In this patent appeal, the holders of a patent — United States Patent No. 4,022,065 —contest the district court’s grant of summary judgment invalidating the patent under the “on sale” bar of 35 U.S.C. § 102(b). Because we agree with the patent holder that a genuine issue of material fact existed concerning the “on sale” status of the patented invention, we reverse the district court and remand for trial.
I. The Case Below
Appellants invented and manufactured a sample measuring syringe for use in the laboratory testing of minute quantities of chemicals. Approximately one year later, on February 19, 1976, they applied for and *1097received a patent on this device. In 1979, they learned that Beckman Instruments was manufacturing a substantially similar syringe, so they filed suit in district court alleging direct patent infringement. The district court, however, granted summary judgment for Beckman Instruments and invalidated appellants’ patent because of the statutory “on sale” bar, which requires the patentee not to sell or offer commercially an invention for more than one year before seeking a patent.1 Specifically, the district court found, as appellants conceded, that appellants had sold one of the devices before the critical date of February 19, 1975; the court rejected the appellants’ defense that the device sold was exempted from the “on sale” bar because the device was only a faulty prototype that did not reduce the invention to practice. The court agreed that the “on sale” bar did not apply to inventions not reduced to practice, but decided that appellants had failed to raise a genuine issue of material fact concerning the “not reduced to practice” defense. See 505 F.Supp. 1035 (1981). With this latter conclusion we disagree.
II. The Invention
During a telephone conversation on or about October 29, 1974, appellants conceived the idea behind their invention. They wanted to create a syringe capable of measuring chemical test samples without having to compensate for the amount of chemical left in the internal passages of the test equipment. The syringe would accomplish this by virtue of an offset in its measurement calibrations, so that it automatically held an amount equal to the volume of the test equipment’s passages in addition to the amount shown by the calibrations. Because the offset would depend on the volume of the test equipment’s passages, the syringe was intended for use with a test valve and adapter built by appellants.
Appellants immediately began to construct the first prototype, but after two weeks they decided to make a radical change in the design of the syringe and adapter. On November 16,1974, appellants sketched the new design and ordered the construction of the second prototype. Several of these second prototype syringes and adapters were manufactured, and on February 4, 1975, two of them were ordered by Dr. Silvestre Tejada of the Environmental Protection Agency for use in the Research Triangle Park laboratories. This order was received by one of appellants’ authorized sales representatives, thus consummating a sale before the critical date of February 19, 1975.2
However, on February 11, 1975, appellants noticed that the second prototype syringes would not make accurate measurements because the large bore needle allowed air bubbles to enter with the sample. They therefore changed the syringe’s specifications by reducing the inside diameter of the needle from .020 inches to .006 inches. Several of these third prototype syringes were manufactured, but they were not completed until after the critical date. Subsequently, the third prototype design became appellants’ commercial product.
Appellants did not send any syringes to EPA until February 21, 1975, after they had completed the third prototype syringes. They thus could have filled EPA’s order with the finally completed invention, but they sent the second prototype syringes as ordered. Dr. Tejada used the second prototype syringes and observed that these syringes made inaccurate measurements because of the air bubble problem caused by *1098the large bore needle. However, Dr. Teja-da did not consider the problem to be serious, since he was using the syringes for qualitative analysis, which does not require accurate measurement.3
III. Reduction to Practice
At trial, the district court examined all of these facts and decided that the invention had been reduced to practice in both the second and third prototypes. First, the court held that the appellants’ patent did not refer to any particular needle bore size and was not limited to quantitative analysis. Thus, the second prototype’s air bubble problem did not defeat the patented invention’s purpose and prevent its reduction to practice. In addition, the court found that the perfection of the invention in the third prototype reduced it to practice, because it involved only the correction of an obvious defect and because the correction was begun prior to the critical date. The court therefore granted summary judgment on the “on sale” claim.
But as this Court views the same facts, there is a genuine issue of material fact concerning the invention’s reduction to practice in both the second and third prototypes, so that summary judgment on the “on sale” issue was not proper.4 We reach this conclusion not because we disagree with the district court’s findings of fact, but rather because we disagree with that court’s interpretation of those facts under the law of “reduction to practice.”
A. The Second Prototype
First, we think that an issue of fact existed as to whether the second prototype reduced the invention to practice. The district court erroneously found the second prototype to be functional because the patent was not limited to quantitative analysis — in other words, because the patent also described a syringe useable for transferring unmeasured amounts of chemicals for qualitative analysis. Thus, according to the district court’s reasoning, the second prototype’s air bubble problem did not prevent it from reducing the invention to practice. This reasoning, however, reads the patent and the law of “reduction to practice” too narrowly.
“[A] patent is to be construed as a contract, with the intent of the parties as the lodestar. It is the real invention claimed and granted protection which we seek to determine.” Laitram Corp. v. Deepsouth Packing Co., 443 F.2d 928, 933 (5th Cir. 1971). Using this reasoning, we think that appellants’ patent should be read more realistically as a claim for a quantitative measuring device alone, not for a quantitative and qualitative device. Of course, any syringe, including appellants’ patented one, can be used to transfer variable, approximately measured amounts of chemicals to the testing equipment, thus making it useful for qualitative analysis. But the very reason for the existence of appellants’ patent is its unique method for making accurate measurements: the calibrations on the syringe have been offset to compensate for the quantity of chemical sample that remains in the internal passages of the test equipment after injection. Indeed, the district court admits as much when it notes that “[t]he critical element of the patent was the ‘offset’ of the syringe calibration scale to compensate for the volume of sample which is not transferred into the column, but remains in the valve after it has been actuated.” The “Background of the Invention” statement contained in the patent also explains this quantitative purpose. Although the claims of the patent do not specifically limit the patent to any purpose, *1099they do reveal the quantitative purpose when construed in light of the common knowledge of a person skilled in the art of chemical analysis. Thus, appellants’ patent should not be read to include a claim for a qualitative analytical device.
When the patent is read in this way, the law of “reduction to practice” seems to dictate a conclusion different from that reached by the district court. In other words, a device intended for quantitative analysis does not seem reduced to practice when it cannot make accurate measurements. As this Court held in In Re Yarn Processing Patent Validity Litigation, 498 F.2d 271, 280 (5th Cir. 1974), cert. denied sub nom. Sauquoit Fibers Co. v. Leesona Corp., 419 U.S. 1057, 95 S.Ct. 640, 42 L.Ed.2d 654 (1974), “ ‘reduction to practice’ includes not only [the] reduction [of an idea] to reality but also sufficient testing or experimentation to demonstrate that the device as it exists possesses sufficient utility to justify a patent, i.e., that• the invention is suitable for its intended purpose.” (Emphasis added)5 Appellants’ syringe in the second prototype did not seem to possess sufficient utility to justify a patent or to be suitable for its intended purpose; it may not have been a “functioning reality” or a reality that could “perform the general function for which it was developed,” as defined in In Re Yarn Processing. See 498 F.2d at 283. In short, the syringe’s measuring problem may have defeated the idea behind the invention. Appellants’ testimony on this issue cannot be entirely disregarded, because “[assessment of the probative value of evidence is properly the function of the jury or the judge sitting as a trier of facts,” not of the court for summary judgment. Id. at 287.6 And when combined with Dr. Tejada’s problematic deposition testimony, it certainly raises an issue of material fact sufficient to preclude summary judgment.
B. The Third Prototype
In addition, we do not think that the third prototype reduced the invention to practice. Although the third prototype later proved to be a successful device, it was not completed before the critical date. The parties cite no cases from this Circuit allowing exceptions to the general rule that uncompleted articles are not reduced to practice, and the evidence does not seem to support the application of such an exception here even if we accept the cases from other Circuits. See generally the cases discussed in Annot., 25 A.L.R.Fed. 486, at § 11 (1975). Appellants’ finalized design had not been unequivocally offered for sale as a satisfactory product: they had ordered the construction of only six third-prototype syringes and intended to display these later at a trade show, but they had not begun any sales.7 This evidence simply does not seem strong enough to impose the “reduced to *1100practice” label on the uncompleted third prototype, at least not in summary judgment.
IV. Conclusion
Because this Court reads the patent to be different from the district court’s reading and because we find evidence of factual disputes concerning the reduction to practice of both the second and third prototypes, we must reverse the district court’s summary judgment. We do not, however, reach any conclusion regarding the merits of this case; instead, we leave such a decision to the wisdom of the district court after it has fully heard the case.
REVERSED AND REMANDED.