CN104568709B - Aeration zone soil adsorption parameter tester based on weak adsorption pollutants - Google Patents

Abstract

The invention provides a vadose zone soil adsorption parameter tester based on weakly adsorbed pollutants, which at least includes a sealing cover, an experimental section, a sampling device, a constant water head fixing device, a constant water head water supply device, a constant water head liquid supply device and a water supply Liquid switching device, wherein the experimental section is composed of soil column, gauze, perforated plexiglass plate, quartz sand filter layer and soil column support, the sealing cover is installed on the top of the soil column and seals the top of the soil column; the sampling device is set at On the upper part of the soil column, the fixed head water supply device and the fixed water head liquid supply device are installed on the fixed water head fixing device, and the upper and lower positions of the two can be adjusted; the bottom of the fixed head water supply device is connected with an outlet pipe, and the bottom of the fixed head liquid supply device It is connected with a liquid outlet pipe, and the water outlet pipe, liquid outlet pipe and input pipe are all connected to the water supply and liquid supply switching device. This device can accurately measure the retardation coefficient and distribution coefficient of weakly adsorbed pollutants in the soil in the vadose zone, and at the same time The method is simple and reliable.

Description

Determination of Adsorption Parameters in Vadose Zone Soil Based on Weakly Adsorbed Pollutants

technical field

The invention relates to a device for measuring adsorption parameters of vadose zone soil, in particular to a device for measuring adsorption parameters of vadose zone soil based on weakly adsorbed pollutants.

Background technique

With the rapid development of modern soil physics and computer technology, more and more mathematical models are used to study and master the migration process of chemical fertilizers and pesticides in unsaturated soil in the quantitative study of solute transport. Therefore, it is necessary to correctly determine the necessary parameters in the mathematical model, such as the retardation factor and partition coefficient of the governing equation of solute transport—the convection-diffusion equation. It is of great significance to the research on the pollution of groundwater by chemical fertilizers and pesticides, the law of regional water and salt movement, and the technology of sewage treatment and reuse.

Adsorption is an external manifestation of the physical and chemical interaction between the solid and liquid phases in soil. It participates in the migration process of solutes in the soil and blocks the migration of solutes. When solving the soil solute transport equation, if the solute will be adsorbed by the soil, the retardation factor R _d and the distribution coefficient K _d of the solute in the soil-water system must be determined through experiments. The retardation factor R _d of solute migration, that is, the ratio of water flow velocity to solute migration velocity, is dimensionless and is a parameter that characterizes the ability of soil to retard solute migration. The distribution coefficient K _d of the solute in the soil-water system, also known as the adsorption ratio, refers to the concentration ratio of the solute in the soil and water when the soil-water two-phase system reaches an equilibrium state, and the common unit is cm ³ /g, It is an important physical and chemical characteristic parameter describing the adsorption behavior of solutes in soil.

In physical chemistry and soil science, the static experimental method is often used to determine the soil adsorption parameters in the vadose zone. However, a large number of studies have shown that the results of static experiments are significantly higher than the results of experiments under osmotic conditions. This is because, in a static experiment, the entire surface of the soil particle interacts with the solution, while under osmotic conditions, the soil sample is not stirred, and the adsorption occurs only on the part of the surface of the particle that is in contact with the solution.

Contents of the invention

The invention provides a vadose zone soil adsorption parameter tester based on weakly adsorbed pollutants. The device solves the problems in the prior art and can accurately measure the retardation of weakly adsorbed pollutants in the vadose zone soil coefficient and distribution coefficient, and the method is simple and reliable.

The technical scheme adopted to realize the above-mentioned purpose of the present invention is:

An instrument for measuring adsorption parameters of vadose zone soil based on weakly adsorbed pollutants, comprising a sealing cover, an experimental section, a sampling device, a constant water head fixing device, a constant water head water supply device, a constant water head liquid supply device, and a water supply liquid supply switching device, wherein The experimental section is composed of soil column, gauze, perforated plexiglass plate, quartz sand filter layer and soil column support. The soil column is a cylindrical plexiglass column with soil samples placed inside, and the soil column support is located at the bottom of the soil column. , the sealing cover is installed on the top of the soil column and seals the top of the soil column; the sampling device is arranged on the upper part of the soil column for sampling from the soil column; gauze, perforated plexiglass plate, and quartz sand filter layer are all located , and the gauze is provided with upper and lower layers, and the bottom of the soil sample is the upper gauze, the quartz sand filter layer, the perforated plexiglass plate and the lower gauze, in which the upper gauze is in contact with the soil sample. The perforated plexiglass plate is located at 8-12 cm above the bottom surface of the plexiglass column. An inlet is arranged on the bottom surface of the plexiglass column, and an inlet is connected to an input pipe; the fixed head fixing device consists of a fixed head fixing frame and an iron bar. It consists of 4 iron rods and is installed vertically on the fixed head fixed frame. The fixed head water supply device and the fixed water head liquid supply device are installed on the fixed water head fixed device, and the upper and lower positions of the two can be adjusted; the fixed water head water supply device The bottom of the fixed head liquid supply device is connected with a water outlet pipe, and the bottom of the fixed water head liquid supply device is connected with a liquid outlet pipe. The water outlet pipe, the liquid outlet pipe and the input pipe are all connected to the water supply and liquid switching device. , control valve A and control valve B, the water outlet pipe, the liquid outlet pipe and the input pipe are all connected to the three-way joint, there is a control valve A on the water outlet pipe, and a control valve B on the liquid outlet pipe; the control valve A is open, When control valve B is closed, it is the state of water supply to the soil column; when control valve A is closed and control valve B is open, it is the state of liquid supply to the soil column.

The sealing cover is composed of a circular plexiglass plate, a silicone ring and a fixing ring. The silicone ring is fixed on the bottom surface of the circular plexiglass plate, and the diameter of the silicone ring is the same as the inner diameter of the soil column; the fixing ring is a hollow ring plexiglass plate, The fixing ring is fixed on the top edge of the soil column with chloroform; there are 6 corresponding screw holes on the fixing ring and the circular plexiglass plate, and the sealing cover is covered on the soil column, and the silicone ring is just buckled into the soil column. The sealing of the soil column can be guaranteed by fixing screws.

Described sampling device is made up of copper pagoda and sealing head, and copper pagoda is contained in apart from soil column top 5cm place, and the screw thread that matches with copper pagoda is arranged on sealing head.

The fixed head water supply device is made up of baffle plate, water supply overflow pipe, water supply pipe, water outlet pipe and inner screw. The board is vertically fixed in the cuboid box, and divides the whole constant head water supply device into left and right chambers. There is a water outlet hole on the far left at the bottom of the left chamber, and the outlet pipe is connected to the water outlet hole; there is a water supply hole on the right side of the bottom of the left chamber, and the water supply pipe It is connected with the water supply hole; there is an overflow hole at the bottom of the right chamber, and the water supply overflow pipe is connected with the overflow hole; the rear wall of the fixed head water supply device has two holes for iron bars to pass through, and the fixed head can be fixed by tightening the inner screw. The water head water supply device is fixed on the fixed head fixing frame.

The fixed water head liquid supply device is composed of a baffle plate, a liquid supply overflow pipe, a liquid supply pipe, a liquid outlet pipe and an inner screw. The fixed water head liquid supply device is a cuboid box with a height of 10 cm without a top surface. The height is 8cm, the baffle is vertically fixed in the cuboid box, and the whole constant water head liquid supply device is divided into left and right chambers, the bottom of the left chamber is equipped with a liquid outlet hole on the far left, and the liquid outlet pipe is connected to the liquid outlet hole; the bottom of the left chamber There is a liquid supply hole on the right side, and the liquid supply pipe is connected with the liquid supply hole; there is an overflow hole at the bottom of the right chamber, and the liquid supply overflow pipe is connected with the overflow hole; there are two iron supply pipes on the rear wall of the constant water head liquid supply device. The hole through which the stick passes, the constant water head liquid supply device can be fixed on the constant water head fixing frame by tightening the inner screw.

The operating principle of a kind of vadose zone soil adsorption parameter measuring instrument based on weakly adsorbed pollutants of the present invention is as follows:

Solute transport is retarded when the rate of movement of solutes is slower than the rate of movement of water in the soil. The difference between the movement speed of water in the soil and the migration speed of solute is caused by the adsorption of solute by the soil, and the retardation factor R _d is usually used to evaluate the migration speed of the hindered solute:

In the formula: R _d — is the retardation factor of solute migration in soil;

υ _w — is the average pore flow velocity of water in the soil, cm/min;

υ _c — is the average migration velocity of soil solute, cm/min.

By measuring υ _w and υ _c , R _d can be calculated. Then, the distribution coefficient K _d can be calculated using formula (2):

In the formula: R _d — is the blocking factor;

ρ _d - is the dry density of the soil, g/cm ³ ;

θ—is the volume moisture content of the soil, cm ³ /cm ³ ;

K _d ——distribution coefficient of solute in soil-water system, cm ³ /g.

Compared with the prior art, the vadose zone soil adsorption parameter detector based on weakly adsorbed pollutants of the present invention has the following advantages:

(1) The measuring instrument of the present invention is the adsorption parameter of aerated zone soil determined by the dynamic soil column adsorption method, and the adsorption is only carried out on that part of the surface where the particle contacts the solution, while the whole surface of the static adsorption method soil particle is in contact with the solution. This method is more accurate and reliable than the static adsorption method.

(2) The fixed water head water supply device and the fixed water head liquid supply device of the measuring instrument of the present invention can flexibly control the water supply and liquid supply to the soil column through the water supply and liquid supply switching device.

(3) The measuring instrument of the present invention is simple in structure, easy to use, and has wide application prospects. It can be widely used in the hydrological research of the vadose zone. As a teaching instrument, it can help students better understand the principle and scope of application of soil adsorption parameters in the vadose zone of weakly adsorbed pollutants; and it can be used to measure weakly adsorbed pollutants in production. The adsorption parameters of the vadose zone soil provide research parameters for farmland soil improvement, and provide research parameters for the migration of weakly adsorbed pollutants in the vadose zone soil.

Description of drawings

Figure 1 is a schematic diagram of the structure of an instrument for measuring adsorption parameters of vadose zone soil based on weakly adsorbed pollutants.

Fig. 2 is a schematic diagram of the top cover structure of the present invention.

Fig. 3 is a top view of the constant water head water supply device in the present invention.

In the figure: 1-soil column, 2-sampling device, 3-sealing cover, 4-gauze, 5-quartz sand filter layer, 6-perforated plexiglass plate, 7-tee joint, 8-control valve A, 9-control valve B, 10-fixed water head liquid supply device, 11-baffle plate, 12-liquid supply overflow pipe, 13-liquid supply pipe, 14-liquid outlet pipe, 15-fixed water head water supply device, 16-water supply overflow Flow pipe, 17-water supply pipe, 18-outlet pipe, 19-fixed water head fixing frame, 20-inlet, 21-soil column support, 22-copper pagoda, 23-sealing head, 24-plexiglass plate, 25-silicone ring , 26-fixing ring, 27-screw hole A, 28-fixing screw, 29-screw hole B, 30-back wall, 31-iron rod, 32-inner screw, 33-overflow hole, 34-water supply hole, 35-water outlet hole.

detailed description

The present invention will be described in detail below in conjunction with specific examples, but the protection scope of the present invention is not limited to the following examples.

The present invention provides an instrument for measuring adsorption parameters of vadose zone soil based on weakly adsorbed pollutants, the structure of which is shown in FIG. 1 . It includes a sealing cover, an experiment section, a sampling device, a constant water head fixing device, a constant water head water supply device, a constant water head liquid supply device and a water supply liquid supply switching device. The sealing cover is composed of a top cover 3, a fixing ring 26 and a fixing screw 28, as shown in FIG. 2 . The top of the sealing cover 3 is a circular plexiglass plate 24 with a diameter of 28cm, and the bottom is a silicone ring 25 with the same diameter as the experimental soil column inner diameter as 20cm; the fixed ring 26 is a hollow ring plexiglass plate with an inner diameter of 22cm and an outer diameter of 28cm , the fixed ring 25 is fixed on the soil column 1 with chloroform; the fixed ring 26 is provided with corresponding 6 screw holes A27 of 0.5 cm, and the top cover 3 plexiglass plate is provided with corresponding 6 screw holes of 0.5 cm. The positions of hole B29, screw hole A27 and screw hole B28 correspond to each other; the top cover 3 is covered on the soil column 1, and the silicone ring 25 is just buckled in the soil column 1, and the sealing of the soil column 1 can be guaranteed by the fixing screw 28.

The experimental section is composed of a soil column 1 , a gauze, a perforated organic glass plate 6 , a filter layer of quartz sand 5 , a soil column support 21 and an inlet 20 . The soil column 1 is a plexiglass column with an inner diameter of 120 cm and a wall thickness of 1 cm. A sampling device 2 is arranged on the top of the soil column; a perforated plexiglass plate 6 is arranged at 10 cm from the bottom of the soil column, and a gauze 4 is stuck under the plexiglass plate. ; On the perforated plexiglass plate 6, be provided with 5cm thick quartz sand filter layer 5; Quartz sand filter layer 5, perforated plexiglass 6, gauze 4 can buffer water flow; Soil pillar support 21 is four high 5cm organic The glass plate is connected to the bottom of the soil column 1 through chloroform; an inlet 20 is provided at the bottom of the soil column for water and liquid to enter the soil column.

The sampling device 2 is composed of a copper pagoda 22 and a sealing head 23 . Copper pagoda 22 is contained in apart from earth column 1 top 5cm places, and the screw thread that matches with copper pagoda 22 is arranged on sealing head 23.

Described fixed water head fixing device is made up of fixed water head fixing frame 19 and iron bar 31. Fixed water head fixed mount is made by angle steel, is provided with 4 iron bars 31 on fixed water head fixed mount 19.

The structure of the fixed head water supply device as shown in Figure 3, consists of baffle plate 11, water supply overflow pipe 16, water supply pipe 17, water outlet pipe 18, overflow hole 33, water supply hole 34, water outlet hole 35 and inner screw 32 compositions. The fixed head water supply device is a cuboid box with a height of 10 cm and a top surface missing. In the middle part of the fixed head water supply device, a baffle plate 11 with a height of 8 cm divides the entire fixed head water supply device into two chambers, and the bottom of the left chamber is provided with a water outlet on the far left. Hole 35, water outlet pipe 18 is connected with water outlet hole 35; the right side of the left chamber bottom is provided with water supply hole 34, water supply pipe 17 is connected with water supply hole 34; right chamber bottom is provided with overflow hole 33, water supply overflow pipe 16 and overflow Hole 33 links to each other; The rear wall 30 of fixed water head water supply device has two holes for iron bars 31 to pass through, and fixed water head water supply device can be fixed on the fixed water head fixed mount 19 by tightening inner screw 32. The structure of the constant water head liquid supply device is the same as that of the constant water head water supply device.

The water supply and liquid supply switching device is composed of a three-way joint 7, a control valve A8, and a control valve B9. The liquid outlet pipe 14 and the water outlet pipe 18 are connected to the soil column inlet 20 through the three-way joint 7; there is a control valve A8 on the water outlet pipe 18, and a control valve B9 on the liquid outlet pipe; the control valve A8 is opened, and the control valve B9 is closed. It is the state of supplying water to the soil column; the control valve A8 is closed, and the control valve B9 is opened, which is the state of supplying water to the soil column 1.

The working steps and experimental results of a vadose zone soil adsorption parameter measuring instrument based on weakly adsorbed pollutants provided by the present invention are organized as follows:

1. Working steps

1. Collect samples

Collect the samples required for the experiment and set them aside; at the same time, use a ring knife to take the original sample, and use the drying-weighing method to measure its dry density ρ′ _d .

2. Preparation of samples

The sample was air-dried, crushed and sieved (with a pore size of 2mm), and set aside.

3. Wet sample

Spread a layer of sieved and air-dried samples in the soil mixing container, and sprinkle water evenly with a watering can; then, spread another layer of samples, and then sprinkle another layer of water, so as to wet the samples completely, and then cover them and keep stuffy overnight , the outer layer is covered with a plastic bag to prevent evaporation and redistribute water in the sample.

4. The initial mass moisture content of the test sample

Fully mix the wet samples, take a small amount of soil samples from different positions and put them into small aluminum boxes of known mass, and make 5 parallel samples. After weighing with a balance, place it in an oven and dry it at 105°C for 12 hours to a constant weight. After cooling, weigh the mass and calculate the mass moisture content θ _m .

5. Filling soil column

Install a 5cm quartz sand filter layer at 10cm from the bottom of the soil column; the middle section is a sample section of 100cm, according to the diameter D, length L, dry density ρ′ _d and initial moisture content θ _m of the sample, _fill according to the formula m=[( πR ² )/4]·ρ′ _d ·(1+θ _m ) Calculate the filling mass, fill the soil column with equal mass in sections (5cm per layer), and roughen the layers to prevent the formation of loss between layers.

6. Saturated soil column

Connect the fixed head liquid supply device with the fixed head water supply device and the inlet of the lower end of the soil column, open the control valve A, close the control valve B, and gradually saturate the soil column with distilled water from bottom to top to remove the air in the soil column and make the soil column The pores are fully saturated with water. 7. Leaching experiment

Saturated until the soil column has a constant weight, continue to supply water at a fixed head. After the flow rate of distilled water is stable, close the control valve A, open the control valve B, and input the existing solute standard working solution (concentration is C _into g/L) at the same constant water head, and at the same time Start to collect the effluent, input the solute standard working solution for a period of time (the input time is determined by the type of solute, the physical properties of the sample, the seepage velocity, etc.), open the control valve A, close the control valve B, continue to supply water at a fixed head, and use distilled water for leaching . Record the water supply flow Q.

8. Collect water samples

Samples are taken at timing t _i (i=0, 1, 2, 3...) at the sampling device to measure its concentration C _i (i=0, 1, 2, 3...) and draw its breakthrough curve.

9. Determination of dry density and porosity of samples

After sampling, record the end time t _n of the experiment; then, in the soil column, use the ring knife to take the original sample, and use the drying weighing method to measure its dry density ρ _d and porosity n.

10. Determination of water samples

Use an ion meter to measure the concentration C _i of the solute in the water sample. Water samples or soil samples that need to be pretreated shall be measured after pretreatment.

2. Arrangement of experimental results

1. Calculate the average pore velocity of water in the soil column

Use formula (3) to calculate

In the formula: υ _w — is the average pore flow velocity of moisture in the soil column, cm/min;

V— is the average Darcy velocity in the soil column, cm/min;

n—is the porosity of the sample in the soil column, %/%.

2. Calculate the average migration velocity of pollutants in the soil column

(1) Calculated according to the penetration curve

① Take the relative concentration C _i /C _in as the ordinate (C _i is the solute concentration in the seepage water, C _in is the solute concentration in the infiltration water), and the breakthrough time t is the abscissa, draw the solute concentration breakthrough curve.

② Read _the t _0.5 value when C/C = 0.5 on the concentration breakthrough curve, which is the time for the solute to penetrate the sample in the soil column.

③Use formula (4) to calculate the average migration velocity of the adsorbed pollutants

υ _c ＝L/t _0.5 (4) In the formula: υ _c —— is the average migration speed of the adsorbed pollutants in the soil, cm/min;

L—is the length of the sample in the soil column, cm;

t _{0.5 —time} corresponding to relative concentration C/C _in =0.5, min.

3. Calculate the blocking factor

By substituting υ _w and υ _c into formula (1), the retardation factor R _d can be calculated.

4. Calculate the distribution coefficient

Substituting R _d , ρ _d , θ (when the sample is saturated with water, its moisture content is equal to its porosity n) into formula (2), the distribution coefficient K _d of weakly adsorbed pollutants in the soil-water system can be calculated.

The measuring instrument of the invention is simple in structure, easy to use and has wide application prospects. If it can be widely used in hydrological research in the vadose zone, as a teaching instrument, it will help students better understand the principle and scope of application of soil adsorption parameters in the vadose zone of weakly adsorbed pollutants; and it can be used to measure weakly adsorbed pollution in production The adsorption parameters of the soil in the aerated zone provide research parameters for farmland soil improvement, and provide research parameters for the migration of weakly adsorbed pollutants in the aerated zone soil.

Claims (5)

1. a kind of based on weakly stable pollutant aerated zone soil absorption parameter determinator it is characterised in that：Including closure, reality Test section, sampler, determine head fixing device, determine head water supply installation, determine head liquid feed device and water supply feed flow switching device, Wherein experimental section is made up of earth pillar, gauze, poly (methyl methacrylate) plate with holes, quartzy sand filter blanket and earth pillar support, and earth pillar is put for inside Be equipped with the Organic glass column of the cylindrical shape of soil sample, earth pillar support be located at earth pillar bottom, closure be installed on earth pillar top and Top sealing by earth pillar；The top that sampler is arranged at earth pillar is used for sampling from earth pillar；Gauze, lucite with holes Plate, quartzy sand filter blanket are respectively positioned in earth pillar, and gauze is provided with upper and lower two-layer, and the lower section of soil sample is from top to bottom followed successively by upper strata yarn Net, quartzy sand filter blanket, poly (methyl methacrylate) plate with holes and lower floor's gauze, its at the middle and upper levels gauze contact with soil sample, organic glass with holes Glass plate is located at the top 8～12cm of bottom surface of Organic glass column, and the bottom surface of Organic glass column is provided with entrance, and porch is even It is connected to input pipe；Described determine head fixing device by determining head fixed mount and iron rod forms, iron rod is provided with 4 and vertically-mounted On determining head fixed mount, determine head water supply installation, determine head liquid feed device and be mounted on determining in head fixing device and both Upper-lower position scalable；The bottom determining head water supply installation is connected with outlet pipe, and the bottom determining head liquid feed device is connected with Drain pipe, outlet pipe, drain pipe and input pipe are all connected with water supply feed flow switching device, described water supply feed flow switching device It is made up of three way cock, control valve A and control valve B, outlet pipe, drain pipe and input pipe are all connected with three way cock, going out Control valve A is had on water pipe, control valve B is had on drain pipe；Control valve A is opened, control valve B is closed, and is to earth pillar service position；Control Valve A closes, control valve B is opened, and is to supply liquid status to earth pillar.

2. the aerated zone soil absorption parameter determinator based on weakly stable pollutant according to claim 1, its feature exists In：Closure is made up of circular poly (methyl methacrylate) plate, silica gel ring and retainer ring, and silica gel ring is fixed on the bottom of circular poly (methyl methacrylate) plate On face, and the diameter of silica gel ring is identical with earth pillar internal diameter；Retainer ring is hollow ring poly (methyl methacrylate) plate, and retainer ring chloroform is fixed On the top sides edge of earth pillar；Retainer ring and circular poly (methyl methacrylate) plate are provided with corresponding 6 screw, closure is covered On earth pillar, silica gel ring just buckles in earth pillar, can ensure the sealing of earth pillar by fixed screw.

3. the aerated zone soil absorption parameter determinator based on weakly stable pollutant according to claim 1, its feature exists In：Described sampler is made up of copper pagoda and seal head, and copper pagoda is contained at the 5cm of earth pillar top, seal head has with The screw thread that copper pagoda matches.

4. the aerated zone soil absorption parameter determinator based on weakly stable pollutant according to claim 1, its feature exists In：Described head water supply installation of determining is made up of baffle plate, water supply overflow pipe, feed pipe, outlet pipe and interior rotary screw, determines head and supplies Water installations are the cuboid box of the scarce top surface of high 10cm, and the height of baffle plate is 8cm, and baffle plate is vertically fixed in cuboid box, will Entirely determine head water supply installation and be divided into left and right two Room, left room bottom Far Left is provided with apopore, and outlet pipe is connected with apopore；Left Room bottom right is provided with water supply hole, and feed pipe is connected with water supply hole；Right ventricle bottom is provided with spout hole, water supply overflow pipe and spout hole It is connected；The rear wall determining head water supply installation has two holes passed through for iron rod, can be supplied water determining head by screwing interior rotary screw Device is fixed on to be determined on head fixed mount.

5. the aerated zone soil absorption parameter determinator based on weakly stable pollutant according to claim 1, its feature exists In：Described head liquid feed device of determining is made up of baffle plate, feed flow overflow pipe, feed pipe, drain pipe and interior rotary screw, determines head and supplies Liquid device is the cuboid box of the scarce top surface of high 10cm, and the height of baffle plate is 8cm, and baffle plate is vertically fixed in cuboid box, will Entirely determine head liquid feed device and be divided into left and right two Room, left room bottom Far Left is provided with liquid outlet, and drain pipe is connected with liquid outlet；Left Room bottom right is provided with confession fluid apertures, and feed pipe is connected with confession fluid apertures；Right ventricle bottom is provided with spout hole, feed flow overflow pipe and spout hole It is connected；The rear wall determining head liquid feed device has two holes passed through for iron rod, can will determine head feed flow by screwing interior rotary screw Device is fixed on to be determined on head fixed mount.