Monitoring moisture condition

Place your sensors where needed and physically revisit the subject site time and time again to take the readings of relative humidity, air temperature, surface temperature and surface EMM reading. This could be time-consuming. You could be limited regarding the times of day when inspection could take place. Sites might be distant from your office. To obtain reliable readings over a given period, consider setting up data loggers.

Figure 1: An individual sensor that measures air temperature and humidity may be placed where required, and the results downloaded to a laptop on site. Software is easily loaded and used. This sensor actually shows you the current reading on screen too. Equipment courtesy Gemini Data Loggers (UK) Ltd

Data loggers may be individual sensors or sensors incorporated into a logging system, such as shown in figure 2. They are used in many industries, e.g. food storage, archive storage.

Figure 2: The engine room of data logging for some years. Note laptop, the CD software for the logging unit, and the log unit with connections for a variety of sensors. A thermo-hygrometer of the type already described in this section is plugged into the logger, or may be used at the end of a lead. In front of the logger is a surface temperature sensor on a lead, and a plug with pins for recording standard resistance readings in wood or plaster

The kind of multi-channel data logger shown in figure 2 seems made for today’s laptops. The logger can collect 500 readings from each of its 6 channels and the computer software can automatically calculate dew point and specific humidity to feed into downloaded graphs or tables of results. Each monitoring exercise can be saved on the computer and the results directly fed into reports.The equipment shown above is still used by many surveyors, but we are now moving into the era of wireless monitoring.

The sensors can be set up as far as 10m from the unit. It is not cheap, and you really need 2 units because you will commonly be logging data in a space where moisture is generated as well as a space where resulting condensation has occurred. And when logging moisture data in room X (with a damp problem) it is useful to log data in a room not suffering a damp problem (i.e. a ‘control’ space).

As with all surveying equipment, basic equipment operation is less of a challenge than the analysis of the results. If you are assessing air conditions over time, you really must conquer psychrometrics (the study of air moisture).

Checking your humidity sensors

It is important to check that your electronic hygrometers remain reliable:

• Use a number of hygrometers, and give each a reference number or letter. You will then be able to identify more easily a rogue sensor that has ‘drifted’ i.e. become unreliable.
• From time to time check hygrometers by taking humidity measurements carefully in the office. Jot down readings from each and compare. If one sensor seems out of kilter with the others, don't use it.
• Alternatively you can prepare a humidity chamber.

How to make a humidity chamber Buy a plastic food box. Drill 3 holes into the side in which to fit humidity sensors. Pour in water and salt until some salt remains undissolved. Seal the lid. Fit 3 thermo-hygrometers. Check the humidity as it gradually stabilises. You should achieve a stable 75% RH at 20°C. With a calibrated sensor, your RH reading should be not too far from the expected 75%. Fit bungs into the sleeve holes until the chamber is needed.

Figure 3: Checking out electronic thermo-hygrometers in a humidity chamber and room air

Interpreting the readings obtained

The humidity chamber takes time to settle down and equilibrate, and the sensors themselves need time to settle down in their new environment and give a reliable RH reading.

After 30 minutes it looks as if the humidity in the chamber has nearly stabilised, and a relative humidity of 71% achieved, assuming that the sensors are reasonably accurate. Clearly sensor H below is the odd one out and should not be used again.

Thermo-hygrometers can be checked by just plugging in each sensor one by one and taking a reading – the instrument placed on your office desk. Such a check showed sensor H again out of kilter with the other 2 sensors.

Humidity reading with 3 sensors

Sensor	10 mins	20 mins	30 mins	60 mins	90 mins	2 hrs	Desk check
B	69.8/18.3	70/17.4	72/17.4	72/17.2	71.8/17.5	71.3/17.9	55.2/21
D	69.7/17.9	71/17.3	72/17.2	72/17.1	71.6/17.3	71.1/17.8	53.5/22.5
H	77.3/18.0	83.6/17.3	84.3/17	84.7/17.1	84.6/17.3	84.5/17.7	59.3/22.4

More recent monitoring products

• Wireless sensors can measure humidity and temperature as well as being able to screw in to a substrate to obtain a moisture meter style reading. Such products are of prime interest as a permanent installation for remote sensing in historic buildings, museums, food storage, archive storage facilities, etc. Using such technology, a property manager can be instantly warned of any unwanted change in air moisture condition or dampness of materials. Prompt action can save money and resources.
• Data acquisition gateways are connected to dedicated secure web servers. A gateway can act as a collection point for up to 150 wireless sensors up to 46m away. This is an extremely useful tool in flood remediation, so a property need not be visited in person just to take moisture and humidity readings.

HygroTrac systems can gather data and transmit through a connected LAN line or using telephone telemetry to transmit the results from a site direct to a dedicated web site where the conditions being monitored can be remotely viewed thousands of miles away. There can also be text alerts sent to your mobile phone that provides an alarm mechanism to the recording of excessively high or low readings, which may indicate a dramatic change in dampness and or temperature levels on the site being monitored.

GE Sensing produce the HygroTrac (See Fig 4 below) system with wireless remote sensors containing very long life batteries that can transmit data collected for several years to a gateway collection device. It can store data from several remote sensors on site for later downloading onto a computer with dedicated software to tabulate the data and produced graphed information in real time. Alternatively the data can be transmitted to the dedicated web address for onward transmission.

Figure 4: Courtesy of GE Sensing – HygroTrac system of remote data capture monitoring