Pumice Slurry Research Articles

Effect of Polishing Technique and Brushing on Surface Roughness of Acrylic Resins

This study evaluated the abrasion resistance of acrylic resin to routine dental brushing procedures using different dentifrices after the resin surface had been chemically or mechanically polished. Eighty specimens were prepared using heat-polymerizing (HP) and autopolymerizing (AP) acrylic resin, and immediately submitted to grinding with abrasive stones and disks. The specimens were divided into two groups: the first group was mechanically polished (MP) using pumice slurry, and the second group was chemically polished (CP) using heated monomer. After polishing, the specimens were submitted to 30,000 brushing cycles. Surface roughness was measured after polishing and brushing procedures, using a Surfcorder SE 1700 rugosimeter. Data were statistically analyzed by ANOVA (alpha= 0.05). There were significant differences in surface roughness between polishing with MP and CP (p < 0.05). Significant differences in surface roughness were also found between brushing with MFP dentifrice and Tartar Control dentifrice (p < 0.05). MP produced smoother surfaces than CP, and surface roughness as a result of polishing technique was not influenced by acrylic resin type. Even though MP resulted in smoother surfaces initially, subjecting the acrylic resin to dentifrice brushing negated this advantage.

Infection Control Procedures in Commercial Dental Laboratories in Jordan

The risk of cross-infection in dental clinics and laboratories has attracted the attention of practitioners for the past few years, yet several medical centers have discarded compliance with infection control guidelines, resulting in a non-safe environment for research and medical care. In Jordan, there is lack of known standard infection control programs that are conducted by the Jordanian Dental Technology Association and routinely practiced in commercial dental laboratories. The aim of this study was to examine the knowledge and practices in infection control among dental technicians working in commercial dental laboratories in Jordan. Data were collected from the dental technicians by a mailed questionnaire developed by the author. The questionnaire asked respondents to provide demographic data about age and gender and to answer questions about their knowledge and practice of infection control measures: use of gloves, use of protective eyeglasses and face shields, hepatitis B virus (HBV) vaccination, laboratory work disinfection when sent to or received from dental offices. and regularly changing pot water or pumice slurry. Of the total respondents, 135 were males (67.5 percent) and sixty-five were females (32.5 percent) with a mean age of twenty-seven years. The results showed that 24 percent of laboratory technicians wore gloves when receiving dental impressions, while 16 percent continued to wear them while working. Eyeglasses and protective face shields were regularly worn by 35 percent (70/200) and 40 percent (80/200) of technicians, respectively. Fourteen (14 percent) had received an HBV vaccination, and 17 percent inquired if any disinfection measures were taken in the clinic. Eighty-six percent of the technicians reported that pumice slurry and curing bath water were rarely changed. Only five dental technicians (two males and three females) were considered to be fully compliant with the inventory of infection control measures, a compliance rate of 2.5 percent with no significant difference between males and females (p>0.05). In conclusion, there is lack of compliance with infection control procedures of dental technicians working in commercial laboratories in Jordan.

Influence of simulated microwave disinfection on complete denture base adaptation using different flask closure methods

Denture microwave disinfection may result in dimensional changes that may distort the acrylic resin base, causing discomfort to the patient. The purpose of this study was to determine the effect of simulated microwave disinfection on maxillary denture base adaptation using 2 different flask closure methods. Forty stone cast-wax base sets were prepared for flasking by the traditional flask closure (TFC) and Restriction System flask closure (RSFC) methods (n=20). The RSFC consists of 2 iron plates to hold the flask during definitive flask closure, maintaining the flask in a closed position after release of pressure. Acrylic resin (Classico) was prepared according to the manufacturer's instructions. After polymerization in water at 74 degrees C for 9 hours, the bases were removed following flask cooling and submitted to conventional finishing with abrasive stones and pumice slurry. Ten bases for each TFC or RSFC method (n=10) were submitted to simulated disinfection (SD) in 150 mL distilled water in a microwave oven at 650 W for 3 minutes; control bases for each TFC or RSFC method (n=10) were not disinfected (ND). Three transverse cuts were made through each stone cast-resin base set, corresponding to the distal of canines, mesial of first molars, and posterior palatal region. Measurements were made in the bases using an optical micrometer at 5 points for each cut to determine adaptation: left and right marginal limits of the flanges, left and right ridge crests, and midline. Collected data were submitted to 3-way ANOVA followed by the Tukey HSD test (alpha=.05). Dimension gap values (mm) for ND denture bases prepared by the RSFC method were significantly lower (0.16 +/- 0.05) when compared to the TFC method (0.21 +/- 0.05) (P<.027). Simulated disinfection statistically improved the base adaptation in bases prepared by the TFC method (0.17 +/- 0.03), compared to the ND bases (0.21 +/- 0.05) (P<.027). Simulated disinfection statistically significantly improved base adaptation (P<.0001) only in the distal of canines (ND=0.13 +/- 0.01; SD=0.11 +/- 0.03) and the posterior palatal region (ND=0.25 +/- 0.04; SD=0.21 +/- 0.01) when bases were prepared by the TFC method. Simulated disinfection by microwave energy improved denture base adaptation when the TFC method was used, but did not statistically alter base adaptation for the RSFC method.

Influence of chemical and mechanical polishing on water sorption and solubility of denture base acrylic resins

Influence of polishing methods on water sorption and solubility of denture base acrylic resins was studied. Eighty samples were divided into groups: Classico (CL), and QC 20 (QC) - hot water bath cured; Acron MC (AC), and Onda Cryl (ON) - microwave cured; and submitted to mechanical polishing (MP) - pumice slurry, chalk powder, soft brush and felt cone in a bench vise; or chemical polishing (CP) - heated monomer fluid in a chemical polisher. The first desiccation process was followed by storage in distilled water at 37 +/- 1 degrees C for 1 h, 1 day, 1, 2, 3 and 4 weeks. Concluding each period, water sorption was measured. After the fourth week, a second desiccation process was done to calculate solubility. Data were submitted to analysis of variance, followed by Tukey test (p<or=0.05). Means of water sorption (%) and solubility (%), respectively, were: CL-MP: 1.92 and 0.02; CL-CP: 1.98 and 0.52; QC-MP: 2.31 and -0.05; QC-CP: 2.32 and 0.25; AC-MP: 2.45 and -0.07; AC-CP: 2.43 and 0.41; ON-MP: 2.32 and -0.06; ON-CP: 2.34 and 0.27. Mechanical polishing promoted significantly lower solubility of acrylic resins; initially, water sorption values were higher for chemically polished samples, however, after 4 weeks all groups were similar.

Surface roughness of acrylic resins submitted to mechanical and chemical polishing

Surface roughness is extremely important as it influences micro-organism accumulation and consequently oral health. The aim of the present study was to evaluate the influence of mechanical polishing (MP) and chemical polishing (CP) on surface roughness of four heat-cured denture base acrylic resins. Twenty sample discs were prepared for each of the following acrylic resins: Classico (CL), QC 20 (QC), Acron MC (AC) and Onda Cryl (ON). The first two are boiling cured resins and the latter two are microwave cured. After curing and deflasking, samples were ground and subdivided into two groups: MP, which received pumice slurry, chalk powder, soft brush and felt cone, at a bench vice; and CP, which received fluid specifically for chemical polisher. Surface roughness was measured using a Surf Corder SE 1700 rugosimeter. Data collected were statistically analysed by analysis of variance followed by Tukey's test. Surface roughness means in mum were: CL-MP: 0.0350; CL-CP: 0.2298; QC-MP: 0.0307; QC-CP: 0.1792; AC-MP: 0.0312; AC-CP: 0.2700; ON-MP: 0.0321; ON-CP: 0.2949. It can be concluded that MP produced significantly smoother surfaces than CP and that surface roughness because of MP was not influenced by acrylic resin type, whereas this was not true of CP.

Effect of three endodontic materials on the bond strength of two composite core materials to dentin

This study investigated the effect of dentin treatment with iodine potassium iodide, calcium hydroxide or a biphenol-A- diglycidylether epoxy resin sealer on the dentin bond strength of titanium-reinforced Bis-GMA and urethane dimethacrylate composite materials. Dentin was exposed in four groups of extracted teeth (20 specimens each). Three groups were treated with one of the contaminants and the fourth group was left uncovered and served as the control group. All specimens were stored for 1 week at 37 degrees C and 100% humidity. The dentin surfaces were cleaned with pumice slurry using a prophylaxis rubber cup, rinsed and dried. Then the dentin surfaces were treated using the conditioner and the primer of the GLUMA adhesive system followed by placement of composite core materials. After 20 minutes, debonding of the core materials was accomplished using a shear-peel test. A two-way analysis of variance and Tukey's multiple range test was performed based upon core type and contaminant type (alpha = 0.05). Titanium-reinforced Bis-GMA composite showed significantly greater bond strength values than the urethane dimethacrylate composite material (p < 0.0001). The mean bond strength values in megapascals for the Bis-GMA resin ranged from 8.47 +/-1.52 for the calcium hydroxide group to 9.81 +/-0.94 for the control group. Mean bond strengths for the urethane dimethacrylate resin ranged from 3.35 +/-0.90 for calcium hydroxide to 3.99 +/-0.88 for iodine potassium iodide groups. Within each core group, no difference in bond strengths were noted compared to the uncontaminated control. Pretreatment of the dentin with iodine potassium iodide, calcium hydroxide, or epoxy resin followed by pumicing and using the GLUMA system conditioner and primer had no effect on the bond strength of the two resin composite core materials. However, the titanium reinforced Bis-GMA composite exhibited significantly greater bond strength to dentin than the urethane dimethacrylate based composite.

Micro-tensile bond strengths to sclerotic dentin using a self-etching and a total-etching technique

Objectives: To evaluate the in vitro regional micro-tensile bond strengths of a self-etching/self-priming adhesive system to sclerotic dentin, in the absence or presence of phosphoric acid conditioning. Methods: Naturally-occurring, non-carious cervical lesions on extracted premolars were hand-cleaned with a slurry of pumice and chlorhexidine, then bonded without further cavity preparation. One group was bonded using Clearfil Liner Bond 2V (Kuraray Co. Ltd., Osaka, Japan). The other group was first conditioned with K-etchant; 40% phosphoric acid gel (Kuraray) prior to the application of the self-etching primer. Artificially prepared wedge-shaped cavities were also made in sound premolars and bonded with the two methods as controls. Resin composite build-ups were made using Clearfil Protect Liner F and AP-X resin composite (Kuraray). After storage in water for 24 h, the teeth were sectioned into 0.7×0.7 mm composite-dentin beams along the occlusal and gingival walls, and at the apex of the lesions. The use of two conditioning methods, two substrate types, and three different locations yielded 10–14 beams for each of the 12 groups. After testing for the μTBS, representative beams that were stressed to failure were examined with SEM. Remaining fractured beams were demineralized and processed for TEM examination. Statistical analysis was performed using a three-way ANOVA and Student-Newman–Keuls tests. Results: Regardless of the conditioning methods, bond strengths to sound dentin were significantly higher than to sclerotic dentin ( P< 0.05). With sclerotic dentin, there was no significant difference for the conditioning methods used, except that K-etchant significantly improved the bond strength at the gingival aspect of the lesions. Fractographic analysis revealed that the self-etching primer could not etch beyond the surface hypermineralized layer of sclerotic dentin. Interfacial failure occurred along the surface of the mineralized intermicrobial matrix and/or hypermineralized layer. With the use of phosphoric acid, a hybrid layer was only seen when the surface layers were thin. Incompletely removed sclerotic casts were evident in both groups. Significance: Removal of the surface layers of sclerotic dentin and/or conditioning with stronger acids may be beneficial to obtain stronger bonding to sclerotic dentin.

An ultrastructural study of the application of dentine adhesives to acid-conditioned sclerotic dentine

Objective: This in vitro study examined the ultrastructure of resin-infiltrated sclerotic dentine following the application of a self-etching primer, with or without the adjunctive use of phosphoric acid pre-conditioning. Materials and Methods: Non-carious, natural cervical sclerotic lesions were hand-cleaned with a slurry of pumice and chlorhexidine and bonded without further cavity preparation. One group was bonded using Clearfil Liner Bond 2V (Kuraray Co. Ltd, Osaka, Japan) alone. Specimens from the other group were pre-conditioned with K-etchant (40% phosphoric acid gel, Kuraray) prior to the application of the same self-etching primer. Artificially prepared wedge-shaped lesions were also made in sound bicuspids, bonded using the two methods, and used as controls for the two groups. For SEM examination, each specimen was cryofractured into two halves through a pre-formed slit made on the lingual surface, after the respective conditioning treatment. Different locations within the lesions were examined after rinsing of the phosphoric acid/self-etching primer and specimen dehydration. For TEM investigation, specimens were bonded with the adhesive. Both demineralised and undemineralised ultrathin sections were prepared from the occlusal, gingival and deepest part of the wedge-shaped bonded lesions following specimen fixation, dehydration and resin embedding. Results: A hypermineralised surface layer was present on the surface of etched sclerotic dentine. This layer was thicker in the deepest part of the natural lesions, where bacterial colonisation of the lesion surface was also apparent. Both treatment protocols were unable to effectively dissolve sclerotic casts that occluded the dentinal tubules. Depending upon the thickness of the surface layers at different locations in the natural lesion, self-etching primer treatment alone resulted in reduction of the thickness of the authentic hybrid layer (i.e. hybridised intertubular dentine). This was also true of phosphoric acid pre-conditioning along the deepest part of the natural lesions. Within this region, intertubular dentine completely devoid of an authentic hybrid layer could be seen in both treatment groups. Resin tags were also sparsely observed in such regions. Conclusions: Adhesive strategies that rely mostly on micromechanical retention alone may be compromised by the sporadic absence of the hybrid layer and resin tags in sclerotic dentine. Based on the ultrastructural features presented, it is further speculated that adaptive strategies such as removal of the surface layers and extended etching time may not be completely effective in improving bonding efficacy in highly sclerotic dentine. Interdisciplinary research should be continued to develop alternative procedures for bonding resins equally well to sound and sclerotic dentine.

MINIMIZING PATIENTS' EXPOSURE TO UNCURED COMPONENTS IN A DENTAL SEALANT

The authors conducted research to examine the effectiveness of six surface treatments in reducing the oxygen-inhibited layer of a commercially available, freshly polymerized, light-cured unfilled dental sealant. Surface treatments of a light-cured sealant (Delton Light Curing Pit & Fissure Sealant, Dentsply Ash) included no treatment (the control treatment), a 20-second exposure to an air/water syringe spray, 20 seconds' manual application of a wet or dry cotton roll, 20 seconds' manual application of pumice with a cotton pellet, and 20 seconds' application of a water/pumice slurry using a prophy cup on a slow-speed handpiece. The authors used high-pressure liquid chromatography to analyze the amount of monomers--bisphenol A glycidyl dimethacrylate, or Bis-GMA; triethylene glycol dimethacrylate, or TEGDMA; and bisphenol A dimethacrylate, or Bis-DMA--remaining after each treatment. A one-way analysis of variance indicated that use of only an air/water spray removed the least (P = .0001) amount of all monomers (only 68.3 percent of the control value). Application of wet or dry cotton reduced equivalent amounts of monomers (86.1 to 88.9 percent of the control value), and the amount of monomer remaining was less than that for the air/water syringe treatment (P = .0001). The use of pumice on either a cotton ball or in a prophy cup achieved the greatest reduction (P = .0001) in total amount of residual monomer (92.9 to 95.3 percent of the control value). Treatment that used pumice eliminated the greatest amount (from 93 percent to 95 percent of the untreated control values) of any type of residual monomer. A slurry of pumice is significantly more effective in removing the oxygen-inhibited layer from freshly cured sealants than either an air/water spray or wet or dry cotton alone. Clinicians can most effectively reduce patients' exposure to the uncured components in the oxygen-inhibited layer of sealants by using a mild abrasive, such as pumice, either on a cotton applicator or in a prophy cup.

Microbiological study of selected risk areas in dental technology laboratories

Objectives: To investigate the microbiological status of certain risk areas in the dental technology laboratory, namely pumice slurry, impression agar and curing water baths. Methods: Samples were inoculated onto selective and non-selective media. Resultant fungal and bacterial colonies were counted and identified to genus or species level. Results: Pumice slurry freshly made up using disinfectant was free from contamination, but colony counts approached 10 9 g −1 after 3 days' use. Impression agar was free from contamination, probably because of its method of preparation and maintenance, which appears to achieve sterility. Staphylococci were most commonly isolated from curing water baths (and from the air). Candida spp. and other yeasts were common in pumice slurry. Conclusions: Personnel and managers should be aware of the potential cross-contamination hazards posed by the presence of a range of opportunist pathogens in dental technology laboratories.

The effects of etching, micro-abrasion, and bleaching on surface enamel.

Conservative techniques for improving the appearance of discolored teeth have become popular in the past decade. These include: in-office bleaching with 30% hydrogen peroxide, which is applied on etched enamel with a gauze pad and then exposed to a bleaching light; home bleaching with a mild form of peroxide, such as 10% carbamide peroxide, which is applied on the tooth surface with custom-made mouthguards; and enamel micro-abrasion with 18% hydrochloric acid, which is applied in a pumice slurry. In this study, the in-office bleaching and enamel micro-abrasion techniques were performed on extracted teeth for investigation of their microscopic effects on the surface enamel. Specimens treated only with 37% phosphoric acid showed an enamel loss of 5.7 +/- 1.8 microns. The specimens treated with 37% phosphoric acid followed by 30% hydrogen peroxide showed enamel loss of 5.3 +/- 1.6 microns; this loss was probably not caused by the hydrogen peroxide, but rather by the etching with 37% phosphoric acid which preceded the hydrogen peroxide application. A direct application of 18% hydrochloric acid for 100 s resulted in a loss of 100 +/- 47 microns. The extent of enamel loss was much greater when the 18% hydrochloric acid was applied in a pumice slurry for the same period of time (360 +/- 130 microns), and the effect was time-dependent. Thus, the pumice and rotary prophy cup used in conjunction with the 18% hydrochloric acid contributed markedly to the loss of surface enamel, enhancing the non-selective stain-removing action of the hydrochloric acid. Therefore, the hydrochloric acid-pumice technique must be used clinically with caution.

Resin-to-enamel bonds: effect of cleaning the enamel surface with prophylaxis pastes containing fluoride or oil

A thorough dental prophylaxis before acid-etching of enamel is often recommended. In the method most commonly used, a slurry of pumice is applied with a rubber cup. Oil-based and fluoride-containing prophylaxis pastes are not generally recommended because of fears that they might hinder the etching process. This study assessed the 24-hour shear strength of resin (Scotchbond 2/Silux Plus) bonded to etched enamel which had been cleaned before acid-etching with a slowly rotating rubber cup loaded with: (1) wet pumice; (2) fluoride-containing non-oil-based pastes (En-De-Kay fluoride paste, Nupro fluoride paste); (3) non-fluoride oil-based paste (Kemdent); and (4) nonfluoride non-oil-based pastes (Nupro, Orapol). The bond strength values obtained after using the various cleaning agents were similar. Furthermore, bond failure in all the test groups was of the cohesive type. It therefore appears that prophylaxis pastes containing fluoride or oil may be used to clean the enamel before acid-etching.

Evaluation of finishing and polishing techniques on surface roughness of chromium-cobalt castings

The effect of finishing and polishing techniques on surface roughness of a chromium-cobalt alloy was evaluated by means of a stylus profile instrumentation and scanning electron microscopy. Scanning electron micrographs, surface profile tracings, surface roughness recordings, and statistical analysis of data support the finding that the best surface finish is obtained when sandblasting, hard stone, medium abrasive disk, second sandblasting, electropolishing, hard rubber point, hard felt disk with pumice slurry, and felt disk and soft brush with polishing paste are used progressively. The results of this study indicate that the finishing procedure should be carried out in a logical, systematic sequence of steps.

Cross-infection hazards associated with the use of pumice in dental laboratories

The bacteriological contamination of pumice slurry in polishing units in a dental clinical area (high risk), a production laboratory (medium risk), and a non-clinical teaching laboratory (low risk), was investigated. Slurry samples taken from all three areas were found to be heavily contaminated with pathogenic organisms. The investigations were repeated following the addition of a disinfectant with both bacteriocidal and virucidal properties to the pumice. Lower bacterial counts were obtained indicating that routine disinfection of pumice slurries is desirable.

An electron microscopic evaluation of the enamel surface subsequent to various debonding procedures

In this study, enamel surface roughness was investigated following the use of various combinations of bonding agents, burrs and polishing procedures. Orthodontic brackets were bonded to 135 premolar teeth extracted from adolescents. Subsequent to debonding and finishing, the enamel surface was gold coated and examined in the scanning electron microscope. Each 200x photomicrograph was graded according to surface roughness and assigned to one of four selected standard grades. The use of different composites and different burs showed no significant effect on the finish of the enamel surface using Chi-square tests. Only two finishing procedures had a significant effect on surface roughness. The use of a Soflex disc followed by pumice slurry resulted in the roughest enamel surface and the use of pumice alone produced the smoothest enamel surface.

Three-year evaluation of sealant: effect of isolation on efficacy

This study analyzes the clinical effectiveness of a sealant (Delton) and its requirements for retreatment to maintain the optimum protection from pit and fissure caries. One hundred paired occlusal surfaces were treated in 29 patients. Two contralateral, partially erupted, noncarious molars with deep occlusal fissures were randomly selected to receive a sealant; one had a rubber dam and the other, cotton rolls for isolation. Each tooth received a prophylaxis with a rubber cup and a pumice slurry. The dryness of the field was evaluated. Each tooth was etched with 37% phosphoric acid for 60 seconds, rinsed for 15 seconds, and dried for 30 seconds. Sealant (Delton Tinted) was applied with a syringe according to manufacturer's instructions. Sealants were evaluated and reapplied where indicated at 6-month intervals. The average 6-month retention rate over 36 months was 95%, regardless of the method of isolation used when the sealant was applied initially. No tooth under treatment with a sealant became carious. The retreatment rate was highest at baseline (8%) and at 6 months (11.3%). Of the total number of retreated sealants, 61% were from the mandibular arch. During the 36 months of this study, 31% of the treated teeth required at least one retreatment.

Adhesive bonding of dental luting cements; influence of surface treatment

Tensile bond strength of four different luting cements to smooth dentin surfaces was measured. A chisel edged, stainless steel ring was cemented to the butt end of a dentin cylinder. The dentin was polished to a plane and smooth surface before cementation. The cements were also applied to dentin surfaces that were treated with a pumice slurry, etched with different acid solutions, or covered with different liners. The results showed that the polycarboxylate cement had a tensile bond strength to smooth, untreated dentin of approximately 4 MN/m2. The zinc phosphate and EBA cements had a bond strength of 0,6 MN/m2 and the composite resin cement had no measurable bond to untreated dentin. All dentin treatments showed in general a decreasing effect on the bond strength of zinc phosphate, polycarboxylate and EBA cements, whereas that of composite resin cement showed a slight increase.

Evaluation of Acid Resistance of Enamel Using Microdetermination of Phosphorus

This investigation was undertaken to evaluate acid resistance of the surface enamel in vivo by the agar plate method reported by TAKEUCHI and KAWASAKI in 1963.The agar plate used in this study were made of agar 0.5% weight/volume) and 0.1M lactic acid-sodium lactate buffer solution, pH 4.5, mixed together by heating in a water bath for 20 minutes and left standing for 24 hours to solidity in a glass vessel of 2mm in depth. Circular plates of 6.5mm in diameter were cut out for experimental use. The surface of the upper central incisor was polished with a slurry of pumice and glycerin and then washed with re-distilled water and dried with compressed air. The plate was placed on the tooth surface for various periods of time to determine the effective time for application in vivo. After application, the phosphorus dissolved from the enamel was estimated by spectrophotometric determination with molybdate-safranin.The subjects were 366 children chosen at random from 1, 036 pupils of the 4th grade (9-10 years old) of elementary school. Fortyseven children were subjected to the application of an agar plate for 5 minutes and 317 of children for 3 minutes.In the case of 5-minute application, the mean value of phosphorus dissolved from the enamel was 1.87μg and the standard deviation was 0.39μg. For the 3-minute application, the values of mean and standard deviation were 1.60μg and 0.38μg, respectively.The data revealed that the distribution of phosphorus dissolved from the enamel showed a fairly normal pattern. With these experiments, the possibility of evaluation of acid resistance of enamel in vivo system by means of the agar plate method devised in this laboratory was demonstrated.Moreover, six female subiects aged 18-20 were studied to determine an appropriate time for in vivo application. The experimental times were set from one to five minutes for each analysis. The results showed that phosphorus dissolved from the enamel in these samples increased with increasing the time of application, notwithstanding the magnitude of acid resistance of individual enamel.From the data in this study, the 3-minute application of the agar plate in vivo seemed to be the best, since it obtains enough phosphorus for analysis with a minimum of trouble to the patient and takes into account possible error in measuring the time.