A capillary viscometer in which the liquid meniscus is detected by sensing electrical resistance changes in hermetically sealed electrical resistors which are spaced apart at the two levels in the measuring tube in which the meniscus is to be detected. The resistors are encased in glass and the electrical leads to these resistors are passed through glass tubes to a cable connector.

BACKGROUND OF THE INVENTION 
The invention disclosed herein is a viscometer for measuring the viscosity 
of liquids, particularly, liquids with Newtonian or approximately 
Newtonian flow properties. 
In capillary viscometers the test liquid is allowed to flow through an 
outlet tube (measuring tube), which is narrowed into a capillary tube 
above the outlet. There are two axially spaced apart reference marks in 
the tube and the space or volume between the two reference marks is 
precisely known. During measurement of viscosity, the sample liquid is 
allowed to flow through the measuring tube and the time that it takes for 
the liquid meniscus to drop from the upper to the lower reference mark is 
accurately determined. From the measured time, the viscosity of the liquid 
to be investigated can be calculated with the formula v=t.multidot.K where 
v=kinematic viscosity (mm.sup.2 per sec.) 
t=measured flow time 
K=instrument constant 
In prior art viscometers two annular reference marks on the measuring tube 
are used and the flow time of the liquid meniscus between the two marks 
was measured manually with a stop watch. Numerous other methods for 
observation or for sensing when the liquid meniscus passes a reference 
mark have also been suggested and used. 
Optical sensing of the reference marks is described by Kirchner, Chem. Eng. 
Techn. 31, 525 (1959) and Hughes and Rohen, J.Sc. Instr. 2, 12 (1969). 
Detecting the meniscus at the reference marks using fiber optics has been 
proposed by Smith, Analyst Ang. 95, 743 (1970) and in the German GM 7 104 
411. German Patent No. 832,691 discloses how to sense the meniscus using 
high voltage sparks. U.S. Pat. No. 3,798,960 describes a capillary 
viscometer into which resistances are sealed by means of resin, 
particularly, an epoxy resin, for sensing the meniscus. 
Since the majority of liquids are clear or translucent, the meniscus is 
detected mostly by visual observation or photoelectric sensing. In the 
category of photoelectric sensing, the method using photoconductive fiber 
optics has prevailed. This method is used widely in automated viscosity 
measurement procedures. For liquids with high electrical resistance, the 
high voltage spark method is usually used as disclosed in German Patent 
No. 832,691 for measuring the viscosity of mineral oils. 
These known methods have one of several disadvantages: 
1. All methods employing visual observation are subject to subjective 
measuring errors by the measuring individual. 
2. The photoelectric methods cannot be used with black and/or opaque 
liquids. 
3. The high voltage spark method cannot be used with conductive specimens 
such as aqueous systems or mineral oils containing some water, or 
rubbed-off metallic particles or conductive lubricating oil additives. 
4. Viscometers having detector elements which are sealed in with organic 
substances such as resins which are exposed to the sample liquids are not 
resistant to solvents and/or chemical solutions at the junction point. 
The objective of the invention described herein is to provide a viscometer 
which excludes the possibility of making subjective measurement errors 
that can determine the viscosity of black and/or opaque liquids that can 
be used for conductive specimens and, above all, is resistant to any types 
of chemicals, with the exception of hydrofluoric acid since all surfaces 
exposed to the test specimen are glass. These objectives are achieved, 
according to the invention, with a capillary viscometer which is 
characterized by the fact that glass-encased electrical resistors with 
positive or negative temperature coefficient are fused hermetically into 
the viscometer at the location of the usual reference marks. 
The principle involved is to mark occurrence of the meniscus by detecting a 
change in electric resistivity as the meniscus passes a thermally 
responsive resistive element. Because of the differing thermal 
conductivity of air and the liquid specimen, the liquid meniscus, 
generates a sharp change in the measured resistance as the meniscus passes 
the resistive elements. The concept of detecting a meniscus by detecting 
the resistivity change in an element such as a thermistor is disclosed in 
U.S. Pat. No. 3,798,960. However, in that patent the detector elements are 
in direct contact with the liquid whose viscosity is being measured which 
means that liquids which might attack the resistive elements must be 
excluded from making viscosity measurements with the patented viscometer. 
The invention is illustrated herein in greater detail in a viscometer of 
the Ubbelohde type. Viscometers of this type are described in more details 
in German Patent No. 673,185 as well as in German Standards DIN 51 562 and 
U.S. Standard ASTM D 2515.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a viscometer in which the new measuring elements are 
incorporated. This viscometer is a modified Ubbelohde type comprising a 
measuring tube 1, a feed tube 2, a ventilation tube 3 and a flushing tube 
4. The measuring tube 1 contains a capillary tube 5 and a measuring bulb 
6. Glass-encased negative temperature coefficient resistances 7 and 8 are 
hermetically fused into a bent glass shape and serve as reference marks in 
the measuring planes, that is, planes at which the meniscus of the fluid 
in the measuring tube is to be detected. Elements 7 and 8 are preferably 
resistors comprised of semi-conductive material such as thermistors. 
Viscometers such as the depicted viscometer are invariably immersed in a 
temperature stabilizing liquid bath, not shown. To insulate against 
outside temperature influence, in particular, to the tempering liquid of 
the bath, the electric leads leading to the temperature sensors 7 and 8 
are directed away laterally in glass tubes 9 and 10 and led upwards in an 
insulating fashion in a single tube 11. The two pairs of conductors in 
tube 11 terminate in a moisture proof quadruple pin electrical connection 
jack 12 which is high enough to extend out of the liquid tempering bath in 
which the viscometer is immersed. A four conductor cable, not shown, would 
be plugged into jack 12 when the viscometer is in use. The signals due to 
the temperature responses of the resistive elements 7 and 8 are conducted 
to a device, not shown, which is triggered to start measuring time when 
the meniscus passes resistive sensor 8 and to terminate measuring the time 
interval when the liquid passes resistive sensor 7. 
FIG. 2 shows how typical resistive device 7 is coated with glass so as to 
be electrically isolated from the sample liquid in the measuring tube 1. 
However, since the glass coating on the resistive element 7 can be quite 
thin, the test liquid and sensing elements are in good heat exchange 
relationship. 
The glass-encased sensor elements 7 and 8 which project into the viscometer 
are either fused in at a right angle to the tube axis as shown or at an 
acute angle to this axis and constructed either straight or bent to adjust 
to the particular type of viscometer, so that a complete, smooth flow-off 
of the measured material in the viscometer is assured. 
The viscometer according to the invention offers a multiplicity of 
significant advantages. Because the detector elements are completely 
encased in glass and fused hermetically into the viscometer tube, it is 
possible that specimens to be measured which are very aggressive 
chemically and have powerful solvent capacity can be measured in the new 
viscometer, which is not possible with prior art viscometers into which 
the resistances are sealed in with synthetic resin. 
The new viscometer is resistant to all solvents, solutions and chemicals 
except hydrofluoric acid. It has an advantage over viscometers that depend 
upon the high voltage spark method for locating the meniscus in that 
electrically conductive specimens can be measured. This is particularly 
advantageous with oils or lubricating agents which contain water and/or 
rubbed off metallic particles and/or conductive additives. 
The new viscometer design has the advantage that entirely opaque and/or 
black liquids can be measured which is not possible in prior viscometers 
that depend on visual observation or photoelectric sensing to detect the 
meniscus. 
The invention makes it possible for the first time to measure a used 
mineral oil containing contaminants such as water or metal particles or 
containing materials that make the mineral oil opaque such as by the 
presence of the combustion product, carbon. There is no need to replace 
the sensing elements after a contaminating test fluid has passed through 
the viscometer. It is only necessary to rinse out the viscometer after a 
use and proceed with the next measurement with confidence that the 
response characteristics of the sensing elements have not changed. 
Those skilled in the art will recognize that the invention can be applied 
to capillary viscometers of any type, for example, to the Cannon-Fenske 
type as described in German standards DIN 51 366 and ASTM D 2515, to 
viscometers of the Ostwald type and to other flow viscometers as well as 
the Ubbelohde type which was used to exemplify application of the 
invention herein.