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1958 - recently introduced refinement to protect expensive equipment from damage caused by instrument failure


CD 2055026 E&MP3.010


April 3, 1958

Constant voltage reference supply fits directly into space formerly occupied by dry cell battery.

Leads from the supply are attached to power and ground terminals within the instrument.

Two terminal posts on top of supply provide 6 volts to servo null balance circuit.

additional text found

Instrumentation Technique Adds System Reliability

With the ever-increasing use of modern instruments, engineers and technicians tend to take this equipment for granted.

Although instrumentation techniques have reached a high state of development, continuous refinements must be made to extend their use and efficiency.

Performance Measurements Co., Detroit, Michigan, recently introduced such a refinement to protect expensive equipment from damage caused by instrument failure.

This technique uses a constant voltage reference supply to replace short-life dry cell batteries in various types of instruments.

Dry cell batteries are commonly used in null balance servo systems requiring a constant current dc voltage in a resistance bridge load.

Voltage across one resistor is kept constant as a reference for measurements. This type of circuit is found in many controlling, recording and indicating instruments.

Null balance servos using dry cells usually have a slide wire standardizing device. Even with alert preventive maintenance, the capacity of this cell often quickly dwindles to low levels. Depending on the use of the instrument, this drift can easily endanger equipment costing thousands of dollars.

The new constant voltage reference supply, known as the CVR-10, is intended to eliminate this problem.

Considering the cost of a spoiled batch, or process interruption, caused by a dead dry cell in control equipment, the substitution of the constant voltage reference supply is very economical, and at times, even the routine, periodic changing of dry cells causes a furnace to “go off the line.” As much as 30 minutes may be required to get it going again; this is a serious problem to production people.

The CVR-10 is designed to replace No. 6 dry cells with no modification of the instrument, Fig. 1. Dimensions of the new constant voltage reference supply are the same as those of the dry cell. In addition, two knurled positive and negative terminals correspond to those of the battery. The CVR-10 also has three leads to make connections to 115 volt line and ground terminals within the instrument.

One typical installation of the constant voltage reference supply is in a temperature controller-recorder, Fig. 2. This particular instrument is used to monitor and control critical temperatures in a gas furnace.

Previously, the dry cell battery was the only practical device for supplying the necessary low voltage to the null balance servo system of the instrument.

So, a planned program of preventive maintenance was needed for reliable temperature readings. In this temperature controller-recorder, normal maximum life for the battery was approximately 90 days; replacement was setup on the basis of 60 days to provide a safety margin.

Scheduled replacement was also backed up with constant checks on battery level between replacement periods. This was necessary, since if the battery failed, giving an incorrect temperature reading, expensive damage could have resulted; or the furnace could have shut off completely causing a considerable amount of down-time. The constant voltage reference supply eliminates these problems.

Operating at 1.5 volts and 6 milliamperes, the CVR-10 has a +- ¼ percent stability over AC line voltage variations between 108 and 125 v. This is particularly important where line voltage fluctuates over a wide range.

Original Caption by Science Service
©Performance Measurements Co.

National Museum of American History


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