Scipy FFT Frequency Analysis of very noisy signal

You are right there is something wrong. One needs to explictiy ask pandas for the zeroth column:

Hn = np.fft.fft(Moisture_mean_x[0])

Else something wrong happen, which you can see by the fact that the FFT result was not symetric, which should be the case for real input.

Seems like @tillsten already answered your question, but here is some additional confirmation. The first plot is your data (zero mean and I changed it to a csv). The second is the power spectral density and you can see a fat mass with a peak at ~0.3 Hz. I 'zoomed' in on the third plot to see if there was a second hidden frequency close to the main frequency.

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
from scipy import signal

x = pd.read_csv("signal.csv")
x = np.array(x, dtype=float)[:,0]
x = x - np.mean(x)
fs = 1e2

f, Pxx = signal.welch(x, fs, nperseg=1024)
f_res, Pxx_res = signal.welch(x, fs, nperseg=2048)

plt.subplot(3,1,1)
plt.plot(x)

plt.subplot(3,1,2)
plt.plot(f, Pxx)
plt.xlim([0, 1])
plt.xlabel('frequency [Hz]')
plt.ylabel('PSD')

plt.subplot(3,1,3)
plt.plot(f_res, Pxx_res)
plt.xlim([0, 1])
plt.xlabel('frequency [Hz]')
plt.ylabel('PSD')

plt.show()

Hn = fft.fft(x)
freqs = fft.fftfreq(len(Hn), 1/fs)
idx = np.argmax(np.abs(Hn))
freq_in_hertz = freqs[idx]
print 'Main freq:', freq_in_hertz
print 'RMS amp:', np.sqrt(Pxx.max())

This prints:

Main freq: 0.32012805122
RMS amp: 0.0556044913489