Everything is Relative … Mainly Humidity

Who doesn’t know the problem? If you put several different hygrometers in a humidor, each instrument shows a different value. Frustrating. The precise measurement of relative humidity is difficult, however we would still like to attempt to explain it as simply as possible. The term humidity refers to the moisture content of a substance.

Like every other substance, air also has a limited capacity. The hotter the air is, the more water it can hold. If, at a given air temperature, there is so much water (vapor) that it can no longer be absorbed any more, we speak of saturation or maximum humidity. Below this saturation value, moist air cannot be discerned from dry air by eye. Above the saturation value, the excess water content precipitates in the form of fine water droplets (fog).

And with hygrometers, too, if you buy cheap, you have to buy twice.

It is relative humidity that we’re interested in; this is what we want to measure. There are several different methods of measurement. Here we are only taking into account the ones that have a certain relevance to cigar storage. Analog hygrometers have an indicator to which a measurement mechanism is attached at the back. Changes to humidity cause movement in the mechanics, which move the indicator via the dial.

Metal-Paper Coil Hygrometer

Measurement errors of up to 20 percent are the norm. Ninety-five percent of all humidors have a standard metal-paper coil hygrometer. Not because it is the best, but because it is the cheapest. A rolled metal strip coated in plastic is used as the measuring instrument, which reacts to a change in humidity by a change in length, thus moving the indicator. Metal-paper coil hygrometers are slow and imprecise. Another problem is the calibration of these mostly very small hygrometers. My recommendation: Get rid of them.

Hair or Fiber Hygrometers

If real hair or fiber hygrometers are precisely calibrated, the measurement error is about +/- three or four percent. With this instrument, the hair or fiber bundle reacts to the change in humidity values by a change in length: as the hair stretches, a simple pointer on a dial indicates the change in tension.

Photo: Marc André

Even though hair tension hygrometers often top the tests, I wouldn’t recommend using them in a humidor. The change in linear expansion of the hair is degressive, which means that at low humidity the linear expansion is greater than at high humidity.

This can also be seen on the percentage scale of the hygrometer: the increments between 10 and 20 percent are markedly smaller than the ones between 70 and 80 percent. The smaller the increments, the more imprecise, however, the measurement.

In addition, hair tension hygrometers need to be regenerated every four to six weeks so that the hair doesn’t become brittle (wrap them in a moist cloth for a few hours).

I recommend a fiber hygrometer. This works like a hair tension hygrometer, but the measuring medium is not hairs but fibers.

The advantage here is that the linear expansion over the entire measuring range is almost linear, which can be seen in the nearly identical distances on the scale.

Psychrometers

The psychrometer has almost been forgotten. What’s interesting about it is that the humidity is deter mined by means of two thermometers via the evaporation of water. A psychrometer consists of two thermometers; on one of them, a sock or wick covers the mercury end, which is dipped in distilled water. The evaporation of the water from the wick lowers the temperature, which thus shows a lower temperature than the dry thermometer.

Here, the temperature difference is the measure for the relative humidity, which can be read on a psychometric chart (see photo). Its somewhat bulky structure is more suited to larger humidor cabinets or walk-in rooms. These devices may seem antiquated, but they are very precise and don’t need to be calibrated. The most precise psychrometers are the so-called aspiration psychrometers. These psychrometers have high accuracy, of below +/- one percent, but cost about 1,500 to 2,000 euros. The most common electronic measurement methods are those using capacitive or resistive sensors.

Resistive Hygrometers

Photo: Marc André

The resistive hygrometer is the one most frequently encountered. A water-absorbing layer (e.g. aluminum oxide or hygroscopic polymer) is used as a sensor between two DC electrodes.

When the ambient humidity rises, the hydrophilic material binds more humidity, the electric resistance lessens and more electricity flows. This change is shown on a display as humidity in percent. The favorable price is the advantage. But this is offset by some disadvantages:

• Over time, deposits on the sensor limit accuracy
• High fluctuation variability of up to 15 percent

Capacitive Hygrometers

This – very precise – measuring method uses an electrically non-conductive material (dielectric) between two electrodes. When the humidity changes, the characteristics of the dielectric also change (e.g. the length), thus changing the electrical capacity of the sensor. An electronic circuit can convert this change in capacitance into a measurable voltage.

The advantage of such sensors is the long-term stability and the hardly upward-changing drift. In addition, the measurement exactitude is constantly at values of around 1.5 to 2 percent. The disadvantage is the price. Depending on the version, these sensors are priced between about 50 to 100 times more than the resistive sensors. And with hygrometers, too, if you buy cheap, you have to buy twice.

Information:

The following weights refer respectively to one cubic meter (33,814 US fl.oz.) of air.

Maximum Humidity
At 32°F, air can absorb a maximum amount of 0.17 oz. of water; at 68°F, it is 0.61 oz. This temperature-dependent amount is called maximum humidity.

Absolute Humidity
Absolute humidity denotes the concentration of water content in ounces. That is, it also indicates the volume of how much water is actually dissolved in the air.

Relative Humidity
Relative humidity (abbrev.: RH) denotes the degree of saturation in the air in percent. At 50% RH, the air contains half of the maximum amount of water vapor volume it can hold (depending on the current air temperature).

Two Examples
At an air temperature of 68°F and absolute humidity of 0.46 oz. the relative humidity is 75.1%. Why? The air, at this temperature could absorb 0.61 oz. of water (= maximum humidity). At an air temperature of 32°F and absolute humidity of 0.13 oz. the relative humidity is likewise 75.1%. Why? At this temperature, the air could absorb 0.17 oz. of water (= maximum humidity).

This article was published in the Cigar Journal Summer Edition 2014. Read more