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Simplified integrator for net radiometer
$olat, Enetfy. V ol. 12. pp. l l T- I I 9.
PeqlalnonPress. 19611. PrintedinGreatBrilain
T E C H N I C A L NOTE SlmldBed Integrator for Net Radlemeter F. GOLDWATER* (Received 12 February 1968) INTRODUCTION THE AUTHOR has previously described an integrator for a net radiometer designed as part of a system including a self-balancing potentiometer recorder [ l]. An application arose which required the operation of an integrator at a place where power lines were not available and a batteryoperated indicator-integrator was designed and built. DESCRIPTION The integrator consists of a Nexus Type C I A - I battery-powered amplifier, together with balancing and feedback resistors, an indicating meter (50/~4), and an integrating motor type 912L manufactured by Ether Ltd.. Stevenage, Herts.. England (See circuit diagram. Fig. I.). A reversing switch is provided so that a left-hand zero meter can be used as an indicator for both +15V INPUT S
u
p
~
)
l
~ p ~'100K
Ether 912 L
oooooJ 2K 200 K
Fig. 1. positive and negative net radiative inputs. This reversing switch is not in the motor circuit and thus does not affect the correct registering of positive and negative inputs corresponding to positive and negative net radiative fluxes. The zero-setting switch provided also increases the sensitivity of the meter so that it will be sufficiently accurate for setting the integrator motor input to zero for zero radiative input. *Department of Meteriology, Hebrew University, Jerusalem, Israel. 117
1! 8
F. GOLDWATER
The amplifier chosen is only one of many types available. Its choice was influenced by two considerations. The first is the zero stability required, particularly with respect to ambient temperature changes. The net radiometer for which this instrument was designed had an output of approximately 25 mV per cal cm -2 rain -~ so that if 1 per cent be the allowable error at full input (1-5 cal cm -2 min-'), the temperature drift of the amplifier must be not greater than 400/~V for the full temperature range, or, assuming a temperature variation of 25°C, 16/~ V per deg. The second point to be considered in the choice of an amplifier is the current drawn from the battery in operation. Amplifiers are available with very low quiescent current drains and since the output required is small, this current is the major factor in determining battery life. However, the cost of the amplifier operating with lower quiescent current will be higher than that of the standard amplifier and this difference in cost must be balanced against the additional cost of batteries and the inconvenience in replacing them. The integrator motor requires an input of approx 12 V max and the amplifier must be capable of delivering this voltage. The circuit is straightforward and requires no modification for different types of amplifiers, since the gain of all commercial types is sufficiently large as not to influence the design of the feedback network. Hence amplifiers such as the Nexus SQ l0 a and even integrated-circuit amplifiers, such as the Motorola Type MC1433G, can be used. The circuit diagram (Fig. l) shows the connections to the amplifier and other components. The radiometer is connected to the + (non-inverting) input of the amplifier and the 2000 fl feedback potentiometer to the - (inverting) input. The 25,000 I~ zero-adjust potentiometer is connected to the trim connections of the amplifier as specified by the manufacturer. 15,0001) series resistors are used here to make the zero setting less critical. The output of the amplifier is connected to the feedback network, the meter network and the integrating motor. Mercury batteries would have been desirable for use with this instrument, since their output voltage is more constant with life. These are not available in Israel and hence regular carbon-zinc dry cells were used, without difficulty. Since the amplifier requires approximately 5 mA, a set of mercury batteries, Type RM42, costing about $59.00, will last about 3000 hr. The dry cells used, 6 V lantern type batteries, Type 918, cost $10.00 per set and last about 600hr. It is evident that the indicating meter shown can be replaced by a battery or spring-driven recorder if a recording is required. Depending upon the characteristics of this recorder, it may be necessary to change the resistor values in series with it. High-grade, low-temperature coefficient resistors should be chosen for the zero-adjustment and feedback circuits so as to minimize changes in zero point and gain with ambient temperature. The resistors in series with the indicating meter are less critical.
ADJUSTMENT The adjustments required are simple: First, the zero adjust control (25 kl~) is adjusted until the meter reads zero for zero voltage input. Second, the feedback potentiometer (2 kl)). is adjusted so that with an input voltage corresponding to 1.0 cal cm -2 min -1 radiative input to the radiometer being used the integrator motor turns at the rate of 5.0 turns per min giving a scale factor of 0.2 cal cm -2 turn -1. Sufficient time should be allowed for this test to ensure minimal errors in reading the number of turns on the motor. With this input voltage, the variable resistor in series with the meter ( 100 kl)) is then set to provide the desired scale reading.
TESTS The instrument was tested by applying an input voltage corresponding to definite values of net radiation and the time required for a given output (2 cal cm -2) measured. The results of these tests are indicated in Table I. The temperature drift of the entire instrument was measured and found to be approximately 0.0002 cal cm -2 min -~ deg -~ for the range I 0-50°C. These errors are well within the limits of error to be expected from a net radiometer in continuous use in the field-see, for example, errors found in net radiometers as described in reference[2]. The indicator is not sufficiently sensitive and has too small a scale for the errors to be detectable in its reading.
Simplified integrator for net radiometer
119
Table 1 Input (cal cm -z min-9
Input (mV)
Indicator reading
0.5 1.0 1-5 2.0
13-7 27.4 41-1 54.8
0-5 1.0 1.5 2.0
Time for I 0 turns (2 cal em-D 4.000 rain 1.992 1.327 0.992
Input (cal cm-~)
Error (cal cm-~)
2.000 1.992 1.990 1.984
0.000 0.008 0.010 0-016
CONCLUSIONS An inexpensive, simple, battery-operated integrator constructed from standard, commercially-available components operates satisfactorily over normally encountered ambient temperatures with error considerably less than that which may be expected from a net radiometer. Both positive and negative radiation inputs are integrated with correct sign and the integrator counter shows integrated net radiation.
Acknowledgement-This integrator was constructed for and will be used by Midrashat Sde-Boker. REFERENCES [ i ] F.J. Goldwater, Solar Energy 9, 1 (1965). [2] J. R. Latimer, Solar Energy 10,4, 187 (1966).
PeqlalnonPress. 19611. PrintedinGreatBrilain
T E C H N I C A L NOTE SlmldBed Integrator for Net Radlemeter F. GOLDWATER* (Received 12 February 1968) INTRODUCTION THE AUTHOR has previously described an integrator for a net radiometer designed as part of a system including a self-balancing potentiometer recorder [ l]. An application arose which required the operation of an integrator at a place where power lines were not available and a batteryoperated indicator-integrator was designed and built. DESCRIPTION The integrator consists of a Nexus Type C I A - I battery-powered amplifier, together with balancing and feedback resistors, an indicating meter (50/~4), and an integrating motor type 912L manufactured by Ether Ltd.. Stevenage, Herts.. England (See circuit diagram. Fig. I.). A reversing switch is provided so that a left-hand zero meter can be used as an indicator for both +15V INPUT S
u
p
~
)
l
~ p ~'100K
Ether 912 L
oooooJ 2K 200 K
Fig. 1. positive and negative net radiative inputs. This reversing switch is not in the motor circuit and thus does not affect the correct registering of positive and negative inputs corresponding to positive and negative net radiative fluxes. The zero-setting switch provided also increases the sensitivity of the meter so that it will be sufficiently accurate for setting the integrator motor input to zero for zero radiative input. *Department of Meteriology, Hebrew University, Jerusalem, Israel. 117
1! 8
F. GOLDWATER
The amplifier chosen is only one of many types available. Its choice was influenced by two considerations. The first is the zero stability required, particularly with respect to ambient temperature changes. The net radiometer for which this instrument was designed had an output of approximately 25 mV per cal cm -2 rain -~ so that if 1 per cent be the allowable error at full input (1-5 cal cm -2 min-'), the temperature drift of the amplifier must be not greater than 400/~V for the full temperature range, or, assuming a temperature variation of 25°C, 16/~ V per deg. The second point to be considered in the choice of an amplifier is the current drawn from the battery in operation. Amplifiers are available with very low quiescent current drains and since the output required is small, this current is the major factor in determining battery life. However, the cost of the amplifier operating with lower quiescent current will be higher than that of the standard amplifier and this difference in cost must be balanced against the additional cost of batteries and the inconvenience in replacing them. The integrator motor requires an input of approx 12 V max and the amplifier must be capable of delivering this voltage. The circuit is straightforward and requires no modification for different types of amplifiers, since the gain of all commercial types is sufficiently large as not to influence the design of the feedback network. Hence amplifiers such as the Nexus SQ l0 a and even integrated-circuit amplifiers, such as the Motorola Type MC1433G, can be used. The circuit diagram (Fig. l) shows the connections to the amplifier and other components. The radiometer is connected to the + (non-inverting) input of the amplifier and the 2000 fl feedback potentiometer to the - (inverting) input. The 25,000 I~ zero-adjust potentiometer is connected to the trim connections of the amplifier as specified by the manufacturer. 15,0001) series resistors are used here to make the zero setting less critical. The output of the amplifier is connected to the feedback network, the meter network and the integrating motor. Mercury batteries would have been desirable for use with this instrument, since their output voltage is more constant with life. These are not available in Israel and hence regular carbon-zinc dry cells were used, without difficulty. Since the amplifier requires approximately 5 mA, a set of mercury batteries, Type RM42, costing about $59.00, will last about 3000 hr. The dry cells used, 6 V lantern type batteries, Type 918, cost $10.00 per set and last about 600hr. It is evident that the indicating meter shown can be replaced by a battery or spring-driven recorder if a recording is required. Depending upon the characteristics of this recorder, it may be necessary to change the resistor values in series with it. High-grade, low-temperature coefficient resistors should be chosen for the zero-adjustment and feedback circuits so as to minimize changes in zero point and gain with ambient temperature. The resistors in series with the indicating meter are less critical.
ADJUSTMENT The adjustments required are simple: First, the zero adjust control (25 kl~) is adjusted until the meter reads zero for zero voltage input. Second, the feedback potentiometer (2 kl)). is adjusted so that with an input voltage corresponding to 1.0 cal cm -2 min -1 radiative input to the radiometer being used the integrator motor turns at the rate of 5.0 turns per min giving a scale factor of 0.2 cal cm -2 turn -1. Sufficient time should be allowed for this test to ensure minimal errors in reading the number of turns on the motor. With this input voltage, the variable resistor in series with the meter ( 100 kl)) is then set to provide the desired scale reading.
TESTS The instrument was tested by applying an input voltage corresponding to definite values of net radiation and the time required for a given output (2 cal cm -2) measured. The results of these tests are indicated in Table I. The temperature drift of the entire instrument was measured and found to be approximately 0.0002 cal cm -2 min -~ deg -~ for the range I 0-50°C. These errors are well within the limits of error to be expected from a net radiometer in continuous use in the field-see, for example, errors found in net radiometers as described in reference[2]. The indicator is not sufficiently sensitive and has too small a scale for the errors to be detectable in its reading.
Simplified integrator for net radiometer
119
Table 1 Input (cal cm -z min-9
Input (mV)
Indicator reading
0.5 1.0 1-5 2.0
13-7 27.4 41-1 54.8
0-5 1.0 1.5 2.0
Time for I 0 turns (2 cal em-D 4.000 rain 1.992 1.327 0.992
Input (cal cm-~)
Error (cal cm-~)
2.000 1.992 1.990 1.984
0.000 0.008 0.010 0-016
CONCLUSIONS An inexpensive, simple, battery-operated integrator constructed from standard, commercially-available components operates satisfactorily over normally encountered ambient temperatures with error considerably less than that which may be expected from a net radiometer. Both positive and negative radiation inputs are integrated with correct sign and the integrator counter shows integrated net radiation.
Acknowledgement-This integrator was constructed for and will be used by Midrashat Sde-Boker. REFERENCES [ i ] F.J. Goldwater, Solar Energy 9, 1 (1965). [2] J. R. Latimer, Solar Energy 10,4, 187 (1966).