The thermometer displays 24°C. The hygrometer shows a relative humidity level of 58%. The CO2 controller reads 1048 ppm. It looks like everything is fine. The plants are comfortable and in the best conditions. Meanwhile, the leaves are shrivelling and mould is setting in. What is happening? It could be that the gardener’s measuring devices give false readings. Could the calibration of some meter and control be wrong?

The goal of a measuring device is to give true values. This is achieved by adjusting such device against error proof standardized measurement tool. Inaccurate calibration causes false readings, sometimes on crucial values. This causes deplorable situations that are often difficult to identify.

A good calibration is important!

Plants are somewhat picky about their needs for ideal climatic conditions. Temperature wise, a plant is comfortable at +/- 1°C from its ideal temperature. If a plant requires a temperature of 24°C, the plant will be comfortable between 23 and 25°C. As soon as the temperature reaches out these limits, the plant becomes uncomfortable, even at 22 or 26°C. A precise thermometer is essential for gardening. As for the relative humidity, the hygrometer’s precision is just as important. A hygrometer with a precision of +/- 10% from the real humidity level can causes important problems that are difficult to figure out. For example, the plant could dry up or stop transpiring without allowing time to understand that the problem is associated to the relative humidity in the garden. This reasoning is applicable to CO₂ as well. A controller drifting 100 ppm or more under the real concentration might result into an excess of CO₂ enrichment. This wasted CO₂ has no benefits to plants; even worse, if the concentration is too high, the plants will stop transpiring and the leaves will start curling on themselves. The last thing one wants to happen in an indoor garden is the plants being affected by a negative climate and have difficulties to grow. In gardening, as well as many other fields, we often obtain better results using superior quality tools. The climate measuring devices such as the thermometer, the hygrometer and the CO₂ concentration controls contribute to harvest maximum yields, as long as they provide good measurements and work according to them, of course! The first step towards measurement precision is the equipment’s calibration on a regular basis!

Temperature

Figure 1 gives a good example of a calibration related problem. Four thermometers, three digital, though at the same location, give different results: 22°C from the first, 22.8°C from the second, the third, a digital thermostat, has two different measurements: 22,78°C and 21,67°C, and 22.5°C for the fourth. Although located at the same spot, these thermometers are as precise as their sensor and programming (converting electrical measurement into a temperature value). As they say: “You get what you pay for’’. However, any of these devices could be utilized as long as they’re properly and regularly calibrated with a reliable thermometer.

Figure1: Readings of 4 temperature measuring devices commonly used in indoor gardening. Which on is right?

Such precise reference thermometers are expensive. The mercury type is the most accurate (Figure 2). Often yellow, it should be more then 12 inches long (30 cm) and have a precise graduated scale (Figure 3). It should come with a Laboratory’s Standard Calibration Certificate. One can be bought from a good laboratory equipment retailer. Once the garden’s thermometer difference to the true value is known, one can note it for future reference and correct the reading by adding or subtracting either by mental calculation or by adjusting the thermometer accordingly.

Figure 2: min-max thermometer

Figure 3: Laboratory thermometer with calibration certification

What does it change to know the real temperature?

Whether read on a thermometer or an adjusted climate control, 1 or 2 degree difference from true temperature will generally not have a big impact on plants. However, it will make all the difference in the world on a demanding plant by providing the best growth and developing conditions.

Relative humidity

Again, four different readings of the actual relative humidity are shown in figure 4. The two at the right are specialized horticultural controllers. They have been factory calibrated. From the left, the second unit, a needle hygrometer (58%) is far from being precise with a difference of over 15% compared to the closest one at 43%. Getting accurate relative humidity measurements is difficult. A psychrometer is the tool used to determine with precision the specific and the relative humidity (Figure 5 and http://en.wikipedia.org/wiki/Psychrometry). An affordable unit retails in the range $100/$150 and +/- 3,5% precision is the best one can expect. For higher precision, one must pay much more. As shown, the needle hygrometer is not expensive but is not worth much either. Indoor gardening demands the best possible relative humidity control and measuring devices to create the best artificial conditions for plants, which are very sensitive to variations.

Figure 4: From left to right: four hygrometers readings with respective measurements of 43%, 58%, between 35 and 40% and last 41%. Which one to rely on?

Figure 5a): Motorized psychrometer like the ones found in weather station

Figure 5 b: A “sling” psychrometer with a wet bulb thermometer.

Horticultural climate controls with sensors cost as much(or more) then residential hygrometers.Simply because they are more precise. These controls are factory calibrated and verified with laboratory quality devices. Once again, it is better to verify indoor garden’s hygrometer with a reliable device. To satisfy the cultivated plants’ needs, it is better to invest on a good digital hygrometer or a factory calibrated digital climate control to get a good relative humidity measurement and a good control of the humidification or dehumidification in the garden.

CO₂ concentration

A NDIR (Non Dispersive Infrared) CO₂ sensor is delicate. It will easily loose its calibration when shocked like in transportation or at the installation. A NDIR CO₂ sensor has a calibrated light source and mirrors easily affected by the environment. As time goes by, the sensor drifts away from its “zero” and the measurements are not true to the real value. In addition, the CO₂ and the humidity in the air create carbonic acid that damage the sensor’s mirrors. Also in presence of humidity, the usage of evaporated sulphur damages the sensor. Finally, the room’s temperature where the sensor is installed also has an impact on the displayed measurement. One should calibrate the CO₂ sensor in conditions similar to the ones where the sensor will be used. According to our experience, a NDIR CO₂ sensor should be calibrated every three months.

Figure 6: Three horticultural CO2 concentration controls, using sensors from the same manufacturer, give three different readings. The first two from the left just got calibrated and the third one has been calibrated about six months ago. The difference between the two first controllers is only 3 ppm, which is normal for sensors that has a precision of +/- 75 ppm from the true value. The controller on the right has drifted away at least 100 ppm since its last calibration: its time to calibrate it!

When to calibrate?

It is recommended to calibrate the controller and its sensor when purchased and at the beginning of each crop to insure a precise control of the CO₂ enrichment. Not only to avoid wasting but insure an ideal CO₂ concentration for a fast and healty growth of the plants. All used bought CO₂ controllers should also be calibrated before installing in the garden. In case of doubt on the calibration’s precision, whether its due to a long utilisation or a non-utilisation, a shock or any other reason, better calibrate it again!

How to calibrate?

Which reference is safe enough to rely on to calibrate a CO₂ sensor? The ideal calibration method is to expose the sensor or the control to a gas mix with a known and certified CO₂ concentration. Some sensors have a built-in calibration circuit in which one vaporizes a known concentration gas mix but most of the horticultural controls used in indoor gardening do not have such a circuit. One method is to place the controller in a sealed bag to protect it against human breathing, remove the air from the bag and replace it with a known CO₂ concentration mix (figure 7). Then, just follow the user’s manual calibration instructions. Like every precision measuring devices, one have to remember that a CO₂ sensor will be inaccurate at its measurement scale limits, for example around 0 and 5000 ppm. To obtain a calibration as precise as possible, the ideal way is to calibrate the controller with a gas mix that has a concentration close to the one desired in the garden (See Plug’N’Grow’s calibration kit).

Figure 7: Calibration gas examples: to the left, a certified 1755 ppm gas bottle allowing one calibration to controls with a built-in calibration circuit; on the right, a certified gas mix with a concentration of 1000 ppm.

Figure 8: Three controls from different manufacturers, using a NDIR sensor from the same manufacturer, have been calibrated within 30 minutes with a gas mix of 1000 ppm of CO2 and installed in the same spot for the picture. Observe two different results: the middle horticultural controller has a difference of 71 ppm with the other two. The design of horticultural controls vary with the manufacturer. Despite the calibration, the programming and the electronics must well translate and display the real CO2 concentration value.

Why not calibrating CO₂ sensors outdoor?

For many years, CO₂ controls manufacturers have recommended the outdoor calibration method, for want of anything better. This method suggests an average outdoor CO₂ concentration from 350 to 400 ppm, which is never truly right for all locations depending on the area’s various CO₂ emitting sources. For example, vehicles reject approximately 20% of their exhaust in CO₂. Imagine the CO₂ quantity rejected during rush hour in the cities, and this is without taking in consideration the CO₂ rejected from factories. One can also think about those wood, fuel oil and gas heating systems. Depending on the time of the day, the outdoor real CO₂ concentration in the air is between 400 and many thousands ppm. One also have to consider that in cold weather, the difference between the outdoor temperature and the garden’s temperature will falsify the calibration as the electronic circuit operates in different conditions. So it is better to avoid this method.

Why not calibrating with another controller’s reading?

In scientific calculation, errors add up. When a sensor with a precision of +/- 75 ppm is calibrated against the reading of another sensor with the same precision, in the worst case, the difference to the real measurement could reach 150 ppm, even if the sensors are placed in a bag. It is also easy to influence the sensors only by breathing around them. A human breathing can contain a CO₂ concentration between 30 000 and 40 000 ppm. It is also important to consider that the affordable CO₂ monitors (600-1000$ : Figure 9) are built with the same sensors as the ones used in controls and do not provide a better precision. These sensors also need to be calibrated before each use to insure a good measurement. A control calibrated in an atmosphere with a known CO2 concentration is just as good as reference than a portable battery operated monitor.

Figure 9: A portable battery operated CO2 measurement device

A good calibration is important!

Plants are somewhat picky about their needs for ideal climatic conditions. Temperature wise, a plant is comfortable at +/- 1°C from its ideal temperature. If a plant requires a temperature of 24°C, the plant will be comfortable between 23 and 25°C. As soon as the temperature reaches out these limits, the plant becomes uncomfortable, even at 22 or 26°C. A precise thermometer is essential for gardening. As for the relative humidity, the hygrometer’s precision is just as important. A hygrometer with a precision of +/- 10% from the real humidity level can causes important problems that are difficult to figure out. For example, the plant could dry up or stop transpiring without allowing time to understand that the problem is associated to the relative humidity in the garden. This reasoning is applicable to CO₂ as well. A controller drifting 100 ppm or more under the real concentration might result into an excess of CO₂ enrichment. This wasted CO₂ has no benefits to plants; even worse, if the concentration is too high, the plants will stop transpiring and the leaves will start curling on themselves. The last thing one wants to happen in an indoor garden is the plants being affected by a negative climate and have difficulties to grow. In gardening, as well as many other fields, we often obtain better results using superior quality tools. The climate measuring devices such as the thermometer, the hygrometer and the CO₂ concentration controls contribute to harvest maximum yields, as long as they provide good measurements and work according to them, of course! The first step towards measurement precision is the equipment’s calibration on a regular basis!