AMETEK 7100 Leak Detect Stik - Manual (Page 10)

8
Chapter 2: Interfacing to Probe
The pulse signals coming from the probe are superimposed onto the nominal 24 VDC power supply
connections. As shown in Figure 3, the pulses are negative-going and are large in amplitude, nominally 20
volts peak. The peak is negative going and only a few microseconds in duration. The leading edge is the
proper edge to use for timing purposes. The large signal amplitude offers high noise immunity.
Figure 3: Pulse Signals
A simple approach to getting 5 volt logic levels from the input signal is to simply set a comparator level as
shown in Figure 6. The comparator output can then be used to create a gate signal for a counter circuit. The
circuit example in Section 2.1 (page10) has a variation of this concept using a 5:1 pulse transformer to elimi-
nate the reference to 24 VDC and make the comparison relative to circuit common (with inverted polarity).
Probe power supply impedance will have an effect on pulse amplitude. Wire lengths will vary the pulse ampli-
tude. Wire has an associated inductance, resistance and capacitance which will change the amplitude of the
coupled pulse. Users may wish to load their circuit to minimize wire effects.
The chart below gives relative times by which a user can distinguish timing for various probe lengths.
Length Float Position Tstart Tstop D t=Position
12" 1" 160msec 180msec 20msec
12" 12" 50msec 290msec 240msec
30" 1" 340msec 360msec 20msec
30" 30" 50msec 650msec 600msec
100" 1" 1040msec 1060msec 20msec
100" 100" 50msec 2050msec 2000msec
200" 1" 2040msec 2060msec 20msec
200" 200" 50msec 4050msec 4000msec
24 Volt Power
Supply Level
Pulse Pair
24 VDC
0 VDC