An investigation on the effect of humidity on the zero signal of a strain gauge measuring system

Univ. Sci. 23 (1): 129-139, 2018. doi: 10.11144/Javeriana.SC23-1.aint Bogotá original article An investigation on the effect of humidity on the zero signal of a strain gauge measuring system K.M. Khaled1, *, G.M. Mahmoud1 Edited by Juan Carlos Salcedo-Reyes () 1. National Institute for Standards (NIS), Giza, Egypt, P.O.136, Code 12211. * Received: 17-07-2017 Accepted: 30-03-2018 Published on line: 11-05-2018 Citation: Khaled KM, Mahmoud GM. An investigation on the effect of humidity on the strain gauge measuring system, Universitas Scientiarum, 23 (1): 129-139, 2018. doi: 10.11144/Javeriana.SC23-1.aint Funding: N.A. Electronic supplementary material: N.A. Abstract Humidity is one of the most important ambient factors influencing torque calibration readings. The present study addresses the effects of relative humidity on the readings of reference torque transduction and amplification devices. The results revealed a linear effect of increasing humidity on the readings of a DMP40 torque amplifier and a torque transducer at different temperatures. Furthermore, humidity effects were smaller on the DMP40 amplifier than on the torque transducer, even at different temperatures. The results also showed that the summed effect of humidity changes on torque calibration readings of the amplifier and the torque transducer, when assessed simultaneously, but with each device under different controlled humidity conditions, is greater than the observed effect on the readings when both devices were subjected to humidity changes together. Therefore, it is recommended to test the effect of humidity in setups where these two devices are both under common humidity conditions. This is likely to lower the uncertainty parameter of the obtained torque calibration readings and is a useful consideration for inter-laboratory comparisons and performance testing. Keywords: Strain gage; torque amplifier; relative humidity; ambient temperature; international system. Introduction Torque measuring systems mainly consist of a torque transducer and a measurement amplifier and may be used inside laboratories or as part of outdoor setups [1]. In either case, knowing the influence of environmental factors such as relative humidity (RH) on the measuring instruments is central to understanding how they perform [2, 3]. The investigation of the effect of relative humidity on the zero signal of a torque measuring system is relevant to determine how it contributes to the error component and drift of the measuring system readings [4]. This can also explain differences between Universitas Scientiarum, Journal of the Faculty of Sciences, Pontificia Universidad Javeriana, is licensed under the Creative Commons Attribution 4.0 International Public License 130 Humidity on the strain gauge measuring calibration measurements carried out under laboratory and on-site industrial conditions [5, 6]. The influence of RH on torque transducers has been considered earlier for inter-laboratory comparisons using a climate-controlled cabinet [7]. Current torque transducers are isolated from the environment such that the effect of RH on the sensing element is minimized [8]. Nonetheless, changes in RH can considerably affect transducer’s sensitivity [9]. Thus far, complicated and extensive methodologies have been used to measure humidity effects, but Bruge [8] proposed a simpler approach measure this effect on torque transducers inside calibration laboratories, with fewer resources and at a lower cost. The effect of humidity on the zero signal of a reference torque transducer in known to vary from + 3.3 to – 9 nV/V % RH, and the influence of humidity on a DMP40 measurement amplifier, studied at 22 ◦ C, was found to vary from 0.27 to – 1.8 nV/V % RH [9]. In the present work, the effects of relative humidity on the zero signal of a torque transducer and a DMP40 measuring amplifier were studied under increasing RH conditions at four different constant temperatures. The readings of these two devices were taken when subjected to these experimental conditions either individually or together. Materials and Methods Apparatus The following equipment was used in this experimental work: · A Votsch Stable Climatic Chamber (SCC) (model VC34060,. Germany) with internal dimensions of 0.7 x 0.8 x 0.8 m, a temperature range of 283 to 363 K, and an RH range from 15 % to 95 %. · An HBMDMP40 amplifier (model DMP40S2,. Germany) with the following settings: filter of 0.2 Hz Bessel, signals reading set to “absolute”, the range of measurement was 2.5 mV/V and the excitation voltage was 5 V. The excitation of the DMP40 amplifier was 220 VAC with 50 Hz. · An HBM BN100A stable bridge calibration unit (Germany). The DMP40 amplifier and the BN100A were connected with a shielded cable. · An HBM reference torque transducer (model TN, Germany), with capacity of 1 kN·m, IP: 20, and an accuracy class of 0.05). Universitas Scientiarum Vol. 23 (1): 129-139 http://ciencias.javeriana.edu.co/investigacion/universitas-scientiarum 131 Khaled & Mahmoud · Two sets of humidity and temperature ALMEMO sensors (model FHAD3Rx, Germany). One set was placed inside the SCC and the other outside. The sensitivity of each set was of 0.1 % for RH and ± 0.1 K for temperature. Experimental procedure In the first measurement round, the DMP40 amplifier was placed outside the SCC while the reference torque transducer was placed inside the SCC. The SCC conditions were set to 40 % RH and temperature of 15 ◦ C for 12 hours, followed by a programmed RH step-wise increase of 5 % every 4 hours until reaching 80 % RH. This procedure was repeated at 22 ◦ C, 31 ◦ C, and 40 ◦ C with an initial RH of 35 %. The following measurement cycle was done as already explained, except that both the DMP40 amplifier and the torque transducer were placed inside the SCC. Finally, the third measurement cycle proceeded as in the previous two, but the DMP40 amplifier was placed inside the chamber and the BN100A unit outside. The BN100A calibration unit was set to an excitation voltage of 0 mV/V, which is the nearest point to the observed torque transducer zero signal (0.012130 mV/V). Warm up times for the DMP40 amplifier and the BN100A calibration unit were recorded, and so the time needed for the DMP40’s low pass filter to work. All of the experiments were conducted under controlled environmental conditions with stabilities of ± 1 K and ± 5 % RH outside the chamber, and ± 0.1 K and ± 0.5 % RH inside the chamber. All measurements were repeated three times. These measurements were employed to assess the effect of the evaluated experimental conditions on the RH readings, reflecting the difference between the DMP40 amplifier readings at each RH value of and the readings taken at initial RH conditions, namely the response difference (nV/V) for the three measurements. Results and Discussion The effect of humidity on the readings of the torque transducer and the DMP40 amplifier was proportional to the increase (...truncated)