of 5 °C degrees were generated in a range of T between 10 °C and 45 °C,
allowing 30 minutes for each step to let the T reach a steady
value. Twenty measurements were collected for each condition to calculate the average and standard deviation.
The measurement performed with the novel RTD showed
a high accuracy, with standard deviation lower than the ones
obtained with the Pt1000 (Table 1). Furthermore, data fitting
showed a very good linearity (R2= 0.999972).
Humidity sensor static characterization was performed inside
a humidification environment in the climatic chamber, interfacing sensors with two multimeters (Tektronix DM2510G)
and a PC, where it was possible to read real time RH values,
thanks to a customized LabVIEW interface using the equation reported in the datasheet of the reference sensor from
Honeywell.
H steps of 10% RH were then generated in a range between
10 and 70% RH, keeping T constant at 20 ± 0.5 °C.

IEEE Instrumentation & Measurement Magazine	81
Instrumentation & Measurement Magazine 23-4

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