Handheld thermohygrometers

A handheld thermohygrometer answers a different question from a logger. The logger answers what the room does over a year. The handheld answers what is happening in this corner, right now, while you are standing in it. Both are needed and they are not interchangeable.

What it is genuinely good for

Surveying a building quickly. Walking twenty rooms with a handheld in an afternoon gives a rough map of where the problems are, which then tells you where to put the loggers. Doing the same with loggers would take a year and twenty instruments.

Finding the local anomaly. A cold spot behind a cupboard, a damp patch at floor level in one bay, the difference between the top and bottom of a tall store. These are questions about position rather than about time, and a handheld answers them in minutes.

Verifying a logger. Holding a reference instrument beside a logger at every download catches a gross failure long before the annual calibration would. It is not a substitute for a proper salt solution check, but it catches the sensor that has failed outright.

Checking a case after work on it. When a case has been opened, resealed or had its gel changed, a spot reading confirms it is heading in the right direction without waiting a month for the logged record.

What to look for

Response time. This is the specification that most distinguishes a useful handheld from a frustrating one. A sensor that takes four minutes to settle after moving between two spaces makes a twenty-room survey impossible. Look for a stated response time; anything much over about a minute to reach most of the final value will slow the work considerably.

Accuracy that is stated across a range. As with loggers, the headline figure is usually quoted at a comfortable mid-point. If you will use the instrument in a cold store, the accuracy at 10 °C is the one that matters.

A probe rather than a body-mounted sensor. An instrument where the sensor sits on a stalk can be pushed into a gap, behind a rack or through a case aperture. One where the sensor is behind a grille in the body of the meter reads the space your hand is warming.

Dew point calculation. Being able to read dew point directly saves a great deal of arithmetic when the question is whether outside air will dry a room or wet it. This is the single most useful derived value on a handheld, and reading climate data explains why.

Hold and min/max functions. Reading a display in an awkward position is easier when the value can be frozen.

How to use it without misleading yourself

Let it settle. Almost every wrong handheld reading in practice is a reading taken before the sensor equilibrated with the space, usually after carrying the instrument from a warm office into a cold store. Two minutes standing still costs nothing.

Keep your hand and your breath away from the probe. Both are warm and wet, and both will move the reading noticeably.

Measure where the objects are, not where it is convenient to stand. The placement rules apply just as much to a spot reading as to a fixed sensor.

Write down the time, the position and the outdoor conditions. A spot reading without context cannot be compared with anything later, which makes it a fact about an afternoon rather than evidence.

The one to buy as a reference

If you are buying two instruments, make one of them a better one and keep it aside. A single well-specified handheld with a calibration certificate, used only to check the working fleet and never carried around as a general tool, gives a whole set of cheaper loggers a traceable anchor. That is a far more efficient use of a calibration budget than sending every logger away every year.