Sensitive dial-type absolute pressure indicators and slide rule calculators.
6
Various manufacturers have been building absolute-pressure measurement and control equipment for some time. The lower limit for full scale range is usually in the vicinity of 100 mm. of mercury. Wallace & Tiernan Products, Inc., Belleville, N. J., now makes absolute pressure indicators with ranges ab low as 0 t o 20 mm. of mercury. The mechanism of one of these indicators is shown in Figure 1. The pressure-sensitive element is constructed of beryllium copper diaphragms soldered together. Beryllium copper is used because of its excellent fatigue-resisting properties, which minimize the effect of hysteresis. The capsule formed by the diaphragms is evacuated and sealed so that the pressure inside is practically zero. The top of capsule A is firmly clamped to the back supporting plate. The bottom is free t o move and is connected to rod C yhich moves longitudinally a s the capsule expands. The opposite end of the rod is connected through a flexible link t o a lever attached to sector gear F , which rotates about flexure pivot E. The sector gear engages pinion H on the pointer shaft and thus causes it t o rotate a s the capsule expands and contracts a s a result of pressure changes in the case. Counterweight G is provided t o balance the lever system and minimize position error. Backlash is eliminated by the following arrangement: A sheave of the same pitch diameter as pinion H is mounted on the
occasions we have described devices for measuring absolute pressures in the range from zero to a few hundred millimeters. These deviccs included both manually operated and recording types of ionization gages, Pirani gages, and McLeod gages for the low range, the float or DuBrovin gage for intermediate pressures, and the closed-end, G-tube manometer for higher pressures (advertising section page 37, March, 1945, and page 91, November, 1945). Gages with Bourdnn tubes, bellows, or diaphragms are often used for measuring pressures below atmospheric. Usually these measure gage pressure-that is, the difference between the actual and atmospheric pressure. When low pressures are being measured, variations in atmospheric pressure become of the same order of magnitude as the low pressure; for this reason atmospheric pressure is not a suitable reference standard. I t becomes necessary, therefore, to use another reference point. The most prartical solution of this problem is to use, as a standard, a vessel evacuated t o such an extent that the pressure within it can be considered zero for the purposes of the measurement. The gage should then be arranged to measure the difference in pressure between the evacuated space used as standard and the space containing the unknown pressure. A gage of this type mcasures absolute pressure. N vARioCs
H
L K
Figu:e 1. Absolute Pressure Indicator
(Continued on p ~ 78) ~ e 7s
pointer shaft directly under the pinion. (This is not visible in the photograph.) A nylon thread is wrapped around the sheave and fastened to a wire spring, J,affixed to the sector. This spring puts tension on the nylon thread and keeps the sector and pinion firmly meshed. When the capsule expands and thus moves the sector to the right, the thread unwinds and the pressure of the sector against the gear remains the same. This arrangement, according to the manufacturers, is superior to a conventional hair spring, for the latter causes tension which changes with rotation of the sector. Lever K is t,he zero adjustment which cthanges the tension on spring L connected to the capsule. T h e mechanism is mounted in a vacuum-tight case, 3 l l Sinches in diameter and 3 inches deep. The vacuum to be measured is applied to the case through a '/-inch female pipe connection on the rear of the case. If the vacuum is )Tithin the range of the instrument, it will cause the capsule to expand and move the pointer. The pointer indicates the difference in pressure betn een the evacuated interior of the capsule and the pressure in the case. The instrument is therefore an absolute pressure indicator. The capsule is provided wit.h internal stops so t h a t neither capsule nor lever system is distorted or otherwise damaged when the instrument is exposed to atmospheric pressure, even if the exposure is sudden, such as when a leak occurs in a vacuum system. The absolute pressure indicator is available in the ranges 0 to 20, 50, 100, and 200 mm. of mercury. The scale can be provided with a hundred graduations. The makers state t h a t the instrument is sensitive to 1 part in 500 of the range, and accurate to 1 part in 300. Gages covering these ranges with such sensitivity and accuracy should find many uses in plant and laboratory. hlost of us are not accustomed to thinking of dial-type pressure gages scnsitive to 0.04-mm. variations in pressure with ranges as low as 20 mm. Wallace & Tiernan also supplics a similar gage for differential pressure measurements. The type of construction just described is employed. The minimu4 range, sensitivity, and accuracy are also the same as those for the absolute pressurc indicator.
slide Rwle )jbeontrd Q&
sip
Control instrumentation frequently requires the calculation of the proper size valve to control the flow of process materials. A valve of improper size can spoil the results obtained from an otherwise well designed control installation. The Brown Instrument Company, Philadelphia 44,Pa., now has available a 6-inch slide rule designed for quick determination of the approximatc control valve size required for various applications. The rule can be used for a wide range of liquid, steam, or gas flows. The usual data about operating conditions, such as pressure drop, downsteam pressure, temperature, and specific gravity of the matcrial being controlled, are required. This small calculator is a n ingenious device for storing a large amount of information on control valve characteristics in compact, readily portable form. Engineers working on control problems will find it valuable for quickly approximating valve sizes.
M e i J * For many purposes it is desirable to see at, a glance whether material is flowing through a process line, yet the expense of a flowmeter is not justified. Sometimes visual examination is dcsirable. Catalog section 93-B, published by Fisher & Porter Company, Hatboro, Pa., covers a wide variety of sight flow indicators to meet such needs. One of the simplest of these is the flapper type, a conventional sight flow glass with a flapper pivoted over the inlet in such a way t h a t the angle a t n.hich it hangs indicates flow. A slight modification enables the device to serve the dual purpose of flow indicator and check valve. For installations in which visibility is poor, a rotating-wheel flow indicator is provided. Simple drip flow indicators are also available. I n case the fluid is so dark t h a t no'type of internal indicator is satisfactory, a modification of the flapper type can be supplied; in this instrument the motion of the flapper is indicated by a n external pointer coupled to it magnetically. Both round and rectangular sight .windows to be ,installed in process vessels are also listed. Another variation IS the thin-stream observation fittikg in which the liquid flows in a thin layer between two :\qndows so t h a t very dark fluids can be examined. This is accomplished without excessive pressure drop by the use of large diameter sight disks. These devices are available in a wide range of sizes and construction materials.
