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Cycling between a pressure within the specified
range and 1 atm or above may require re-zeroing of the transducer due to mechanical stress.
Like the closed-end manometer, the capacitance manometer is
a direct pressure measuring device. A nice on-line introduction to the technology is given in Baratron Capacitance Manometer Basics.
MKS Instruments is a major manufacturer of process control devices using their "Baratron" pressure transducers based on the capacitance manometer principle.
After putting it back together and wiring up a cable, I
found that there was a response to pressure but it was not possible to zero the output using the externally accessible
adjustment even after a long warmup time.
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To operate, pull as high a vacuum as possible
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Your refrigerator is a cornerstone of your kitchen and household,
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kitchen running smoothly. Why will you keep it on? Any other card like the one above will be assessed a 3.75%
fee and a minimum fee of $3.95. As noted above, one nice thing about these sensors is that all
the "smarts" are inside the pressure transducer. If the pump results in pressure pulsations, these will even be detectable on a
readout with sufficiently fast response. If the noise is accompanied by pooled water, it's likely a faulty drain pump
that needs replacement. The replacement is the 627B and they would happily give me a 15% discount with
trade-in. Without specifications for the capacitance of the diaphragm and/or
frequency calibration, all I could check out was the electronics and as
far as I could tell, this was operating correctly, including the gain (span) and zero circuitry, and temperature regulating heater.
I made some measurements of a few internal signals including the 10 V
and 1.9 V references, the oscillator frequency from the manometer, the output voltage after the frequency-to-voltage converter, and the zero voltage reference (the
1.9 V reference multiplied by a bit over 5, adjustable
via the Zero pot).
A frequency-to-voltage converter (basically a triggered pulse
generator and averager) followed by op-amp circuits
to adjust gain (span) and zero setting provides an output voltage (typically) between 0
and 10 VDC where 10 VDC corresponds to full scale with extremely
good linearity with respect to pressure over the entire range.
These are darn small changes but when their sum is multiplied by the gain of the
output stage (about 15), would appear to account for the zero drift.
Due to the delicate nature of the diaphragm, especially for units with a small full scale
rating like 1 Torr, mechanical vibrations must be minimized by isolating the transducer from the
pump(s) since resonance conditions can occur. A gauge with
a 10 Torr full scale rating would be suitable for these lasers since they operate between a fraction of a Torr and a few Torr.
This is not a problem with newer designs but for the 227A, below, exposing the
transducer to a pressure up to 1,000 Torr won't cause permanent damage but anything above 110 percent of full scale,
even for an instant, may require re-zeroing. I have checked the 227A's calibration against my Hastings VT-4B thermocouple vacuum gauge and once zeroed, the
readings do track within 20 percent over its entire range of 0 to 1 Torr.
Not like those silly thermocouple gauges with their weird
power supplies and custom non-linear meter scales.
The extension to testing of the DV-23 and
DV-24 based Hasting TC gauges is left as an exercise for the reader.
Modern versions of these capacitance manometer based
vacuum/pressure gauges have few deficiencies but for older models, there were several:
- Temperature affects the mechanical structure (and to a lesser extent, the sensing electronics) so
a heater is usually included to maintain a constant temperature.
For continuous manufacturing processes run for days or weeks without
interruption, these deficiencies (the first two in particular) wouldn't matter.
Thus, there is a warmup period and this can be two hours or more
before the system stabilizes. Thus, if you find an older model,
realize that it may have its own set of quirks that will need to be dealt with to maintain your
sanity. Thus, any inexpensive digital or analog panel meter can be used, or
just your Radio Shack multimeter!
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