A brief history of degree-day meters

These days, if you want to measure your own local degree-day figures, you may well have the option of using your building energy management system (BEMS) or an automatic weather station (AWS). It was not so easy 70 years ago...
This design for a degree-day meter is taken from a 1936 paper presented to the Institution of Heating Engineers.

A resistance thermometer (1) measuring the outside air temperature drives a moving-coil meter (2) via a Wheatsone Bridge (3). The needle (4) indicates the temperature on the scale (5).

Above the needle is a wedge-shaped "chopper bar" (6) which oscillates up and down driven by a constant-speed motor (7) cam (8) and light spring (9). The extent of the chopper bar's vertical motion, however, is determined by the position of the meter needle (4). When the needle is over to the right (lower temperatures) the vertical travel is at its maximum; the hgher the temperature, the further over to the left the needle sits, and the less the vertical travel of the chopper bar.

The chopper bar's vertical oscillations are transmitted via a pawl (10) and ratchet wheel (11) to a cyclometer counter (12). It can be seen that the cyclometer will clock up fewer revolutions in mild weather than in cold. The reading on the cyclometer is, in fact, the integral through time of the temperature deficit.


Figure 1

Figure 2 		Figure 2 is a reconstruction of a device that someone described to me during a coffee break at a conference somewhere. I think they attributed it to Honeywell and said that it had been used in the 1960s in the US to help heating-oil suppliers measure degree days so that they could gauge when to send out top-up deliveries to their customers.

Mounted outside, these devices consisted of a continuously-rotating metal disc (1) carrying a snail-shaped insulated area (2). A bimetallic strip (3) with a pickup contact at its free end is mounted next to the wheel, and as the temperature varied, would move the contact in and out along a radius of the wheel. The result is that in hot weather the pickup would sit closer to the centre of the wheel, where it spends more time in contact with the insulated surface. In colder weather the pickup moves out along the radius where the proportion of insulated surface is less and electrical contact is maintained for a greater proportion of the time.

The proportion of time that the electrical circuit between the bimetallic strip (3) and the spindle (4) is "made" thus increases as the weather gets colder. The total contact time is thus proportional to the number of degree days.

Figure 3 represents the first attempt at a digital degree-day meter. Developed by the Shirley Institute (I think in the 1980s) and briefly marketed by JEL as the "DDM1000", this device had a sensor which it used to measure the temperature at intervals, subtracting it from a chosen "base temperature" and totalising the results (in effect replicating the electromechanical method of figure 1).

Like its electromechanical predecessors, the device was relatively inflexible, as one had to declare in advance what base temperature one wanted to use. At a cost of around £1,000 in money of the day, it was also a relatively expensive device and was eclipsed by building management systems that could do the same thing at little or no extra cost.


Figure 3

Figure 4 		I developed the Jackdaw automatic degree-day system (Figure 4) in 1988 with the assistance of my electronics-wizard brother, Robin. Like a BMS or the DDM1000, a Jackdaw unit had an outside temperature sensor, but rather than continuously computing the cumulative degree-day value, we designed the hardware just to record the raw daily maximum and minimum temperature. We left the calculation of degree days to a piece of PC software communicating via a serial link.

The advantage of this approach was that the cost was lower (albeit still over £500 at the time) and the user could choose their own base temperature. We also built in a cooling-degree-day calculation as standard, plus the ability to calculate the result over any number of days.

The Jackdaw never really caught on as a product, although at least one survived in use into the new millennium and I saw an abandoned one in a corner of someone's office in 2004. The widespread adoption of the internet enabled Degree Days Direct to offer weekly published heating and cooling degree days at a fraction of the cost, and the BEMS and AWS routes remain open to users who want their own custom local figures.