Testing Recording Instruments

Press, Volume CVI, Issue 31248, 21 December 1966, Page 18

Testing Recording Instruments

The accuracy of hydrological investigations in New Zealand substantially depends on the servicing of the measuring instruments employed. All these instruments are tested and adjusted at the instrument depot of the Ministry of Works (water and soil division) at Kainga, near Christchurch.

The instruments, many of them with a clockwork mechanism, are used in investigating water resources for electricity schemes and irrigation projects, and in gathering information for catchment, soil conservation and flood control work.

The Ministry of Works imports all hydrological instruments for its own staff and other Government agencies and also for local authorities, to which instruments are made available free.

The Kainga depot services all the instruments issued by the department. Before it was set up they were sent to Australia, Britain and the United States for servicing. This was time-consuming and expensive. The main instruments issued and serviced by the depot are current meters, water level recorders, snow and rain gauges and a large number of stopwatches. Current meters measure the velocity of rivers, enabling their behaviour over years to be assessed. Water-level recorders register rises and falls in river levels and snow gauges the fall and depth of snow. Stopwatches are used in various manual tests. An accurate assessment of the flow which can be expected in rivers and lakes in a given period is a primary requirement in evaluating water resources for an electricity scheme. Both current meters and water-level recorders are used.

Methods of Use

There are several methods of using current meters. In one, twin towers connected by a steel cable are built at selected points along a river. Members of hydrological field parties are pulled across the cableway in a car and lower current meters to record the speed of the river’s flow.

Another method is to suspend a meter from a jet boat, held stationary. The jet boat is used because it can negotiate difficult waters.

Other ways are simply to wade into a river and use a meter, or gauge directly from a highway bridge or a footbridge. The meters take some hard knocks, especially in fastflowing rivers and from debris during floods. Because the worth of the tests depends on precise recordings, the meters are periodically brought back to the depot at Kainga for tests and calibration. I6sft Tank

One of the depot’s workshops houses a concrete tank 165 ft long, six feet wide and six feet deep. The rails which run above the tank have been set with a high degree of accuracy allowing for the curvature of the earth. They are accurate to a tolerance of 4/1000ths of an inch over the full length of the tank.

A "ship’s bridge” platform mounted on the rails is powered to move at up to 20ft per second and contains the instruments which check the accuracy of the current meters under test conditions. A meter under test is attached to the platform and connected to the test instruments on the “bridge” and lowered into the tank. The platform is then driven at various speeds to enable the meter to be accurately calibrated.

The depot has 300 waterlevel recorders on issue. These include 100 which operate on a punch-tape system. Fifty more of these are on order. Most water-level recorders are set in stilling wells in fixed positions in rivers under investigation. Two open pipes, one of them below miniminn water level, run into each well. The level of the water running through the pipes into the well is automatically recorded on the punch-tape type instruments. In the others it is plotted on continuous charts. Remote Control The punch-tape strips are removed and the information

they record electrically translated for feeding into a computer. The information on continuous charts is evaluated by personal calculation. The depot already has a few instruments for “telephoning” the punch-tape recorders which, by remote control signals or “bleeps,” relay back their information. In this way the state of a river’s level recorded at 15minute intervals —some recorders are set for five and 10-minute recordings is obtained without going to a recorder location and retrieving the punch-tapes. Snow Gauge

Current meters and waterlevel recorders are in use on hydro - electricity investigations in several parts of New Zealand. The main rivers being investigated in the South Island are the Waitaki, Clutha, Waiau and Buller. Those in the North Island include the Wanganui, the Upper Waikato, the Rangitikei, the Mohaka, the Wairoa and several rivers in the Bay of Plenty. The depot has so far imported only one punch-tape snow gauge, which is at Craigieburn, 43 miles from Arthur’s Pass. It measures the fall of snow and rain by weight. The other gauges are manual snow-sampling equipment. Calibrated plastic poles, some of them now on the Tasman glacier, measure the depth of snow. They are inspected every few weeks and extra sections of pole added as required. Snow samplers are also used. These are tubes which are forced into the snow to get samples from which its density can be calculated. From this the water equivalent of the snow can be estimated. Depths of snow, calculations of its density and other information obtained over a period enable hydrologists to estimate the pattern of flow in snow-fed rivers under investigation for electricity schemes.

Such investigations and others in the geological and engineering fields often continue for 10 or more years before enough data is

obtained on which to base a recommendation. The photograph shows the 165 ft long concrete tanks at the Ministry of Works instrument depot at Kainga, and the “ship’s bridge” platform, mounted on rails.