A method for manufacturing hygristors includes preparing a hygroscopic gel containing a substantially greater proportion of carbon than the prior art, and milling the gel for a period of at least 24 hours to produce a smooth mixture. Substrates are dipped into the milled gel at controlled rates to coat the substrates. The coated substrates are cured under controlled temperature and humidity. Adhesion of the coating is improved by the smoother mixture resulting from the longer milling time, and by the differences in components of the mixture compared to the prior art. Long term stability and dynamic range are both increased.