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Choice of datalogger |
I divide data loggers according to their suitability for four different tasks:
Quality control of museum climate usually requires measuring points that are fixed for years. This is best achieved by wiring in transmitters for temperature, relative humidity and maybe light. The word transmitter is industry jargon for a box on the wall which includes the sensors and the necessary electronics to massage the sensors' output into a robust signal that can be transmitted over relatively long distances, up to 1 km, without degradation. These transmitters are widely used in industrial buildings. Service will be reasonable and obsolescence will come gradually. The installation price is high, typically 1000 dollars per measuring site. A central computer will digest the data, store it and issue the necessary alarms. The operating cost is low and the benefits of instant, centralised reaction to problems is a great advantage. Such arrangements are therefore optimistically entitled Building Management Systems
There are at least two manufacturers offering radio transmitting sensors with a centrally placed receiver/datalogger. Such an installation should give exactly the same performance as the hard wired type described above, with the advantage that one can adjust the position of the sensors if the exhibition changes. It may anyway be necessary to adjust the position of the sensors to get the signal through to the receiver. The range is less than that of wired systems, though repeaters can be installed.
I advise caution in choosing such systems, particularly if you are far from the company headquarters. The hard wired systems have been developed by large, conservative engineering companies. The radio systems are at present the preserve of small companies operating with admirable entrepreneurial energy and about one engineer/programmer each. After a flirt with such a system my own institution is continuing with wired sensors, even in our historic buildings.
A hybrid alternative, suitable for isolated sites, is to use a data logger which serves a cluster of sensors by wire and sends the data by standard radio telephone. The Campbell Scientific logger, described later, can do this.
Data loggers for packing with art in transit are usually designed to reveal their sufferings at the terminus, rather than squeal in pain at anxious moments during the trip. The minimal data logger for such adventures is represented by the Orion Tinytalk (There are so many of this type of data logger that the biggest difficulty in making them, apparently, is choosing a name).
This device is a 35 mm film can which contains both sensor and electronics. In some models the sensor sits outside on the base, or hangs on a cable. It measures with a precision limited to 256 steps, which is usually enough. It is programmed and relieved of its data by taking off the cap and sticking in a connector which goes to a computer. It is cheap, about $100, and easily transportable by ordinary post. It is excellent for any climate measurement service where instant knowledge of the climate is not necessary, or would be useless. Typical uses would be monitoring conditions under transport and evaluating the natural climate in a church with a view to designing a heating system.
Here in Denmark, we send these little loggers out in pairs (T and RF) in a small perforated box. They are used to measure climate in churches and historic houses. Once a month we send a new set and an envelope for returning the previous set. All data collection and calibration is centralised, ensuring, or at least permitting, good quality control of the performance of the data logger. There is enough memory for hourly readings, with one month reserve for delays in the post or forgetfulness in sending replacements. A vast memory is not necessarily an advantage. Data should be gathered and interpreted reasonably frequently so that the measuring frequency, or the placing of the logger, can be adjusted as the climatic picture builds up.
There are a great many transportable loggers of this kind, because they are used in much bigger industries, such as the transport of ice cream, but many are expensive in relation to what they can do, which is not much. They sit and measure one, two or maybe four data points and wait for a computer to come and suck the data out once in a while. A few have a display showing the current measurement, which is of some psychological value to the local operator, who feels more involved in the process than the Tinytalk allows.
Investigations often require placing sensors in walls or woodwork where the standard logger or transmitter won't fit. Unusual sensors may be needed, for surface temperature or surface condensation, for example. This needs a data logger that can handle many types of sensor. There are not so many of this type. The leader in transportable, programmable, work anywhere, connect anything data loggers is Campbell Scientific.
Campbell instruments are the standard data gatherer in weather stations. They can also control equipment. There is a wide range of connection methods, by wire, radio, telephone, satellite.
The price is reasonable when calculated as dollars per measurement: the CR10 (notice the more conservative naming tradition for semi-industrial equipment) costs $1500 for up to twelve data points (depending on the sensor type). Such a logger becomes the cheapest option in projects where several sensors are needed close enough together (within 30 metres say) to be connected to one collecting point. A considerable hidden extra cost is the effort required to learn the programming language. This is a toy for conservators with a background in engineering or science. The later sections of this article deal with choice and installation of sensors for this type of logger.
The "traditional" electronic data logger was designed as laboratory equipment. This type of logger has been largely replaced by a card which plugs directly into a computer. These cards typically accept voltage or pulse input. This is a good option in a conservation workshop or laboratory where climatic experiments are in progress or where tricky processes are underway, such as moisture relaxation of full size oil portraits of emperors on horseback.
The Cambell Scientific data logger also has a niche here, because it is battery operated and less of a fire hazard than a computer spinning its disk in the small hours.
Temperature sensors for electronic data loggers
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