An instrumentation skill you will need in the first lab. and in your technical c

Question

An instrumentation skill you will need in the first lab. and in your technical career, is the ability to read an analog meter. An analog meter is a pointer that deflects an amount proportional to some input signal that is being measured. It might be responding to a voltage, a force, a pressure, etc. Reading the meter is not as obvious as a first glance might suggest. It will often have more than one scale, so you must know the correct one to use; and there is usually a multiplier that must be applied to obtain the correct order of magnitude for the quantity being measured. Look at the meter below. It has two scales (-3 to 3 and -10 to 10). What is it reading? The answer is, "It depends." This meter is part of a measuring device that has multiple ranges. Typically, there is a button or knob on the device for selecting the range that is appropriate for the measurement being made. The ranges arc labeled by their full-scale-values. This means, for a given range, the meter will deflect full scale (along the dotted line in the diagram), if the input signal is equal in magnitude to the range value. It is generally best practice to select the range that gives the maximum deflection of the needle without exceeding full-scale deflection. Let's say that this meter belongs to a device that can be used to measure potential, in volts. It has a selector knob for choosing the six listed ranges: 1 V, 10 V, 100 V, 0.3 V, 3 V and 30 V. Let's say the meter is set on the 100 V range. Although either of the two scales can be used for any range selected, it is much simpler to read the scale whose value at full scale deflection is a power of ten multiple of the range. In this case use the top scale, because the range. 100 V, is a simple factor of 10 greater than the top scale's maximum. Having chosen the scale to read, the multiplier to be applied to each reading is the range divided by the scale's maximum, 100 V + 10 = 10 V, in this case. In other words, read off what the pointer indicates on the top scale and multiply by 10 V to obtain the correct reading in Volts. The pointer indicates about +5.5 on the top scale. Therefore, the input signal is +5.5 times 10 V = 55 V. Show your work for each question; that is, show how you obtain the answer by applying a multiplier. For the pointer position in the diagram, what is the input signal if the range is set at 0.3 V? For the pointer position in the diagram, what is the input signal if the range is set at 1 V? For the pointer position in the diagram, what is the input signal if the range is set at 30 V? Let's say you know that the input signal will be about -0.25 V. What range should you select to give maximum deflection without going off scale? Draw a line, with a straight-edge, on the picture above, showing how the pointer would point.

Explanation / Answer

If we set the selector nob at 0.3V, 3V or 30V ...it will be easy to read the input voltage from lower scale &

If we set the selector nob at 1V, 10V or 100V ...it will be easy to read the input voltage from upper scale

1. The selector is set to 0.3V. We choose lower scale.

First find the multiplier (M). M = selector voltage(Range)/Scale's maximum = 0.3/3= 0.1V.

Now find pointers reading. It shows reading is R=1.75

So the input signal is I = MR= 0.1 x 1.75= 0.175 V

2. The selector is set to 1V. We prefer to choose upper scale.

First find the multiplier (M). M = selector voltage(Range)/Scale's maximum = 1/10= 0.1V.

Now find pointer reading. It shows reading is R=5.5

So the input signal is I = MR= 0.1 x 5.5= 0.55 V

3.

The selector is set to 30V. We prefer to choose lower scale.

First find the multiplier (M). M = selector voltage(Range)/Scale's maximum = 30/3= 10V.

Now find pointer reading. It shows reading is R=1.75

So the input signal is I = MR= 1.75 x 10= 17.5 V

4. Input signal is I=0.25.

If we use upper scale and the selctor volgae is at 100V

Multiplier will be (M) = 100/10= 10V

The pointer reading will show , R= I/M= 0.25/10= 0.025..which is hard to determine

If we use upper scale and the selctor volgae is at 10V

Multiplier will be (M) = 10/10= 1V

The pointer reading will show , R= I/M= 0.25/1= 0.25..Again it is hard to determine

If we use upper scale and the selctor volgae is at 1V

Multiplier will be (M) = 1/10= 0.1V

The pointer reading will show , R= I/M= 0.25/0.1= 2.5..which is not a full scale deflection in upper scale

If we want use lower scale and the selctor volgae is at 30V

Multiplier will be (M) = 30/3= 10V

The pointer reading will show , R= I/M= 0.25/10= 0.025..which is hard to determine

If we want use lower scale and the selctor volgae is at 3V

Multiplier will be (M) = 3/3= 1V

The pointer reading will show , R= I/M= 0.25/1= 0.25..Again it is hard to determine

If we want use lower scale and the selctor volgae is at 0.3V

Multiplier will be (M) = 0.3/3= 0.1V

The pointer reading will show , R= I/M= 0.25/0.1= 2.5.. and it is full scale deflection in lower scale.

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