Humidity measurement with capacitive humidity sensors between −70°C and 25°C in low vacuum

A Lorek

doi:10.5194/jsss-3-177-2014

Abstract

Abstract. At the German Aerospace Center (DLR), capacitive humidity sensors are used to measure relative humidity in experiments under extreme atmospheric conditions such as on Mars or in the coldest regions on Earth. This raises the question whether such experiments can be performed using low-cost humidity sensors with a tolerable measurement uncertainty. As part of the standardizing project SMADLUSEA (project no. SF11021A), nine capacitive humidity sensors (Sensirion SHT75) were investigated for pressure ranging from 10 to 1000 hPa (low vacuum) and temperatures from −70 to 25 °C. It has been shown that these sensors worked reliably and with reproducibly measured values over the entire investigated pressure and temperature range. There was no aging of the sensors observable. In addition to the known strong temperature dependency, the SHT75 also shows a pressure dependency below −10 °C. A characteristic curve for the SHT75 was calculated with an expanded uncertainty of 7% of the measured values. In conclusion, low-cost capacitive humidity sensors offer the option to obtain reliably measured values even under extreme conditions with comparatively little effort.

Highlights

What are the most useful sensor principles and their potential measurement ranges under Martian conditions? This was a fundamental question for the development of the in situ trace humidity measuring system called MiniHUM, designed for the ExoMars lander to measure the humidity of the nearsurface Martian atmosphere (Koncz, 2012)
This has a direct influence on the precision and accuracy of the relative humidity measurements
Pt100 and SHT75-sensors work reliably in the operating range given by manufacturer and the SHT75 even outside

Summary

Introduction

What are the most useful sensor principles and their potential measurement ranges under Martian conditions? This was a fundamental question for the development of the in situ trace humidity measuring system called MiniHUM, designed for the ExoMars lander to measure the humidity of the nearsurface Martian atmosphere (Koncz, 2012). What are the most useful sensor principles and their potential measurement ranges under Martian conditions? The coulometric and capacitive sensor principles were chosen to develop a lightweight and low-energy device. The coulometric sensors (Lorek et al, 2010), which we will not further described in this paper, measure the absolute humidity and have the potential to detect trace humidity below frost points of −100 ◦C. Our own measurements have shown low chemical activity at temperatures less than −50 ◦C which leads to a limited functionality for this type of sensor. At standard environmental conditions (25 ◦C and 1013 hPa), the capacitive sensors measure reliably in the range from 10 to 90 % relative humidity (Uw,i). There are only insufficient data, even for coulometric sensors, about the behavior in low vacuum (1 to 1000 hPa) and temperatures between −70 and 25 ◦C. While some of them have been investigated for temperatures down to −70 ◦C (Miloshevich et al, 2001; Hudson et al, 2004), there are not sufficient data on their behavior in a low vacuum especially down to 10 Pa

Methods

Results

Conclusion